Investigating Coronal Holes and CMEs as Sources of Brightness Depletion Detected in PSP/WISPR Images

Sérgio Sacani
Sérgio SacaniSupervisor de Geologia na Halliburton à Halliburton

The Parker Solar Probe (PSP) mission provides a unique opportunity to observe the solar corona from distances below 20 R☉. In this work, we utilize white light images from the Wide-field Imager for Solar PRobe aboard the PSP from solar encounters 10 through 13 to examine the causes of brightness depletions of the corona during the rapid transit of PSP through the perihelia of its orbit. We analyze the effect of (1) coronal holes (CHs) and (2) energetic coronal mass ejection (CME) events on the observed brightness of the images. We speculate on the causes of the brightness depletions, ascribing them to the evacuation of (1) free electrons (reduced K-corona) and (2) interplanetary dust (reduced F-corona). In particular, we show that (1) the presence of CHs in all of the orbits is directly correlated with the depletion of the global white light emission recorded, and (2) a huge CME event in encounter 13 caused a very deep depletion in its wake that removed the electron content as well as some of the interplanetary dust.

Investigating Coronal Holes and CMEs as Sources of Brightness Depletion Detected in
PSP/WISPR Images
Guillermo Stenborg1
, Evangelos Paouris1,2
, Russell A. Howard1
, Angelos Vourlidas1
, and Phillip Hess3
1
Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA; guillermo.stenborg@jhuapl.edu
2
George Mason University, 4400 University Dr, Fairfax, VA 22030, USA
3
Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
Received 2023 March 27; revised 2023 May 3; accepted 2023 May 3; published 2023 May 31
Abstract
The Parker Solar Probe (PSP) mission provides a unique opportunity to observe the solar corona from distances
below 20 R☉. In this work, we utilize white light images from the Wide-field Imager for Solar PRobe aboard the
PSP from solar encounters 10 through 13 to examine the causes of brightness depletions of the corona during the
rapid transit of PSP through the perihelia of its orbit. We analyze the effect of (1) coronal holes (CHs) and (2)
energetic coronal mass ejection (CME) events on the observed brightness of the images. We speculate on the
causes of the brightness depletions, ascribing them to the evacuation of (1) free electrons (reduced K-corona) and
(2) interplanetary dust (reduced F-corona). In particular, we show that (1) the presence of CHs in all of the orbits is
directly correlated with the depletion of the global white light emission recorded, and (2) a huge CME event in
encounter 13 caused a very deep depletion in its wake that removed the electron content as well as some of the
interplanetary dust.
Unified Astronomy Thesaurus concepts: Solar F corona (1991); Solar K corona (2042); Solar coronal holes (1484);
Solar wind (1534); Solar coronal mass ejections (310); Interplanetary dust (821)
1. Introduction
White light (WL) imaging constitutes the main means of
tracking coronal mass ejection (CME) events from the inner
corona into the interplanetary medium (e.g., Webb &
Howard 2012). The recorded signal consists of photospheric
light scattered both by the free electrons in the corona via
Thomson scattering (the K-corona) and by the dust particles
orbiting the Sun (the F-corona) together with planetary and
galactic sources. The F-corona generally dominates the signal
beyond about 5 R☉ and it, therefore, must be removed to allow
for the tracking of coronal structures. For observations taken
from a fixed distance from the Sun and within the ecliptic
plane, the F-corona contribution is relatively stable, with the
variations being mainly due to the ecliptic longitude of the
observer with respect to the ascending node of the zodiacal dust
cloud (Stenborg & Howard 2017). When the observer is within
the plane of symmetry of the zodiacal dust cloud, the
longitudinal dependence disappears (see, e.g., Stenborg et al.
2018, and references therein).
The Parker Solar Probe (PSP; Fox et al. 2016) mission is
deepening our understanding of the near-Sun environment
(Raouafi et al. 2023). The PSP spacecraft (S/C) carries only
one remote sensing instrument suite, the Wide-field Imager for
Solar Probe (WISPR; Vourlidas et al. 2016), which consists of
two WL imaging telescopes. The high ellipticity and low
perihelion of the PSP orbit translates into a high speed of the
S/C close to perihelion (>150 km s−1
) and results in a
continuously variable background scene within each tele-
scope’s field of view (FOV). Within the 10 days nominal PSP
science windows in particular (i.e., when the S/C is below
0.25 au), there is a rapid change of heliocentric distance while
the S/C approaches (and recedes from) perihelion along with a
large range of ecliptic longitudes. Thus the volume of space
covered by the WISPR FOV changes greatly from image to
image. Moreover, since PSP is embedded in the solar corona
and is quickly moving through it, rather than viewing distant
structures like most previous coronal imagers, WISPR becomes
a local imager. As one result, the F-corona brightness varies
from image to image, and the standard techniques employed to
reveal the faint, dynamic K-corona features in WL images used
by instruments at 1 au (e.g., base or running differences/ratios)
no longer work. An additional complication arises from the
continually changing spatial resolution of the images as PSP
approaches and then recedes from the Sun. This has created
new issues that needed to be solved for the interpretation of
these exciting observations (see, e.g., Liewer et al. 2019, 2020;
Nisticò et al. 2020).
Most of the attention to date in analyzing the WISPR data
has been on the structures that increase brightness such as
coronal streamers and coronal rays (e.g., Liewer et al. 2022;
Poirier et al. 2022), CME events (e.g., Braga & Vourlidas 2021;
Wood et al. 2021), Venus (Wood et al. 2022), asteroids and
comets (Battams et al. 2022, 2020), and interplanetary dust
(Stenborg et al. 2022, 2021b, 2021a). As PSP transits through
its orbit about the Sun, WISPR records these brightness
enhancements along with regions of lower brightness. But what
other structures could be affecting the WISPR observations?
For example, from 1 au, regions of depleted brightness are also
routinely observed in WL coronagraphs, such as observations
of coronal holes (CHs) over the Sun’s poles. These are regions
of very low brightness caused by the very low electron density
in CHs (Altschuler et al. 1972). Moreover, WL coronagraphs
have also recorded brightness depletions following the passage
of streamer blowout (SBO) events through the FOV (e.g.,
Vourlidas & Webb 2018, and references therein). Here, the
depletion is caused by the evacuation of the electron corona in
the CME volume by the transit of the SBO through the FOV. A
The Astrophysical Journal, 949:61 (12pp), 2023 June 1 https://doi.org/10.3847/1538-4357/acd2cf
© 2023. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms
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1
similar brightness depletion was first seen from the SOLWIND
instrument on a low-Earth-orbiting satellite (P78-1; Sheeley
et al. 1980), on the images taken just after the S/C emerged
from orbit night (Howard et al. 1985). This depletion was
called a remnant CME.
Another possible mechanism to decrease the observed
brightness is to reduce the F-corona, i.e., to reduce the density
of the interplanetary dust in orbit about the Sun. This was
proposed in 2003 by Ragot & Kahler (2003), who suggested
that CMEs may interact with the interplanetary dust and carry
the dust outward. Such an effect has never been detected from
1 au observations. This failure was probably due to observing
factors such as the difference in geometry affecting the
scattering efficiency for dust and electron scattering, but
perhaps more due to the long integration of dust along the
line of sight (LOS) from 1 au obscuring the transient effect of
the CME passage. Thanks to the geometry of the PSP orbit, we
now have a much better platform to search for dust depletion
after the passage of big CME events. We point out that WISPR
has already detected a rather stable dust depletion zone (DDZ)
extending from 19 R☉ down to the dust-free zone at about 5 R☉
(Stenborg et al. 2022, and references therein) in contrast to the
more ephemeral depletion zone that might occur due to a CME.
Therefore, in this paper we examine the brightness
depletions observed during the nominal science encounters
10 through 13 with a two-fold objective: (1) to see if/when
they can be ascribed to evacuation of free electrons (i.e., the
K-corona) along with their cause(s), and (2) to check if there is
any evidence for evacuation of interplanetary dust (i.e., the
F-corona). To achieve these objectives, we need to obtain a
background that accounts for the variability of the K-corona
without placing an implied bias on the F-corona.
The paper is organized as follows. Section 2 describes the
observations used and the methodology implemented to
achieve the objectives pursued. Our results are presented in
Section 3, where we first exhibit the observational signatures
followed by a comprehensive analysis of these signatures
(Sections 3.1 and 3.2). The findings are discussed and put into
context in Section 4. Finally, our conclusions are summarized
in Section 5.
2. Observations and Methodology
After the most recent Venus flyby (2021 October 16), PSP
reached a perihelion distance of 13.29 R☉ on 2021 November
21 (orbit 10). The geometry of the PSP orbit will remain
invariant until the 6th Venus flyby, which is scheduled for 2023
August 21. During this time interval, a set of seven orbits (10
through 16) will have reached the same perihelion distance.
The constancy of these orbits provides a valuable data set to
analyze the K- and F-corona background environment. Here,
we analyze the observations of the WISPR inner telescope
(hereafter WISPR-I) during the nominal science encounters 10
through 13 (hereafter E10, E11, E12, and E13).
The WISPR suite is mounted on the ram side of the S/C, i.e.,
it looks in the direction that the S/C is moving. The WISPR-I
FOV has an angular extent of about 40° in both the horizontal
and vertical directions, covering the region between 13°
.5 and
53° elongation along the projected Venus orbital path (see, e.g.,
Figure 1 in Howard et al. 2022). Due to the geometry of the
orbit, the constant angular FOV corresponds to different ranges
of heliocentric distances (see, e.g., Figure 1 in Stenborg et al.
2022—exemplified for E09) and spatial resolution. (At the
perihelion distance reached during orbits 10 through 13, the
nominal 2 pixel 1 au resolution of 2 34 becomes 8 8.)
During the encounters under consideration, WISPR-I
acquired images (960 × 1024 pixels2
) with a 15 minutes
cadence, increasing to 7.5 minutes for a couple of days around
perihelion. The images were calibrated with the standard
calibration procedure described in Hess et al. (2021). For this
work, we had to minimize the effects of the K-corona
variability, which is mainly caused by CMEs and streamers
rotating in and out of the FOV. To accomplish this, we
considered the data from all four encounters together to
estimate the minimum coronal signal (K+F) at any given
heliocentric distance. This approach adopts the same philoso-
phy that is widely used to generate the quicklook images from
the SOHO/LASCO (Brueckner et al. 1995) and STEREO/
SECCHI (Howard et al. 2008) coronagraphs, but with a
different methodology. The minimum coronal signal from
imagers at 1 au can be estimated by taking the minimum value
of the observations covering one solar rotation. This is not
possible when the observer rapidly changes its heliocentric
distance; hence a different approach must be used, i.e., we
consider the set of observations obtained from any given
distance independently from one another to create the
minimum background corona at each distance. Since the taking
of the WISPR images is scheduled by time rather than by
distance from the Sun, the S/C radial distance from which the
images were taken in each encounter can vary slightly from
each other. Thus, we grouped the images in sets of four images
(one image per encounter acquired at a similar heliocentric
distance and similar roll angle). We used the images from E13
as the reference and then compared the distances and the roll
angles of the images from the other three encounters to the
values of the reference set. To create the sets of four images
taken at a “similar” distance, we found the corresponding
images from the other three encounters that were within ±0.5%
of the E13 reference image. Similarly, the S/C roll angle had to
be within ±7.5% of the E13 reference image. If any one of the
encounters did not have a corresponding point then a set for
that distance was not used. The distance and roll angle
tolerances were adjusted to obtain a significant number of
image sets from nearly cospatial viewpoints while avoiding
uneven gaps among the resulting cadences of the sets (i.e., to
avoid too many large gaps in the distance coverage). The above
tolerances for the distances and roll angles resulted in time
series of 952 images per encounter.
For each set of four images, Ij
d
(with j = 1...4 for each
d = 1...952) we compute the background brightness, Bd
, as the
minimum brightness of each set at each pixel position. The
background images mainly reflect the contributions of the
diffuse, pseudo-stationary K-corona and the F-corona. This
approach does retain some remnant discrete features, but the
contribution of transient K-structures is greatly diminished. We
then compute the excess K-coronal brightness, DK
d
j
(in
percentage) of each image, i.e., D = I B
K
d
j
d d
j
[ −1] × 100,
relative to the corresponding background image.
In the next section, we use latitude–time (LT) maps created
from the excess brightness images to investigate the plausible
connections between regions of minimum excess brightness
with the crossing of the LOS through (i) the extension of CHs
in the lower corona and (ii) the post-CME regions.
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The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
3. Results
To visualize the time evolution of the global excess
brightness at a glance for the four encounters, we created the
maps shown in Figure 1. Each column (1024 pixels), displayed
as a function of hours from perihelion, corresponds to the
median of each WISPR-I excess brightness row (960 pixels), so
the 960 columns of each image are collapsed into just one
column. These maps preserve the latitudinal variation in the
image frame but collapse the variation in the perpendicular
direction (i.e., in elongation) into a single value; hence, we will
simply refer to them, in brief, as LT maps hereafter. The LT
maps show regions exhibiting very low (∼0%) excess bright-
ness (referred to as depletion regions, hereafter), regions with
low (about 2%–5%) excess brightness, and bright regions. The
latter correspond to regions dominated by either the passage of
big CME events or by the presence of coronal streamers. Our
goal is to find the causes for the large, homogeneous depletion
regions, which are identified in the maps, to first approx-
imation, with the yellow circles.
3.1. The Impact of CHs
Observations from 1 au have long established that the WL
emission above CHs is depleted. As mentioned in Section 1,
during solar encounters WISPR becomes a local K-corona
imager as the Thomson sphere (TS; Vourlidas & Howard 2006)
shrinks while the S/C approaches perihelion. Thus, along each
LOS, the TS becomes gradually smaller during the approach to
perihelion (and hence lies gradually closer to the S/C;
Vourlidas et al. 2016). We emphasize that we are referring to
linear distances and not angular separations, which, of course,
remain the same. Notwithstanding, we expect that brightness
depletions due to CHs will manifest in WISPR-I images. In the
following, we assess when and to what extent they affect the
overall brightness observed.
To identify and track the location of the candidate CHs for
each encounter, we use extreme ultraviolet (EUV) observations
in the 193 Å and 211 Å channels from the Atmospheric
Imaging Assembly (AIA; Lemen et al. 2012) instrument on
board the Solar Dynamic Observatory (SDO; Pesnell et al.
2012). After visual identification, the CHs are segmented to
derive their boundaries and the location of the respective
centroids when they are in Earth-facing position (i.e., ±10°
from the central meridian) to minimize projection effects. For
the CH segmentation, we use the Spatial Possibilistic
Clustering Algorithm (SPoCA; Verbeeck et al. 2014), which
is a feature recognition technique implemented in the SunPy
open-source software package (The SunPy Community
et al 2020). The identified CHs are shown in Figure 2, which
displays two SDO/AIA images per encounter (i.e., E10
through E13) obtained in the 193 Å channel at the time of
their central meridian crossing. The boundaries of the CHs are
the helioprojective coordinates (latitude and longitude) of the
points of the polygons delineated with the dashed cyan lines.
The coordinates of the centroid (hereafter cLAT and cLON) are
determined simply as the average of the latitudes and
longitudes of the polygon points. To validate this approach,
we used an alternative methodology, namely an unsupervised
machine-learning technique aimed at identifying clusters of
data objects (K-means clustering, as implemented in the python
scikit-learn library; Pedregosa et al. 2011). The centroid
coordinates obtained with this alternative approach match the
ones obtained with the simple averaging calculation. The
longitudinal extent of the CH is defined as the range covered by
the maximum and minimum values of the longitude of the
polygon points. These values are summarized in Table 1. The
first column identifies the corresponding panel, (a) to (h), in
Figure 2. The second column indicates the encounter number,
while the third one labels the CHs of interest for each
encounter. Columns 4–7 display the reference time stamp (UT)
Figure 1. LT maps showing the time evolution of the median brightness of each WISPR-I image row for E10, E11, E12, and E13. The circles mark the brightness
depletion regions discussed in the text. The vertical white dashed line indicates the time of perihelion, and the horizontal white dashed lines indicate the latitudinal
bands used to compute the proxy, Pch, displayed in Figure 4.
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The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
when the CH was in Earth-directed position, the latitude and
longitude of the centroid, and the latitudinal and longitudinal
extend for each CH according to the polygon boundary points
(see, e.g., the cyan dashed lines in Figure 2), respectively. In
some cases, other CHs may have been seen on the disk in the
time intervals considered. The ones not explicitly shown were
excluded from the analyses either because they were polar CHs
or their position was on the opposite hemisphere to PSP (in
either case they could have not affected the brightness in the
WISPR-I images).
CHs are dynamic structures and their boundaries evolve with
time. We assume, however, that their boundaries do not change
substantially during the time elapsed between their passage
through the central meridian (identification stage) until when
they are in a favorable location to influence the brightness
observed in the WISPR-I images (see, e.g., Wang et al. 2010,
and references therein). Insley et al. (1995) reported that
isolated CHs at low and mid latitudes tend to rotate
differentially with latitude, in contrast to polar CHs that extend
toward equatorial latitudes, which manifest a rather rigid
rotation rate. Since here we are dealing with isolated, nonpolar
CHs, it is thus reasonable to assume that their average rotation
rate depends on latitude. Therefore, after the CHs transit behind
the western limb and are no longer visible in the SDO/AIA
images, we assume that the CHs rotate rigidly (i.e., we neglect
any changes of their shape due to differential rotation, if any)
with a rate (sidereal, in μrad/s) given by:
w f f
= + +
A B C
sin sin , 1
2 4 ( )
where A (2.972) is the equatorial rotation rate, and B (−0.484)
and C (−0.361) account for the differential rate (see, e.g.,
Snodgrass & Ulrich 1990; Beck 2000). The angle f is the
heliographic latitude of the CH centroid. Note that there might
exist a difference between the actual rotation rate of the isolated
CHs and the values estimated from Equation (1). For instance,
Insley et al. (1995) reported an average rotation rate (sidereal)
for isolated CHs of ∼13°
.6 day−1
at 45° latitude compared to
∼13°
.1 day−1
from Equation (1) at the same latitude (i.e., about
3.5% faster).4
Under these assumptions, we can track the CHs
as PSP transits its orbit. From SOHO/LASCO observations,
the expansion of open magnetic flux tubes in the poles is
known to be superradial (e.g., DeForest et al. 2001). Since the
CHs that might influence the brightness observed are at low
and mid latitudes (see Figure 2) we choose a conservative
approach and assume radial expansion. This approach might
underestimate the extent of the region of influence of the CHs,
but it provides a minimum zone of influence that can be used to
assess their effects.
To illustrate the methodology devised, we consider the
depletion region labeled “b1” in E11 (Figure 1, top right panel).
Figure 3 shows a snapshot of the estimated zone of influence of
the CHs candidates in E11 and their intersection with the
WISPR-I FOV. Here we use the Heliocentric Earth Ecliptic
(HEE) coordinate system, where the x-axis points toward Earth
and the z-axis is perpendicular to the plane of the Earth’s orbit
around the Sun (positive north). This system is fixed with
respect to the Earth–Sun line. The CH zone of influence is
represented by the purple and blue shaded areas for CH1 and
CH2, respectively. The longitudinal extents represent the range
covered by the maximum and minimum values of the longitude
of the corresponding polygon points, assuming radial expan-
sion. The WISPR-I FOV is represented by the gray shaded area
delimited by the red lines. The Sun–PSP distance
(in astronomical units and solar radii), the hours relative to
perihelion (negative/positive values respectively correspond to
the inbound/outbound segment of the orbit), and the
boundaries of the projected radial extent of the WISPR-I
FOV (in solar radii) are indicated at the top left corner of this
figure. The brightness of the K-corona along a given LOS is
Figure 2. SDO/AIA 193 Å images showing the CHs considered for analysis at the time of the central meridian crossing. The boundaries of each CH (polygons
delimited by the dashed cyan lines) were obtained by the SPoCA algorithm. The corresponding centroids are depicted with the star symbol.
4
The small discrepancy does not play a role in our results nor their
interpretation.
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The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
modulated by the efficiency of the scattering. From the TS, the
scattering efficiency does not drop steeply along the LOS, but
remains at 0.9 within 30° of the TS and drops to 0.5 within 45°
of the TS (see Figure 2 of Michels et al. 1997). This region of
near constant and high scattering efficiency was termed a
plateau (Howard & DeForest 2012). For simplicity, we assume
an electron scattering efficiency of 1 in the region delimited by
RT ± 25% (0 efficiency elsewhere), where RT is the radius of
the Thomson surface (i.e., half the distance between the Sun
and the observer). This region is represented in the figure by the
light blue annulus. The orange diamond depicts the perihelion
of the orbit and the red star the location of the S/C at the time
of the snapshot. For the analysis, we reproduced this snapshot
at 1 hr cadence for each whole encounter.
A brightness depletion in a WL observation might be
associated with either the effect of a CH, or the evacuation of
material in the wake of a big CME event, or a combination of
both. In the case of the “b1” region, there was no CME. In
Figure 3, we see that the WISPR-I FOV intersects CH1 right at
the TS. The area of this intersection evolves during the
encounter. Our hypothesis is that the “b1” depletion is caused
by the crossing of the WISPR-I LOS with the low electron
density above CH1 right at where the electron scattering
efficiency is maximized. To assess this hypothesis, we devised
a proxy to ascertain the effect of the CHs. The proxy, hereafter
Pch, is simply defined as Pch = A1/A2, where A1 is the area of
the intersection among the WISPR-I FOV, the CH zone of
influence, and the TS; and A2 is the area of the intersection
between the TS and the FOV of the instrument.
We computed Pch at 1 hr time intervals for the four
encounters and assessed its relationship with the evolution of
the median excess brightness (Figure 1) in the latitudinal bands
delimited by the horizontal white dashed lines in Figure 1. In
this way, we collapse the brightness along each column (time
instance) to a single value. Depletion region “c2” is not
accounted for in this analysis because no association with a CH
was found (see Section 3.2). In Figure 4, we display with blue
dots the time evolution of the median excess brightness so
computed for each encounter as a function of the observing
time. The red line depicts the time evolution of the proxy Pch
(scale on the right y-axis). In the top x-axis, we report the S/C
Carrington longitude at the time of each major x tick mark. The
plots show a clear inverse relationship between the proxy and
the median excess brightness. Namely, Pch is high during the
brightness depletion intervals (light green shaded regions)
while it remains small or 0 otherwise.
During the inbound segment of E10, the observed
depletion (“a1” in Figure 1) is matched with a high,
gradually changing Pch. Around 25 hr before perihelion,
there is a sharp increase of the median excess brightness
followed by a sudden drop to 0% for a short period of time
(about 10 hr, marked with the dark green shaded area). This
coincides with a rather abrupt increase of Pch (this null
excess brightness can be noticed in Figure 1 as a very dark
stripe just before the green/red zone in the upper left panel).
Afterwards, Pch drops to 0 (no CH influence) while the
median excess brightness fluctuates well above 0%.
During the inbound segment in E11, the median excess
brightness level is low and highly variable. Pch is also low,
exhibiting a gradual decrease up to about 50 hr before
perihelion (the following increase is due to the predomi-
nance of distinct K-corona features, such as bright coronal
rays). This situation can be appreciated in Figure 1 (top right
panel) within the latitudinal band in the inbound segment.
The background level is fluctuating below about 3%,
occasionally dropping to 0%. About a day before perihelion,
Pch rises sharply, reaching a peak in temporal coincidence
with a short duration depletion (dark green shaded area). The
opposite trend is observed afterwards. A sharp Pch decrease
Table 1
CH Parameters at the Time of the Central Meridian Crossing
Panel in Figure 2 Enc. CH Reference Time Stamp Centroid cLAT/cLON LAT (min/max) LON (min/max)
(a) E10 CH1 2021-11-14T02:22:04 −30.8/0.0 −44.9/−17.4 −11.5/12.7
(b) E10 CH2 2021-11-18T23:18:04 −29.5/0.1 −36.5/−21.3 −9.0/14.5
(c) E11 CH1 2022-2-18T06:00:04 13.9/0.0 −3.5/31.1 −8.5/8.1
(d) E11 CH2 2022-2-24T18:00:04 5.8/3.1 −5.4/13.7 −10.6/14.8
(e) E12 CH1 2022-5-13T12:00:04 16.5/4.6 −3.8/48.0 −14.3/20.1
(f) E12 CH2 2022-5-17T00:00:04 1.7/6.6 −19.1/17.8 −20.6/37.9
(g) E13 CH1 2022-8-27T23:00:04 −54.6/−1.0 −88.8/−28.2 −28.8/28.4
(h) E13 CH2 2022-9-14T07:12:04 28.1/0.0 15.7/38.3 −17.0/19.0
Figure 3. Snapshot of the simulation for the time stamp “2022-2-27T16:00:00
UT”, ∼48.4 hr after E11 perihelion. The assumed radial extension of the CHs
are indicated with the blue and purple shaded areas. The zone of influence of
each CH is represented by the angular width of the respective area. The
perihelion of PSP orbit (orange line) is depicted with the orange diamond and
the position of PSP with the red star. The FOV of WISPR-I is represented with
the gray region and the region where the electron scattering efficiency is around
maximum with the light blue annulus.
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The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
coincides with a sharp increase of the median excess
brightness. A day postperihelion an extensive depletion
interval occurs (region “b1” in Figure 1, top right panel),
matched by a high, constant value of Pch.
The E12 case (“c1” region in Figure 1) is a “textbook”
scenario. The observed depletion during the inbound segment
of the encounter is matched with a rather constant, Pch value.
This is followed by a sharp brightness rise and a sharp drop to 0
in Pch.
For E13, the behavior of Pch explains the three depletion
regions in Figure 1 (bottom right panel). Pch, again, reflects the
appearance, transit, and disappearance of the CH(s) extensions
into the WISPR-I FOV. Interestingly, Pch reaches a minimum
value of almost 0 (i.e., no CHs influencing the brightness)
when the excess brightness increases to a high value (>15%)
within a short period of only ∼2 hr. This time coincides with
the passage of the 2022 September 5 CME event through the
WISPR-I FOV. Right after the CME, we observe a depletion
area for about a day, labeled as “d2” in Figure 1, in coincidence
with a large value of the proxy. This indicates that the
extension of a CH is at least partly responsible for the
brightness depletion observed. As it will be shown in
Section 3.2, the brightness observed in this time interval was
also influenced by the passage of the CME. The rest of the
relative evolution of brightness compared to the proxy explains
the region “d3” in E13.
The discussion around Figure 4 supports our hypothesis that
the extent of CH plasma across the WISPR-I FOV leads to
detectable lower brightness levels in spite of the large
brightness variability observed along the encounters.
3.2. The Effect of CMEs
Not all brightness depletion regions can be explained by the
crossing of the WISPR-I LOS through CH extensions. In
particular, region “c2” in Figure 1 (E12) does not coincide with
the presence of a CH at an appropriate location. Our suspicion
is that it might be due to brightness depletion in the wake of a
CME event. Therefore, to investigate this possibility and to
assess the plausible effect of big CME events on the overall
brightness observed, we created height–time (HT) maps of the
excess brightness for the horizontal slits at rows 256, 512, and
768 for all four encounters to have ample coverage of the
evolution of the transients at all latitudes and a point of
comparison between them. For detailed analysis, we display the
corresponding HT maps for E12 and E13 in Figure 5. The
vertical, white dashed lines pinpoint the time of perihelion and
the labels indicate the corresponding depletion regions in
Figure 1. (Note that the labels are either yellow or greenish.
The yellow color is used to show that the corresponding
depletion region is at least partly due to a CME event, while the
greenish color that is associated only to a CH.)
We distinguish two different regions in the maps; namely
those with nearly zero excess brightness (darker regions) and
those with slanted stripes of various widths exhibiting a
relatively high excess brightness. The latter are the signatures
of CMEs developing across the WISPR-I FOV at the respective
latitudinal height. Here we concentrate on the depletion regions
observed after the big CME events. Figure 5 shows, in
particular, regions exhibiting a depleted brightness (about equal
to the overall background level, i.e., 0%) after the passage of a
CME in E12 just around perihelion for rows 256 and 512, and
Figure 4. Time evolution of (1) the median excess brightness in the area delimited by the dashed white lines in Figure 1 (blue points), and (2) the proxy, Pch, (red
curve, y-axis on the right) for E10, E11, E12, and E13. The top x-axis displays the S/C Carrington longitude at the time of the major x tick marks. The inset labels
mark the corresponding depletion regions in Figure 1.
6
The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
in E13 a couple of hours before perihelion for all three rows.
These depletion intervals are concurrent with the aftermath of
the 2022 June 2 (region “c2”), and 2022 September 5 (region
“d2”) events, respectively. Snapshots of the two events are
shown in Figure 6. Their analysis is beyond the scope of this
paper.
As inferred from their morphology and brightness in the
FOV of the WISPR telescopes, these CME events were large
and massive, and we generally expect brightness depletion in
the wake of such events (e.g., Vourlidas & Webb 2018; and the
discussion in Section 4). The event in E12, appears to evacuate
(partly, at least) the K-corona below the ecliptic plane only.
The upper halves of the images in the aftermath of the event do
not show an associated depletion, although the CME feature
encompasses all latitudes covered by WISPR-I. In contrast, the
2022 September 5 event affects the brightness across the full
latitudinal range of the images. This CME was the fastest and
most energetic event encountered by PSP so far, with a
projected speed in excess of 2000 km s−1
. Interestingly, this
CME was crossed by PSP during its evolution. Detailed
analyses of this event are in preparation and will be published
elsewhere. Here, we focus on the brightness evolution in its
wake to shed light onto the nature of the observed depletion.
To assess the nature of the depletion, we first extract the
brightness profiles along the PSP orbital plane from all
calibrated WISPR-I images in E10–E13, as well as from the
minimum background set (hereafter Ebkg, see Section 2) for
comparison. The resulting 952 profiles from the five data sets
are plotted in log–log scale in Figure 7(a) as a function of
elongation, expressed in solar radii. These distances, which we
will refer to as coronal heights or LOSs, indistinctly, are
computed by normalizing the angular elongations by the
angular radius of the Sun at each PSP heliocentric distance. The
coronal height range is restricted to the heights covered by the
FOV of WISPR-I during the temporal extent of the depletion
“d2” in Figure 1. A scaling factor is applied to each set (namely
f = 2−t
with t = 0, 1, 2, 3, and 4 for E10, E11, E12, E13, and
Ebkg, respectively) to allow for proper visualization. At each
LOS, there is an aggregation of measurements (the same LOS
Figure 5. HT maps for the slits at rows 256, 512, and 768 on the WISPR-I images in E12 and E13. The vertical white dashed line indicates the time of perihelion on
the corresponding encounter. The labels indicate the corresponding depletion regions in Figure 1 (labels in yellow point to depletion regions associated with a
CME wake).
Figure 6. Snapshots of the 2022 June 2 (left panel) and 2022 September 5 (right panel) events in E12 and E13, respectively. The composite images displayed show the
WISPR-I and WISPR-O images projected into the helioprojective Cartesian (HPC) coordinate system. The Sun is to the left at 0° latitude. The white line depicts the
PSP orbital plane. The images have been processed following the procedure described in Appendix A of Howard et al. (2022).
7
The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
is covered by a range of S/C distances), hence the variable
spread.
Since we are trying to derive the baseline (nonvarying)
component of the K+F coronal brightness across the
encounters, we compute a baseline level for each data set by
taking the running 5-percentile, hereafter Bi
p
5
, with i = E10,
E11, E12, E13, Ebkg and then robustly fitting5
them (in log–log
space) with the analytical function given in Equation (2) (see
Stenborg et al. 2022):
= + +
- -
 A e A A x. 2
i
f i A x A x i i
0 4 5
i Ai i
1 2 3 ( )
( )
The fitted baseline levels, i
f
(dashed red lines in
Figure 7(a)), should be largely free from the effects of discrete
K-corona structures crossing the WISPR-I FOV. Note that the
individual encounter curves are scaled with respect to each
other for visualization purposes. To visualize the changes of the
background level among the encounters properly, we compute
the percentage excess brightness (1) D = -
B B 1
i
p
i
p
bkg
p
5 5 5
[ ] for
the data, and (2) D = -
  1
i
f
i
f
bkg
f
[ ] for the fitted levels. We
display them in Figure 7(b): Di
p
5
with the same color code as in
panel (a), and Di
f
in red for all four cases. These quantities are
essentially one-dimensional representations of the “quicklook”
coronagraph images that are used extensively for assessment of
coronal activity and measurements of CME speeds. The
difference is that the coronagraph backgrounds are the monthly
minima instead of the minimum of the four encounters at each
observer’s distance in the WISPR case. The plots indicate that
the fits, i
f
, represent well the data, except for edge effects.
Equation (2) is designed to fit a profile with an extended linear
part (Stenborg et al. 2021b, 2022) but here we considered only
the data up to 20 R☉, where the linear part is not yet fully
manifested. In addition, there are remaining K-corona con-
tributions to the baseline level in all four encounters from
coronal transients and other discrete K-corona structures
(which are not accounted for in Equation (2)). The sudden
increase in E12 around 13 R☉ is such an example. As we are
not considering those coronal heights in our analysis, we ignore
these effects in the remainder of the paper.
Figure 7(b) shows clearly that the E13 background level
(during “d2”) is exceptionally low and equal to the background
minimum within 4.5–14 R☉. This time interval in E13 is
dominated by a CH (see Section 3.1) as well as by the massive
CME on 2022 September 5. In other words, the E13 brightness
levels (within 4.5–14 R☉) defined the minimum background
brightness for all four encounters. The comparison, however,
only establishes a relative level among the four encounters. We
cannot assess if this is total K depletion or if it includes a dust
depletion as well. For that, we have to turn to a model of the
F-corona, in lieu of the unavailability of any other means to
establish a “ground truth” for the F-corona background.
Following the procedure of Stenborg et al. (2022), we
forward-model the F-corona brightness along the orbital plane,
for the same set of S/C heliocentric distances as in Figure 7(a).
As before, we determine the baseline level of the simulated
measurements by computing the running 5-percentile, BF
p
5
, and
robustly fit it with the analytical function given in Equation (3)
(as in Stenborg et al. 2022):
= + +
-
 A e A A x. 3
F
f A x
0 3 4
A
1 2
( )
We then compute the percentage excess brightness between
the fits of the four encounters (and of the empirical background,
as a check) and F
f
(i.e., D = - ´
  1 100
i
F
i
f
F
f
[ ]
/ ). We plot
Di
F
in Figure 7(c) using the same color code as in panel (a).
The red line marks the 0% level (i.e., DF
F
). The comparison
reveals that the E13 baseline level dips below the expected
F-corona level within ∼10–15 R☉ (shaded range). This is an
exciting result because it suggests rather strongly that the 2022
September 5 event may have caused the long-predicted dust
evacuation (Ragot & Kahler 2003). This finding is discussed
Figure 7. (a) Aggregated brightness measurements along the PSP S/C orbital
plane in E10 (orange), E11 (light blue), E12 (green), and E13 (blue), and in the
minimum background set, Ebkg (black). The dashed red line at the bottom of
each set delineates the modeled baseline levels ( i
,
i
f
= E10...E13, Ebkg; see
Equation (2)). Each set of measurements has been rescaled with respect to E10
to allow for proper visualization. (b) Percentage excess brightness of the
baseline level profile of each encounter with respect to that of the background
set ( *
D = -
B B 1 100
i
p
i
p
bkg
p
5 5 5
[ ] with i = E10...E13; same color code). The red
continuous lines depict the modeled excess brightness (i.e.,
*
D = -
  1 100
i
f
i
f
bkg
f
[ ] ). (c) Percentage excess brightness of the modeled
baseline levels with respect to the modeled F-corona baseline level (i.e.,
*
D = -
  1 100
i
F
i
f
F
f
[ ] ) with i = E10...E13, Ebkg, same color code). The red
continuous line at 0% delineates DF
F
.
5
The coefficients of the fitting were obtained by using the “least_square”
function of the “scipy.optimize” package in Python 3.8.8.
8
The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
further in Section 4. Furthermore, Dbkg
F
(black dashed line)
tends to DF
F
beyond ∼16 R☉ (i.e., 
 
bkg
f
F
f
, as expected at
large coronal heights, where the F-corona signal dominates
over the nonvarying K-corona; see, e.g., Stenborg et al. 2022).
We also note that the E11 brightness also seems to reach below
the 0% level. However, this occurs above ∼15 R☉, where the fit
of the baseline level begins to diverge (see Figure 7(b) and
earlier discussion). This result is too uncertain to consider
further here.
4. Discussion
An important feature of the PSP orbit is the high angular
speed of the S/C compared to 1 au orbiters. The view from 1 au
changes ∼13° day−1
compared to about 70° day−1
for PSP at
its current perihelion distance. The S/C crosses a wide range of
ecliptic longitudes, particularly over a couple of days around
perihelion, when the PSP superrotates—its angular speed
exceeds the solar rotation speed (see Figure 8). Consequently,
WISPR sweeps through a rapidly varying coronal scene during
this short interval. This explains the relatively short time
interval of the depletion regions around perihelion compared to
the larger duration of the depletion regions farther away
(Figure 1).
The relatively sharp brightness changes offer a way to derive
the boundaries of CH in the outer corona through the
introduction of the geometric proxy, Pch. As we demonstrated
in Section 3.1, the majority of the brightness depletions was
associated with the passage of CHs through the WISPR FOV.
As shown, Pch and its associated methodology allow us to track
the sources of the CHs in the low corona and assess their extent
and effect in the outer corona. Although brightness reductions
due to CHs are routinely observed from 1 au, they over-
whelmingly arise from polar CH extensions and are generally
observed at high latitudes. Equatorial and mid-latitude CHs are
difficult to identify, and hence, study, from 1 au because their
coronal extensions are masked by intervening coronal streamer
structures. The long LOS also affects polar CH studies. For
example, these LOS effects prevented DeForest et al. (2018) to
separate slow from fast solar wind despite the detailed image
processing methods they applied on deep-exposure COR2
images.
The use of Pch, and the unique location of WISPR within the
solar corona can remove much of the ambiguity in CH studies
from 1 au. The CH boundary identifications and the short LOS
through the corona can better separate fast (CH-associated)
plasma structures in the WISPR images from the slower flows
associated with streamers. The rapid S/C rotation can then be
used to extract 3D information for these structures. Perhaps
more importantly, the identification of CH (and post-CME flow
boundaries) remotely by WISPR can be used to pinpoint the
relevant PSP in situ measurements with much higher precision
that currently possible. In that respect, WISPR is fulfilling its
original overarching objective to serve as a “local” imager and
provide crucial context for the interpretation of the in situ
measurements of the mission.
The analysis of post-CME flows is the second exciting
prospect that is emerging from the presented methodology. We
were able to identify the boundaries of coronal depletions
caused by CMEs in two encounters, E12 and E13. Both were
large events and were expected to have a significant impact in
the ambient corona. Post-CME dimmings have been observed
from 1 au for many years but the most concrete associations
tend to be for the slower, SBO CMEs (e.g., Vourlidas &
Webb 2018, and references therein), which originate from quiet
Sun regions, such as polar crown filaments. EUV full-disk
imaging and irradiance measurements have established low
EUV dimmings as the imprint of faster CMEs in the low
corona (e.g., Reinard & Biesecker 2008; Mason et al. 2016).
More recently, Krista & Reinard (2017) and Dissauer et al.
(2019) performed extensive statistical analyses over a large
number of CME flare events and dimmings. They verify the
long-held idea that the dimming is a result of the opening of the
field lines in the vicinity of the eruption and the evacuation of
mass in high-speed flows. Transient CHs develop much more
quickly than normal CHs, and although they do not last as long,
their decay depends on the size of the evacuation. Dissauer
et al. (2019) find a strong correlation (∼0.6–0.7) between the
depth of EUV dimming and CME mass, indicating that much
of the CME mass detected in the coronagraphs (<20–30 R☉)
must come from the low corona, say, below 2 R☉.
However, none of these studies have investigated the impact
of the CME on the middle and outer corona in a quantitative
manner. The tracking of the evacuation is generally difficult at
these heights because the considerable disturbances caused by
these CMEs tend to obscure the evacuation signatures,
especially early in the event. In many cases, projection effects
further complicate the analysis since the evacuation may occur
along, rather than across, the 1 au LOS. Post-CME rays, when
visible, are the strongest manifestation of the plasma flow from
the low to the outer corona implied by the EUV dimming
analyses (Vrsnak et al. 2009; Webb & Vourlidas 2016). The
rays appear 3–4 hr behind the main body of the CME and last
for about 18 hr, on average. Most of the cases are associated
with streamer disappearances, with the streamer reforming
within about a day. The 1 au analyses, therefore, suggest that
coronal evacuation from CMEs is a relatively short-lived event
and that the evacuation occurs along narrow lanes (the post-
CME rays) that are presumably the plasma sheets around the
post-CME current sheet(s). However, the evacuations seen in
WISPR appear to form much faster and must occupy a wider
area than post-CME rays suggest. It is unclear, at this moment,
how the coronal evacuation proceeds. Is it all via post-CME
flows or does the CME itself carry a significant part as pileup at
its front? Howard & Vourlidas (2018) found no evidence of
coronal pileup at the CME front in the 2–30 R☉ range but
suggested that the pileup rate may be too low to be detected
Figure 8. Carrington longitude of the PSP/SC as a function of time (S/C
heliocentric distance in R☉ on the top x-axis) for E10 (orange), E11 (light blue),
E12 (green), and E13 (blue). The shaded area indicates the PSP superrotation
stage.
9
The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
from 1 au in the presence of the bright CME fronts they
analyzed. Further WISPR analyses of the evolution of CME
fronts and wakes may shed light on this issue thanks to the
greatly improved sensitivity of the WISPR observations when
compared to their 1 au counterparts.
A potentially groundbreaking demonstration of the sensitiv-
ity of WISPR observations to the coronal environment may lie
in the very low brightness level left in the wake of the 2022
September 5 CME (Figure 7(c)). As we noted in that figure, the
empirically determined minimum background level during the
“d2” depletion dips below the expected F-corona level. The
obvious interpretation is that the CME has carried away not
only the electron population but has removed part of the
ionized, interplanetary dust as well. This mechanism had been
suggested twenty years ago (Ragot & Kahler 2003) but has
never been verified with observations.
To explore the WISPR results in more detail, in Figure 9 we
zoom into the distance range where the E13 depletion falls
short of the F-corona background. Panel (a) compares the
minimum brightness, i
f
, of the four encounters and of the
background set against the baseline level of the forward-
modeled F-corona (red dotted line), F
f
, in absolute brightness
units. The x-axis is restricted to allow for an easier comparison
among the encounters. The increasing divergence at lower
heights indicates the increasing effect of K-corona variability
closer to the Sun. It also shows that only E13 dips below the
F-corona level, a signature indicating that dust must have been
depleted. Note also that at the lowest heights, E13 is only
slightly brighter, suggestive that the K-corona has been
depleted at these heights.
The effect of depletion becomes more apparent in
Figure 9(b), where we compare in the restricted x-range the
percentage excess brightness of the encounters (and back-
ground set) baseline level with respect to the modeled F-corona
baseline (i.e., D = - ´
  1 100
i
F
i
f
F
f
[ ]
/ ). As we noted in
Figure 7(c), E13 drops below the model F-corona (red line).
The drop is significant (up to about 2% within 11–14 R☉) and
evident only for E13. It is unlikely to be an observational
artifact. The obvious interpretation is that the drop indicates
dust evacuation. This interpretation hinges, however, on the
reliability of the F-corona model, since no independent
measurement of the F-corona brightness exists.
The F-corona model has been tested against observations
(Stenborg et al. 2021b, 2022) across an extended range of
heliocentric distances and elongations. The observations
showed clear evidence of dust depletion in the circumsolar
environment. The F-corona model was able to fit the
observations from 5 R☉ to 0.3 au with the inclusion of a
DDZ between 5 and 19 R☉. The excess brightness for each
encounter displayed in Figure 9 was estimated against the
F-corona model with a DDZ and thus makes the E13 brightness
depletion even a more significant indication of dust depletion.
Another indirect evidence in support of dust evacuation
comes from the brightness gradient in the observed profiles.
Stenborg et al. (2022) demonstrated that the inflection point in
the brightness profile along the symmetry axis of the F-corona
is an observational signature of the location of the dust-free
zone (DFZ). The presence of K-corona signal on these profiles
shifts the inflection point outwards from the Sun, with the
amount of shift dependent on the amount of K-corona. This
dependency was shown by Stenborg et al. (2022) who by
modeling both the F & K components found that the position of
the maximum in the brightness gradient space is a strong
function of the F/K ratio, with a lower ratio (i.e., more K or
less F) driving the maximum of the gradient out further. In
Figure 10 we compare the gradients of the baseline levels, i
f
,
for the four encounters (E10 in orange, E11 in light blue, E12
in green, and E13 in blue) against the modeled baseline level of
the F-corona (F
f
, in red). The maxima are marked by the
vertical dotted lines and are clearly shifted outwards for all four
encounters, indicating the presence of “contaminating”
K-corona. This is a reasonable conclusion for E10, E11, and
E12 because there were visible K-structures in all of those
encounters. This is not a reasonable conclusion for E13,
Figure 9. (a) Coronal height evolution of the baseline brightness level for E10
( ;
E
f
10 orange), E11 ( ;
E
f
11 light blue), E12 ( ;
E
f
12 green), E13 ( ;
E
f
13 blue),
Ebkg ( ;
bkg
f
dashed black line), and the baseline of the modeled F-corona ( ;
F
f
dashed red line). The x-axis is restricted to the heights where the E13 brightness
drops below the model F-corona to allow for a direct comparison among the
sets. (b) Zoomed-in version of Figure 7(c). The color code is the same as in
panel (a). The E13 minimum brightness is about 2% lower than the modeled
F-corona baseline level in the ∼11–14 R☉ range.
Figure 10. Gradient of the baseline level models i (i = E10—orange, E11—
light blue, E12—green, E13—blue, and F—red). The vertical (colored) dashed
line pinpoint the position of the relative maximum for each set.
10
The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
however. The “d2” period was remarkable by the presence of a
large brightness depletion with a virtually complete absence of
K-coronal structures. Then, the only explanation for the large
shift of the gradient maximum is the reduction in the F-corona
brightness.
The only remaining option for explaining the decrease of the
F-corona brightness without dust evacuation is a change in the
dust scattering function. This implies a change in the scattering
properties of the dust. This can be the result of changes in either
the dust composition and/or the shape of the dust grains.
However, these changes must be the result of interaction with
the CME since the depletion in question occurs clearly in the
CME wake. It is not obvious, to us at least, what physical
process could lead to either a selective liftoff of dust or a
modification of the grain shape as a result of CME–dust
interaction. On the contrary, it is already established that the
near-Sun dust is ionized and therefore it can be dragged out by
the CME magnetic structure, an interaction described by Ragot
& Kahler (2003) and regularly seen in CME–comet interactions
(Vourlidas et al. 2007). The above arguments leave the
reduction in the dust density as the most probable explanation.
5. Summary and Conclusions
We have developed a methodology to identify the sources of
brightness variations—specifically, brightness depletions—in
WISPR images. Using EUV observations of the low corona
along with a simple geometric representation of the coronal
extent of equatorial CHs, we were able to quantify the
intersection between the CH coronal extensions and the WISPR
FOV. We find that the majority of the observed K-corona
brightness depletions could be ascribed to the existence of a
CH region within the FOV of the instrument.
The WISPR observations from “within” the solar corona
offer an unprecedented opportunity to remove the long LOS
confusion that limits 1 au observations and finally allow us to
study the coronal atmosphere above both polar and equatorial
CHs. The techniques presented here become even more
powerful when we take into account the ability of PSP to
perform in situ measurements of the plasma images captured by
WISPR only a few hours earlier. In particular, the geometric
CH localization technique illustrated in Figure 3 allows us to
locate the boundaries of the CH zone of influence in the PSP/
FIELDS (Bale et al. 2016) in situ measurements without much
difficulty (Vamsee Jagarlamudi, private communication). We
will embark on this joint remote/in situ analysis next.
However, CHs are not the only sources of brightness
depletion. CMEs are, in fact, the only sources of coronal
evacuation known from 1 au (e.g., Vourlidas & Webb 2018,
and references therein). WISPR observations have offered
several such examples from an intracoronal viewpoint. In both
E12 and E13, large CMEs caused clear brightness depletions
(Figure 5). Again, our ability to identify the boundaries of the
CME-driven depletion will be extremely helpful in analyzing
the in situ measurements of those regions. The coronal
environment in the wake of CMEs has never been studied with
in situ measurements until now.
In E13, however, the fast CME of 2022 September 5 appears
to have created a depletion so deep that we had to invoke dust
evacuation to explain it. The phenomenon of dust ionization
and depletion has been postulated for twenty years (Ragot &
Kahler 2003) but has never been observed conclusively.
Although it is strongly supported by physical considerations
and observed in comet interactions with the solar wind, the
effect is restricted to the CME envelope. Thus it is very difficult
to observe from 1 au given the small volume of a CME when
compared to the interplanetary dust. There have never been any
identifications of dust evolution from that distance. Our
analysis, based on observations from within less than 20 R☉
provides the strongest evidence yet for this process. Although
this interpretation is based on comparisons to an F-corona
model, it is consistent with the observations and well-under-
stood physical processes. To be conservative, we consider it as
an indirect detection of dust evacuation for now. With the
rising cycle, we expect more CMEs to cross the WISPR FOV
and give us a chance to compare the brightness depletion across
multiple events and coronal conditions. Such studies will
establish whether CMEs can indeed lead to dust evacuation
within the solar corona.
Parker Solar Probe was designed, built, and is now operated
by the Johns Hopkins Applied Physics Laboratory as part of
NASAʼs Living with a Star (LWS) program (contract
NNN06AA01C). Support from the LWS management and
technical team has played a critical role in the success of the
Parker Solar Probe mission. The Wide-Field Imager for Parker
Solar Probe (WISPR) instrument was designed, built, and is
now operated by the US Naval Research Laboratory in
collaboration with Johns Hopkins University/Applied Physics
Laboratory, California Institute of Technology/Jet Propulsion
Laboratory, University of Goettingen, Germany, Centre
Spatiale de Liege, Belgium, and the University of Toulouse/
Research Institute in Astrophysics and Planetology. G.S. and
A.V. were supported by WISPR Phase-E funds to APL. E.P.
acknowledges support from the NASA LWS grant No.
80NSSC19K0069. R.A.H. was supported by NASA grant
No. 80NSSC19K1261. P.H. was supported by the NASA
Parker Solar Probe Program Office for the WISPR program
(contract NNG11EK11I) and the Office of Naval Research.
ORCID iDs
Guillermo Stenborg https:/
/orcid.org/0000-0001-
8480-947X
Evangelos Paouris https:/
/orcid.org/0000-0002-8387-5202
Russell A. Howard https:/
/orcid.org/0000-0001-9027-8249
Angelos Vourlidas https:/
/orcid.org/0000-0002-8164-5948
Phillip Hess https:/
/orcid.org/0000-0003-1377-6353
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Investigating Coronal Holes and CMEs as Sources of Brightness Depletion Detected in PSP/WISPR Images

  • 1. Investigating Coronal Holes and CMEs as Sources of Brightness Depletion Detected in PSP/WISPR Images Guillermo Stenborg1 , Evangelos Paouris1,2 , Russell A. Howard1 , Angelos Vourlidas1 , and Phillip Hess3 1 Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA; guillermo.stenborg@jhuapl.edu 2 George Mason University, 4400 University Dr, Fairfax, VA 22030, USA 3 Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA Received 2023 March 27; revised 2023 May 3; accepted 2023 May 3; published 2023 May 31 Abstract The Parker Solar Probe (PSP) mission provides a unique opportunity to observe the solar corona from distances below 20 R☉. In this work, we utilize white light images from the Wide-field Imager for Solar PRobe aboard the PSP from solar encounters 10 through 13 to examine the causes of brightness depletions of the corona during the rapid transit of PSP through the perihelia of its orbit. We analyze the effect of (1) coronal holes (CHs) and (2) energetic coronal mass ejection (CME) events on the observed brightness of the images. We speculate on the causes of the brightness depletions, ascribing them to the evacuation of (1) free electrons (reduced K-corona) and (2) interplanetary dust (reduced F-corona). In particular, we show that (1) the presence of CHs in all of the orbits is directly correlated with the depletion of the global white light emission recorded, and (2) a huge CME event in encounter 13 caused a very deep depletion in its wake that removed the electron content as well as some of the interplanetary dust. Unified Astronomy Thesaurus concepts: Solar F corona (1991); Solar K corona (2042); Solar coronal holes (1484); Solar wind (1534); Solar coronal mass ejections (310); Interplanetary dust (821) 1. Introduction White light (WL) imaging constitutes the main means of tracking coronal mass ejection (CME) events from the inner corona into the interplanetary medium (e.g., Webb & Howard 2012). The recorded signal consists of photospheric light scattered both by the free electrons in the corona via Thomson scattering (the K-corona) and by the dust particles orbiting the Sun (the F-corona) together with planetary and galactic sources. The F-corona generally dominates the signal beyond about 5 R☉ and it, therefore, must be removed to allow for the tracking of coronal structures. For observations taken from a fixed distance from the Sun and within the ecliptic plane, the F-corona contribution is relatively stable, with the variations being mainly due to the ecliptic longitude of the observer with respect to the ascending node of the zodiacal dust cloud (Stenborg & Howard 2017). When the observer is within the plane of symmetry of the zodiacal dust cloud, the longitudinal dependence disappears (see, e.g., Stenborg et al. 2018, and references therein). The Parker Solar Probe (PSP; Fox et al. 2016) mission is deepening our understanding of the near-Sun environment (Raouafi et al. 2023). The PSP spacecraft (S/C) carries only one remote sensing instrument suite, the Wide-field Imager for Solar Probe (WISPR; Vourlidas et al. 2016), which consists of two WL imaging telescopes. The high ellipticity and low perihelion of the PSP orbit translates into a high speed of the S/C close to perihelion (>150 km s−1 ) and results in a continuously variable background scene within each tele- scope’s field of view (FOV). Within the 10 days nominal PSP science windows in particular (i.e., when the S/C is below 0.25 au), there is a rapid change of heliocentric distance while the S/C approaches (and recedes from) perihelion along with a large range of ecliptic longitudes. Thus the volume of space covered by the WISPR FOV changes greatly from image to image. Moreover, since PSP is embedded in the solar corona and is quickly moving through it, rather than viewing distant structures like most previous coronal imagers, WISPR becomes a local imager. As one result, the F-corona brightness varies from image to image, and the standard techniques employed to reveal the faint, dynamic K-corona features in WL images used by instruments at 1 au (e.g., base or running differences/ratios) no longer work. An additional complication arises from the continually changing spatial resolution of the images as PSP approaches and then recedes from the Sun. This has created new issues that needed to be solved for the interpretation of these exciting observations (see, e.g., Liewer et al. 2019, 2020; Nisticò et al. 2020). Most of the attention to date in analyzing the WISPR data has been on the structures that increase brightness such as coronal streamers and coronal rays (e.g., Liewer et al. 2022; Poirier et al. 2022), CME events (e.g., Braga & Vourlidas 2021; Wood et al. 2021), Venus (Wood et al. 2022), asteroids and comets (Battams et al. 2022, 2020), and interplanetary dust (Stenborg et al. 2022, 2021b, 2021a). As PSP transits through its orbit about the Sun, WISPR records these brightness enhancements along with regions of lower brightness. But what other structures could be affecting the WISPR observations? For example, from 1 au, regions of depleted brightness are also routinely observed in WL coronagraphs, such as observations of coronal holes (CHs) over the Sun’s poles. These are regions of very low brightness caused by the very low electron density in CHs (Altschuler et al. 1972). Moreover, WL coronagraphs have also recorded brightness depletions following the passage of streamer blowout (SBO) events through the FOV (e.g., Vourlidas & Webb 2018, and references therein). Here, the depletion is caused by the evacuation of the electron corona in the CME volume by the transit of the SBO through the FOV. A The Astrophysical Journal, 949:61 (12pp), 2023 June 1 https://doi.org/10.3847/1538-4357/acd2cf © 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 1
  • 2. similar brightness depletion was first seen from the SOLWIND instrument on a low-Earth-orbiting satellite (P78-1; Sheeley et al. 1980), on the images taken just after the S/C emerged from orbit night (Howard et al. 1985). This depletion was called a remnant CME. Another possible mechanism to decrease the observed brightness is to reduce the F-corona, i.e., to reduce the density of the interplanetary dust in orbit about the Sun. This was proposed in 2003 by Ragot & Kahler (2003), who suggested that CMEs may interact with the interplanetary dust and carry the dust outward. Such an effect has never been detected from 1 au observations. This failure was probably due to observing factors such as the difference in geometry affecting the scattering efficiency for dust and electron scattering, but perhaps more due to the long integration of dust along the line of sight (LOS) from 1 au obscuring the transient effect of the CME passage. Thanks to the geometry of the PSP orbit, we now have a much better platform to search for dust depletion after the passage of big CME events. We point out that WISPR has already detected a rather stable dust depletion zone (DDZ) extending from 19 R☉ down to the dust-free zone at about 5 R☉ (Stenborg et al. 2022, and references therein) in contrast to the more ephemeral depletion zone that might occur due to a CME. Therefore, in this paper we examine the brightness depletions observed during the nominal science encounters 10 through 13 with a two-fold objective: (1) to see if/when they can be ascribed to evacuation of free electrons (i.e., the K-corona) along with their cause(s), and (2) to check if there is any evidence for evacuation of interplanetary dust (i.e., the F-corona). To achieve these objectives, we need to obtain a background that accounts for the variability of the K-corona without placing an implied bias on the F-corona. The paper is organized as follows. Section 2 describes the observations used and the methodology implemented to achieve the objectives pursued. Our results are presented in Section 3, where we first exhibit the observational signatures followed by a comprehensive analysis of these signatures (Sections 3.1 and 3.2). The findings are discussed and put into context in Section 4. Finally, our conclusions are summarized in Section 5. 2. Observations and Methodology After the most recent Venus flyby (2021 October 16), PSP reached a perihelion distance of 13.29 R☉ on 2021 November 21 (orbit 10). The geometry of the PSP orbit will remain invariant until the 6th Venus flyby, which is scheduled for 2023 August 21. During this time interval, a set of seven orbits (10 through 16) will have reached the same perihelion distance. The constancy of these orbits provides a valuable data set to analyze the K- and F-corona background environment. Here, we analyze the observations of the WISPR inner telescope (hereafter WISPR-I) during the nominal science encounters 10 through 13 (hereafter E10, E11, E12, and E13). The WISPR suite is mounted on the ram side of the S/C, i.e., it looks in the direction that the S/C is moving. The WISPR-I FOV has an angular extent of about 40° in both the horizontal and vertical directions, covering the region between 13° .5 and 53° elongation along the projected Venus orbital path (see, e.g., Figure 1 in Howard et al. 2022). Due to the geometry of the orbit, the constant angular FOV corresponds to different ranges of heliocentric distances (see, e.g., Figure 1 in Stenborg et al. 2022—exemplified for E09) and spatial resolution. (At the perihelion distance reached during orbits 10 through 13, the nominal 2 pixel 1 au resolution of 2 34 becomes 8 8.) During the encounters under consideration, WISPR-I acquired images (960 × 1024 pixels2 ) with a 15 minutes cadence, increasing to 7.5 minutes for a couple of days around perihelion. The images were calibrated with the standard calibration procedure described in Hess et al. (2021). For this work, we had to minimize the effects of the K-corona variability, which is mainly caused by CMEs and streamers rotating in and out of the FOV. To accomplish this, we considered the data from all four encounters together to estimate the minimum coronal signal (K+F) at any given heliocentric distance. This approach adopts the same philoso- phy that is widely used to generate the quicklook images from the SOHO/LASCO (Brueckner et al. 1995) and STEREO/ SECCHI (Howard et al. 2008) coronagraphs, but with a different methodology. The minimum coronal signal from imagers at 1 au can be estimated by taking the minimum value of the observations covering one solar rotation. This is not possible when the observer rapidly changes its heliocentric distance; hence a different approach must be used, i.e., we consider the set of observations obtained from any given distance independently from one another to create the minimum background corona at each distance. Since the taking of the WISPR images is scheduled by time rather than by distance from the Sun, the S/C radial distance from which the images were taken in each encounter can vary slightly from each other. Thus, we grouped the images in sets of four images (one image per encounter acquired at a similar heliocentric distance and similar roll angle). We used the images from E13 as the reference and then compared the distances and the roll angles of the images from the other three encounters to the values of the reference set. To create the sets of four images taken at a “similar” distance, we found the corresponding images from the other three encounters that were within ±0.5% of the E13 reference image. Similarly, the S/C roll angle had to be within ±7.5% of the E13 reference image. If any one of the encounters did not have a corresponding point then a set for that distance was not used. The distance and roll angle tolerances were adjusted to obtain a significant number of image sets from nearly cospatial viewpoints while avoiding uneven gaps among the resulting cadences of the sets (i.e., to avoid too many large gaps in the distance coverage). The above tolerances for the distances and roll angles resulted in time series of 952 images per encounter. For each set of four images, Ij d (with j = 1...4 for each d = 1...952) we compute the background brightness, Bd , as the minimum brightness of each set at each pixel position. The background images mainly reflect the contributions of the diffuse, pseudo-stationary K-corona and the F-corona. This approach does retain some remnant discrete features, but the contribution of transient K-structures is greatly diminished. We then compute the excess K-coronal brightness, DK d j (in percentage) of each image, i.e., D = I B K d j d d j [ −1] × 100, relative to the corresponding background image. In the next section, we use latitude–time (LT) maps created from the excess brightness images to investigate the plausible connections between regions of minimum excess brightness with the crossing of the LOS through (i) the extension of CHs in the lower corona and (ii) the post-CME regions. 2 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 3. 3. Results To visualize the time evolution of the global excess brightness at a glance for the four encounters, we created the maps shown in Figure 1. Each column (1024 pixels), displayed as a function of hours from perihelion, corresponds to the median of each WISPR-I excess brightness row (960 pixels), so the 960 columns of each image are collapsed into just one column. These maps preserve the latitudinal variation in the image frame but collapse the variation in the perpendicular direction (i.e., in elongation) into a single value; hence, we will simply refer to them, in brief, as LT maps hereafter. The LT maps show regions exhibiting very low (∼0%) excess bright- ness (referred to as depletion regions, hereafter), regions with low (about 2%–5%) excess brightness, and bright regions. The latter correspond to regions dominated by either the passage of big CME events or by the presence of coronal streamers. Our goal is to find the causes for the large, homogeneous depletion regions, which are identified in the maps, to first approx- imation, with the yellow circles. 3.1. The Impact of CHs Observations from 1 au have long established that the WL emission above CHs is depleted. As mentioned in Section 1, during solar encounters WISPR becomes a local K-corona imager as the Thomson sphere (TS; Vourlidas & Howard 2006) shrinks while the S/C approaches perihelion. Thus, along each LOS, the TS becomes gradually smaller during the approach to perihelion (and hence lies gradually closer to the S/C; Vourlidas et al. 2016). We emphasize that we are referring to linear distances and not angular separations, which, of course, remain the same. Notwithstanding, we expect that brightness depletions due to CHs will manifest in WISPR-I images. In the following, we assess when and to what extent they affect the overall brightness observed. To identify and track the location of the candidate CHs for each encounter, we use extreme ultraviolet (EUV) observations in the 193 Å and 211 Å channels from the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) instrument on board the Solar Dynamic Observatory (SDO; Pesnell et al. 2012). After visual identification, the CHs are segmented to derive their boundaries and the location of the respective centroids when they are in Earth-facing position (i.e., ±10° from the central meridian) to minimize projection effects. For the CH segmentation, we use the Spatial Possibilistic Clustering Algorithm (SPoCA; Verbeeck et al. 2014), which is a feature recognition technique implemented in the SunPy open-source software package (The SunPy Community et al 2020). The identified CHs are shown in Figure 2, which displays two SDO/AIA images per encounter (i.e., E10 through E13) obtained in the 193 Å channel at the time of their central meridian crossing. The boundaries of the CHs are the helioprojective coordinates (latitude and longitude) of the points of the polygons delineated with the dashed cyan lines. The coordinates of the centroid (hereafter cLAT and cLON) are determined simply as the average of the latitudes and longitudes of the polygon points. To validate this approach, we used an alternative methodology, namely an unsupervised machine-learning technique aimed at identifying clusters of data objects (K-means clustering, as implemented in the python scikit-learn library; Pedregosa et al. 2011). The centroid coordinates obtained with this alternative approach match the ones obtained with the simple averaging calculation. The longitudinal extent of the CH is defined as the range covered by the maximum and minimum values of the longitude of the polygon points. These values are summarized in Table 1. The first column identifies the corresponding panel, (a) to (h), in Figure 2. The second column indicates the encounter number, while the third one labels the CHs of interest for each encounter. Columns 4–7 display the reference time stamp (UT) Figure 1. LT maps showing the time evolution of the median brightness of each WISPR-I image row for E10, E11, E12, and E13. The circles mark the brightness depletion regions discussed in the text. The vertical white dashed line indicates the time of perihelion, and the horizontal white dashed lines indicate the latitudinal bands used to compute the proxy, Pch, displayed in Figure 4. 3 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 4. when the CH was in Earth-directed position, the latitude and longitude of the centroid, and the latitudinal and longitudinal extend for each CH according to the polygon boundary points (see, e.g., the cyan dashed lines in Figure 2), respectively. In some cases, other CHs may have been seen on the disk in the time intervals considered. The ones not explicitly shown were excluded from the analyses either because they were polar CHs or their position was on the opposite hemisphere to PSP (in either case they could have not affected the brightness in the WISPR-I images). CHs are dynamic structures and their boundaries evolve with time. We assume, however, that their boundaries do not change substantially during the time elapsed between their passage through the central meridian (identification stage) until when they are in a favorable location to influence the brightness observed in the WISPR-I images (see, e.g., Wang et al. 2010, and references therein). Insley et al. (1995) reported that isolated CHs at low and mid latitudes tend to rotate differentially with latitude, in contrast to polar CHs that extend toward equatorial latitudes, which manifest a rather rigid rotation rate. Since here we are dealing with isolated, nonpolar CHs, it is thus reasonable to assume that their average rotation rate depends on latitude. Therefore, after the CHs transit behind the western limb and are no longer visible in the SDO/AIA images, we assume that the CHs rotate rigidly (i.e., we neglect any changes of their shape due to differential rotation, if any) with a rate (sidereal, in μrad/s) given by: w f f = + + A B C sin sin , 1 2 4 ( ) where A (2.972) is the equatorial rotation rate, and B (−0.484) and C (−0.361) account for the differential rate (see, e.g., Snodgrass & Ulrich 1990; Beck 2000). The angle f is the heliographic latitude of the CH centroid. Note that there might exist a difference between the actual rotation rate of the isolated CHs and the values estimated from Equation (1). For instance, Insley et al. (1995) reported an average rotation rate (sidereal) for isolated CHs of ∼13° .6 day−1 at 45° latitude compared to ∼13° .1 day−1 from Equation (1) at the same latitude (i.e., about 3.5% faster).4 Under these assumptions, we can track the CHs as PSP transits its orbit. From SOHO/LASCO observations, the expansion of open magnetic flux tubes in the poles is known to be superradial (e.g., DeForest et al. 2001). Since the CHs that might influence the brightness observed are at low and mid latitudes (see Figure 2) we choose a conservative approach and assume radial expansion. This approach might underestimate the extent of the region of influence of the CHs, but it provides a minimum zone of influence that can be used to assess their effects. To illustrate the methodology devised, we consider the depletion region labeled “b1” in E11 (Figure 1, top right panel). Figure 3 shows a snapshot of the estimated zone of influence of the CHs candidates in E11 and their intersection with the WISPR-I FOV. Here we use the Heliocentric Earth Ecliptic (HEE) coordinate system, where the x-axis points toward Earth and the z-axis is perpendicular to the plane of the Earth’s orbit around the Sun (positive north). This system is fixed with respect to the Earth–Sun line. The CH zone of influence is represented by the purple and blue shaded areas for CH1 and CH2, respectively. The longitudinal extents represent the range covered by the maximum and minimum values of the longitude of the corresponding polygon points, assuming radial expan- sion. The WISPR-I FOV is represented by the gray shaded area delimited by the red lines. The Sun–PSP distance (in astronomical units and solar radii), the hours relative to perihelion (negative/positive values respectively correspond to the inbound/outbound segment of the orbit), and the boundaries of the projected radial extent of the WISPR-I FOV (in solar radii) are indicated at the top left corner of this figure. The brightness of the K-corona along a given LOS is Figure 2. SDO/AIA 193 Å images showing the CHs considered for analysis at the time of the central meridian crossing. The boundaries of each CH (polygons delimited by the dashed cyan lines) were obtained by the SPoCA algorithm. The corresponding centroids are depicted with the star symbol. 4 The small discrepancy does not play a role in our results nor their interpretation. 4 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 5. modulated by the efficiency of the scattering. From the TS, the scattering efficiency does not drop steeply along the LOS, but remains at 0.9 within 30° of the TS and drops to 0.5 within 45° of the TS (see Figure 2 of Michels et al. 1997). This region of near constant and high scattering efficiency was termed a plateau (Howard & DeForest 2012). For simplicity, we assume an electron scattering efficiency of 1 in the region delimited by RT ± 25% (0 efficiency elsewhere), where RT is the radius of the Thomson surface (i.e., half the distance between the Sun and the observer). This region is represented in the figure by the light blue annulus. The orange diamond depicts the perihelion of the orbit and the red star the location of the S/C at the time of the snapshot. For the analysis, we reproduced this snapshot at 1 hr cadence for each whole encounter. A brightness depletion in a WL observation might be associated with either the effect of a CH, or the evacuation of material in the wake of a big CME event, or a combination of both. In the case of the “b1” region, there was no CME. In Figure 3, we see that the WISPR-I FOV intersects CH1 right at the TS. The area of this intersection evolves during the encounter. Our hypothesis is that the “b1” depletion is caused by the crossing of the WISPR-I LOS with the low electron density above CH1 right at where the electron scattering efficiency is maximized. To assess this hypothesis, we devised a proxy to ascertain the effect of the CHs. The proxy, hereafter Pch, is simply defined as Pch = A1/A2, where A1 is the area of the intersection among the WISPR-I FOV, the CH zone of influence, and the TS; and A2 is the area of the intersection between the TS and the FOV of the instrument. We computed Pch at 1 hr time intervals for the four encounters and assessed its relationship with the evolution of the median excess brightness (Figure 1) in the latitudinal bands delimited by the horizontal white dashed lines in Figure 1. In this way, we collapse the brightness along each column (time instance) to a single value. Depletion region “c2” is not accounted for in this analysis because no association with a CH was found (see Section 3.2). In Figure 4, we display with blue dots the time evolution of the median excess brightness so computed for each encounter as a function of the observing time. The red line depicts the time evolution of the proxy Pch (scale on the right y-axis). In the top x-axis, we report the S/C Carrington longitude at the time of each major x tick mark. The plots show a clear inverse relationship between the proxy and the median excess brightness. Namely, Pch is high during the brightness depletion intervals (light green shaded regions) while it remains small or 0 otherwise. During the inbound segment of E10, the observed depletion (“a1” in Figure 1) is matched with a high, gradually changing Pch. Around 25 hr before perihelion, there is a sharp increase of the median excess brightness followed by a sudden drop to 0% for a short period of time (about 10 hr, marked with the dark green shaded area). This coincides with a rather abrupt increase of Pch (this null excess brightness can be noticed in Figure 1 as a very dark stripe just before the green/red zone in the upper left panel). Afterwards, Pch drops to 0 (no CH influence) while the median excess brightness fluctuates well above 0%. During the inbound segment in E11, the median excess brightness level is low and highly variable. Pch is also low, exhibiting a gradual decrease up to about 50 hr before perihelion (the following increase is due to the predomi- nance of distinct K-corona features, such as bright coronal rays). This situation can be appreciated in Figure 1 (top right panel) within the latitudinal band in the inbound segment. The background level is fluctuating below about 3%, occasionally dropping to 0%. About a day before perihelion, Pch rises sharply, reaching a peak in temporal coincidence with a short duration depletion (dark green shaded area). The opposite trend is observed afterwards. A sharp Pch decrease Table 1 CH Parameters at the Time of the Central Meridian Crossing Panel in Figure 2 Enc. CH Reference Time Stamp Centroid cLAT/cLON LAT (min/max) LON (min/max) (a) E10 CH1 2021-11-14T02:22:04 −30.8/0.0 −44.9/−17.4 −11.5/12.7 (b) E10 CH2 2021-11-18T23:18:04 −29.5/0.1 −36.5/−21.3 −9.0/14.5 (c) E11 CH1 2022-2-18T06:00:04 13.9/0.0 −3.5/31.1 −8.5/8.1 (d) E11 CH2 2022-2-24T18:00:04 5.8/3.1 −5.4/13.7 −10.6/14.8 (e) E12 CH1 2022-5-13T12:00:04 16.5/4.6 −3.8/48.0 −14.3/20.1 (f) E12 CH2 2022-5-17T00:00:04 1.7/6.6 −19.1/17.8 −20.6/37.9 (g) E13 CH1 2022-8-27T23:00:04 −54.6/−1.0 −88.8/−28.2 −28.8/28.4 (h) E13 CH2 2022-9-14T07:12:04 28.1/0.0 15.7/38.3 −17.0/19.0 Figure 3. Snapshot of the simulation for the time stamp “2022-2-27T16:00:00 UT”, ∼48.4 hr after E11 perihelion. The assumed radial extension of the CHs are indicated with the blue and purple shaded areas. The zone of influence of each CH is represented by the angular width of the respective area. The perihelion of PSP orbit (orange line) is depicted with the orange diamond and the position of PSP with the red star. The FOV of WISPR-I is represented with the gray region and the region where the electron scattering efficiency is around maximum with the light blue annulus. 5 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 6. coincides with a sharp increase of the median excess brightness. A day postperihelion an extensive depletion interval occurs (region “b1” in Figure 1, top right panel), matched by a high, constant value of Pch. The E12 case (“c1” region in Figure 1) is a “textbook” scenario. The observed depletion during the inbound segment of the encounter is matched with a rather constant, Pch value. This is followed by a sharp brightness rise and a sharp drop to 0 in Pch. For E13, the behavior of Pch explains the three depletion regions in Figure 1 (bottom right panel). Pch, again, reflects the appearance, transit, and disappearance of the CH(s) extensions into the WISPR-I FOV. Interestingly, Pch reaches a minimum value of almost 0 (i.e., no CHs influencing the brightness) when the excess brightness increases to a high value (>15%) within a short period of only ∼2 hr. This time coincides with the passage of the 2022 September 5 CME event through the WISPR-I FOV. Right after the CME, we observe a depletion area for about a day, labeled as “d2” in Figure 1, in coincidence with a large value of the proxy. This indicates that the extension of a CH is at least partly responsible for the brightness depletion observed. As it will be shown in Section 3.2, the brightness observed in this time interval was also influenced by the passage of the CME. The rest of the relative evolution of brightness compared to the proxy explains the region “d3” in E13. The discussion around Figure 4 supports our hypothesis that the extent of CH plasma across the WISPR-I FOV leads to detectable lower brightness levels in spite of the large brightness variability observed along the encounters. 3.2. The Effect of CMEs Not all brightness depletion regions can be explained by the crossing of the WISPR-I LOS through CH extensions. In particular, region “c2” in Figure 1 (E12) does not coincide with the presence of a CH at an appropriate location. Our suspicion is that it might be due to brightness depletion in the wake of a CME event. Therefore, to investigate this possibility and to assess the plausible effect of big CME events on the overall brightness observed, we created height–time (HT) maps of the excess brightness for the horizontal slits at rows 256, 512, and 768 for all four encounters to have ample coverage of the evolution of the transients at all latitudes and a point of comparison between them. For detailed analysis, we display the corresponding HT maps for E12 and E13 in Figure 5. The vertical, white dashed lines pinpoint the time of perihelion and the labels indicate the corresponding depletion regions in Figure 1. (Note that the labels are either yellow or greenish. The yellow color is used to show that the corresponding depletion region is at least partly due to a CME event, while the greenish color that is associated only to a CH.) We distinguish two different regions in the maps; namely those with nearly zero excess brightness (darker regions) and those with slanted stripes of various widths exhibiting a relatively high excess brightness. The latter are the signatures of CMEs developing across the WISPR-I FOV at the respective latitudinal height. Here we concentrate on the depletion regions observed after the big CME events. Figure 5 shows, in particular, regions exhibiting a depleted brightness (about equal to the overall background level, i.e., 0%) after the passage of a CME in E12 just around perihelion for rows 256 and 512, and Figure 4. Time evolution of (1) the median excess brightness in the area delimited by the dashed white lines in Figure 1 (blue points), and (2) the proxy, Pch, (red curve, y-axis on the right) for E10, E11, E12, and E13. The top x-axis displays the S/C Carrington longitude at the time of the major x tick marks. The inset labels mark the corresponding depletion regions in Figure 1. 6 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 7. in E13 a couple of hours before perihelion for all three rows. These depletion intervals are concurrent with the aftermath of the 2022 June 2 (region “c2”), and 2022 September 5 (region “d2”) events, respectively. Snapshots of the two events are shown in Figure 6. Their analysis is beyond the scope of this paper. As inferred from their morphology and brightness in the FOV of the WISPR telescopes, these CME events were large and massive, and we generally expect brightness depletion in the wake of such events (e.g., Vourlidas & Webb 2018; and the discussion in Section 4). The event in E12, appears to evacuate (partly, at least) the K-corona below the ecliptic plane only. The upper halves of the images in the aftermath of the event do not show an associated depletion, although the CME feature encompasses all latitudes covered by WISPR-I. In contrast, the 2022 September 5 event affects the brightness across the full latitudinal range of the images. This CME was the fastest and most energetic event encountered by PSP so far, with a projected speed in excess of 2000 km s−1 . Interestingly, this CME was crossed by PSP during its evolution. Detailed analyses of this event are in preparation and will be published elsewhere. Here, we focus on the brightness evolution in its wake to shed light onto the nature of the observed depletion. To assess the nature of the depletion, we first extract the brightness profiles along the PSP orbital plane from all calibrated WISPR-I images in E10–E13, as well as from the minimum background set (hereafter Ebkg, see Section 2) for comparison. The resulting 952 profiles from the five data sets are plotted in log–log scale in Figure 7(a) as a function of elongation, expressed in solar radii. These distances, which we will refer to as coronal heights or LOSs, indistinctly, are computed by normalizing the angular elongations by the angular radius of the Sun at each PSP heliocentric distance. The coronal height range is restricted to the heights covered by the FOV of WISPR-I during the temporal extent of the depletion “d2” in Figure 1. A scaling factor is applied to each set (namely f = 2−t with t = 0, 1, 2, 3, and 4 for E10, E11, E12, E13, and Ebkg, respectively) to allow for proper visualization. At each LOS, there is an aggregation of measurements (the same LOS Figure 5. HT maps for the slits at rows 256, 512, and 768 on the WISPR-I images in E12 and E13. The vertical white dashed line indicates the time of perihelion on the corresponding encounter. The labels indicate the corresponding depletion regions in Figure 1 (labels in yellow point to depletion regions associated with a CME wake). Figure 6. Snapshots of the 2022 June 2 (left panel) and 2022 September 5 (right panel) events in E12 and E13, respectively. The composite images displayed show the WISPR-I and WISPR-O images projected into the helioprojective Cartesian (HPC) coordinate system. The Sun is to the left at 0° latitude. The white line depicts the PSP orbital plane. The images have been processed following the procedure described in Appendix A of Howard et al. (2022). 7 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 8. is covered by a range of S/C distances), hence the variable spread. Since we are trying to derive the baseline (nonvarying) component of the K+F coronal brightness across the encounters, we compute a baseline level for each data set by taking the running 5-percentile, hereafter Bi p 5 , with i = E10, E11, E12, E13, Ebkg and then robustly fitting5 them (in log–log space) with the analytical function given in Equation (2) (see Stenborg et al. 2022): = + + - -  A e A A x. 2 i f i A x A x i i 0 4 5 i Ai i 1 2 3 ( ) ( ) The fitted baseline levels, i f (dashed red lines in Figure 7(a)), should be largely free from the effects of discrete K-corona structures crossing the WISPR-I FOV. Note that the individual encounter curves are scaled with respect to each other for visualization purposes. To visualize the changes of the background level among the encounters properly, we compute the percentage excess brightness (1) D = - B B 1 i p i p bkg p 5 5 5 [ ] for the data, and (2) D = -   1 i f i f bkg f [ ] for the fitted levels. We display them in Figure 7(b): Di p 5 with the same color code as in panel (a), and Di f in red for all four cases. These quantities are essentially one-dimensional representations of the “quicklook” coronagraph images that are used extensively for assessment of coronal activity and measurements of CME speeds. The difference is that the coronagraph backgrounds are the monthly minima instead of the minimum of the four encounters at each observer’s distance in the WISPR case. The plots indicate that the fits, i f , represent well the data, except for edge effects. Equation (2) is designed to fit a profile with an extended linear part (Stenborg et al. 2021b, 2022) but here we considered only the data up to 20 R☉, where the linear part is not yet fully manifested. In addition, there are remaining K-corona con- tributions to the baseline level in all four encounters from coronal transients and other discrete K-corona structures (which are not accounted for in Equation (2)). The sudden increase in E12 around 13 R☉ is such an example. As we are not considering those coronal heights in our analysis, we ignore these effects in the remainder of the paper. Figure 7(b) shows clearly that the E13 background level (during “d2”) is exceptionally low and equal to the background minimum within 4.5–14 R☉. This time interval in E13 is dominated by a CH (see Section 3.1) as well as by the massive CME on 2022 September 5. In other words, the E13 brightness levels (within 4.5–14 R☉) defined the minimum background brightness for all four encounters. The comparison, however, only establishes a relative level among the four encounters. We cannot assess if this is total K depletion or if it includes a dust depletion as well. For that, we have to turn to a model of the F-corona, in lieu of the unavailability of any other means to establish a “ground truth” for the F-corona background. Following the procedure of Stenborg et al. (2022), we forward-model the F-corona brightness along the orbital plane, for the same set of S/C heliocentric distances as in Figure 7(a). As before, we determine the baseline level of the simulated measurements by computing the running 5-percentile, BF p 5 , and robustly fit it with the analytical function given in Equation (3) (as in Stenborg et al. 2022): = + + -  A e A A x. 3 F f A x 0 3 4 A 1 2 ( ) We then compute the percentage excess brightness between the fits of the four encounters (and of the empirical background, as a check) and F f (i.e., D = - ´   1 100 i F i f F f [ ] / ). We plot Di F in Figure 7(c) using the same color code as in panel (a). The red line marks the 0% level (i.e., DF F ). The comparison reveals that the E13 baseline level dips below the expected F-corona level within ∼10–15 R☉ (shaded range). This is an exciting result because it suggests rather strongly that the 2022 September 5 event may have caused the long-predicted dust evacuation (Ragot & Kahler 2003). This finding is discussed Figure 7. (a) Aggregated brightness measurements along the PSP S/C orbital plane in E10 (orange), E11 (light blue), E12 (green), and E13 (blue), and in the minimum background set, Ebkg (black). The dashed red line at the bottom of each set delineates the modeled baseline levels ( i , i f = E10...E13, Ebkg; see Equation (2)). Each set of measurements has been rescaled with respect to E10 to allow for proper visualization. (b) Percentage excess brightness of the baseline level profile of each encounter with respect to that of the background set ( * D = - B B 1 100 i p i p bkg p 5 5 5 [ ] with i = E10...E13; same color code). The red continuous lines depict the modeled excess brightness (i.e., * D = -   1 100 i f i f bkg f [ ] ). (c) Percentage excess brightness of the modeled baseline levels with respect to the modeled F-corona baseline level (i.e., * D = -   1 100 i F i f F f [ ] ) with i = E10...E13, Ebkg, same color code). The red continuous line at 0% delineates DF F . 5 The coefficients of the fitting were obtained by using the “least_square” function of the “scipy.optimize” package in Python 3.8.8. 8 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 9. further in Section 4. Furthermore, Dbkg F (black dashed line) tends to DF F beyond ∼16 R☉ (i.e.,    bkg f F f , as expected at large coronal heights, where the F-corona signal dominates over the nonvarying K-corona; see, e.g., Stenborg et al. 2022). We also note that the E11 brightness also seems to reach below the 0% level. However, this occurs above ∼15 R☉, where the fit of the baseline level begins to diverge (see Figure 7(b) and earlier discussion). This result is too uncertain to consider further here. 4. Discussion An important feature of the PSP orbit is the high angular speed of the S/C compared to 1 au orbiters. The view from 1 au changes ∼13° day−1 compared to about 70° day−1 for PSP at its current perihelion distance. The S/C crosses a wide range of ecliptic longitudes, particularly over a couple of days around perihelion, when the PSP superrotates—its angular speed exceeds the solar rotation speed (see Figure 8). Consequently, WISPR sweeps through a rapidly varying coronal scene during this short interval. This explains the relatively short time interval of the depletion regions around perihelion compared to the larger duration of the depletion regions farther away (Figure 1). The relatively sharp brightness changes offer a way to derive the boundaries of CH in the outer corona through the introduction of the geometric proxy, Pch. As we demonstrated in Section 3.1, the majority of the brightness depletions was associated with the passage of CHs through the WISPR FOV. As shown, Pch and its associated methodology allow us to track the sources of the CHs in the low corona and assess their extent and effect in the outer corona. Although brightness reductions due to CHs are routinely observed from 1 au, they over- whelmingly arise from polar CH extensions and are generally observed at high latitudes. Equatorial and mid-latitude CHs are difficult to identify, and hence, study, from 1 au because their coronal extensions are masked by intervening coronal streamer structures. The long LOS also affects polar CH studies. For example, these LOS effects prevented DeForest et al. (2018) to separate slow from fast solar wind despite the detailed image processing methods they applied on deep-exposure COR2 images. The use of Pch, and the unique location of WISPR within the solar corona can remove much of the ambiguity in CH studies from 1 au. The CH boundary identifications and the short LOS through the corona can better separate fast (CH-associated) plasma structures in the WISPR images from the slower flows associated with streamers. The rapid S/C rotation can then be used to extract 3D information for these structures. Perhaps more importantly, the identification of CH (and post-CME flow boundaries) remotely by WISPR can be used to pinpoint the relevant PSP in situ measurements with much higher precision that currently possible. In that respect, WISPR is fulfilling its original overarching objective to serve as a “local” imager and provide crucial context for the interpretation of the in situ measurements of the mission. The analysis of post-CME flows is the second exciting prospect that is emerging from the presented methodology. We were able to identify the boundaries of coronal depletions caused by CMEs in two encounters, E12 and E13. Both were large events and were expected to have a significant impact in the ambient corona. Post-CME dimmings have been observed from 1 au for many years but the most concrete associations tend to be for the slower, SBO CMEs (e.g., Vourlidas & Webb 2018, and references therein), which originate from quiet Sun regions, such as polar crown filaments. EUV full-disk imaging and irradiance measurements have established low EUV dimmings as the imprint of faster CMEs in the low corona (e.g., Reinard & Biesecker 2008; Mason et al. 2016). More recently, Krista & Reinard (2017) and Dissauer et al. (2019) performed extensive statistical analyses over a large number of CME flare events and dimmings. They verify the long-held idea that the dimming is a result of the opening of the field lines in the vicinity of the eruption and the evacuation of mass in high-speed flows. Transient CHs develop much more quickly than normal CHs, and although they do not last as long, their decay depends on the size of the evacuation. Dissauer et al. (2019) find a strong correlation (∼0.6–0.7) between the depth of EUV dimming and CME mass, indicating that much of the CME mass detected in the coronagraphs (<20–30 R☉) must come from the low corona, say, below 2 R☉. However, none of these studies have investigated the impact of the CME on the middle and outer corona in a quantitative manner. The tracking of the evacuation is generally difficult at these heights because the considerable disturbances caused by these CMEs tend to obscure the evacuation signatures, especially early in the event. In many cases, projection effects further complicate the analysis since the evacuation may occur along, rather than across, the 1 au LOS. Post-CME rays, when visible, are the strongest manifestation of the plasma flow from the low to the outer corona implied by the EUV dimming analyses (Vrsnak et al. 2009; Webb & Vourlidas 2016). The rays appear 3–4 hr behind the main body of the CME and last for about 18 hr, on average. Most of the cases are associated with streamer disappearances, with the streamer reforming within about a day. The 1 au analyses, therefore, suggest that coronal evacuation from CMEs is a relatively short-lived event and that the evacuation occurs along narrow lanes (the post- CME rays) that are presumably the plasma sheets around the post-CME current sheet(s). However, the evacuations seen in WISPR appear to form much faster and must occupy a wider area than post-CME rays suggest. It is unclear, at this moment, how the coronal evacuation proceeds. Is it all via post-CME flows or does the CME itself carry a significant part as pileup at its front? Howard & Vourlidas (2018) found no evidence of coronal pileup at the CME front in the 2–30 R☉ range but suggested that the pileup rate may be too low to be detected Figure 8. Carrington longitude of the PSP/SC as a function of time (S/C heliocentric distance in R☉ on the top x-axis) for E10 (orange), E11 (light blue), E12 (green), and E13 (blue). The shaded area indicates the PSP superrotation stage. 9 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 10. from 1 au in the presence of the bright CME fronts they analyzed. Further WISPR analyses of the evolution of CME fronts and wakes may shed light on this issue thanks to the greatly improved sensitivity of the WISPR observations when compared to their 1 au counterparts. A potentially groundbreaking demonstration of the sensitiv- ity of WISPR observations to the coronal environment may lie in the very low brightness level left in the wake of the 2022 September 5 CME (Figure 7(c)). As we noted in that figure, the empirically determined minimum background level during the “d2” depletion dips below the expected F-corona level. The obvious interpretation is that the CME has carried away not only the electron population but has removed part of the ionized, interplanetary dust as well. This mechanism had been suggested twenty years ago (Ragot & Kahler 2003) but has never been verified with observations. To explore the WISPR results in more detail, in Figure 9 we zoom into the distance range where the E13 depletion falls short of the F-corona background. Panel (a) compares the minimum brightness, i f , of the four encounters and of the background set against the baseline level of the forward- modeled F-corona (red dotted line), F f , in absolute brightness units. The x-axis is restricted to allow for an easier comparison among the encounters. The increasing divergence at lower heights indicates the increasing effect of K-corona variability closer to the Sun. It also shows that only E13 dips below the F-corona level, a signature indicating that dust must have been depleted. Note also that at the lowest heights, E13 is only slightly brighter, suggestive that the K-corona has been depleted at these heights. The effect of depletion becomes more apparent in Figure 9(b), where we compare in the restricted x-range the percentage excess brightness of the encounters (and back- ground set) baseline level with respect to the modeled F-corona baseline (i.e., D = - ´   1 100 i F i f F f [ ] / ). As we noted in Figure 7(c), E13 drops below the model F-corona (red line). The drop is significant (up to about 2% within 11–14 R☉) and evident only for E13. It is unlikely to be an observational artifact. The obvious interpretation is that the drop indicates dust evacuation. This interpretation hinges, however, on the reliability of the F-corona model, since no independent measurement of the F-corona brightness exists. The F-corona model has been tested against observations (Stenborg et al. 2021b, 2022) across an extended range of heliocentric distances and elongations. The observations showed clear evidence of dust depletion in the circumsolar environment. The F-corona model was able to fit the observations from 5 R☉ to 0.3 au with the inclusion of a DDZ between 5 and 19 R☉. The excess brightness for each encounter displayed in Figure 9 was estimated against the F-corona model with a DDZ and thus makes the E13 brightness depletion even a more significant indication of dust depletion. Another indirect evidence in support of dust evacuation comes from the brightness gradient in the observed profiles. Stenborg et al. (2022) demonstrated that the inflection point in the brightness profile along the symmetry axis of the F-corona is an observational signature of the location of the dust-free zone (DFZ). The presence of K-corona signal on these profiles shifts the inflection point outwards from the Sun, with the amount of shift dependent on the amount of K-corona. This dependency was shown by Stenborg et al. (2022) who by modeling both the F & K components found that the position of the maximum in the brightness gradient space is a strong function of the F/K ratio, with a lower ratio (i.e., more K or less F) driving the maximum of the gradient out further. In Figure 10 we compare the gradients of the baseline levels, i f , for the four encounters (E10 in orange, E11 in light blue, E12 in green, and E13 in blue) against the modeled baseline level of the F-corona (F f , in red). The maxima are marked by the vertical dotted lines and are clearly shifted outwards for all four encounters, indicating the presence of “contaminating” K-corona. This is a reasonable conclusion for E10, E11, and E12 because there were visible K-structures in all of those encounters. This is not a reasonable conclusion for E13, Figure 9. (a) Coronal height evolution of the baseline brightness level for E10 ( ; E f 10 orange), E11 ( ; E f 11 light blue), E12 ( ; E f 12 green), E13 ( ; E f 13 blue), Ebkg ( ; bkg f dashed black line), and the baseline of the modeled F-corona ( ; F f dashed red line). The x-axis is restricted to the heights where the E13 brightness drops below the model F-corona to allow for a direct comparison among the sets. (b) Zoomed-in version of Figure 7(c). The color code is the same as in panel (a). The E13 minimum brightness is about 2% lower than the modeled F-corona baseline level in the ∼11–14 R☉ range. Figure 10. Gradient of the baseline level models i (i = E10—orange, E11— light blue, E12—green, E13—blue, and F—red). The vertical (colored) dashed line pinpoint the position of the relative maximum for each set. 10 The Astrophysical Journal, 949:61 (12pp), 2023 June 1 Stenborg et al.
  • 11. however. The “d2” period was remarkable by the presence of a large brightness depletion with a virtually complete absence of K-coronal structures. Then, the only explanation for the large shift of the gradient maximum is the reduction in the F-corona brightness. The only remaining option for explaining the decrease of the F-corona brightness without dust evacuation is a change in the dust scattering function. This implies a change in the scattering properties of the dust. This can be the result of changes in either the dust composition and/or the shape of the dust grains. However, these changes must be the result of interaction with the CME since the depletion in question occurs clearly in the CME wake. It is not obvious, to us at least, what physical process could lead to either a selective liftoff of dust or a modification of the grain shape as a result of CME–dust interaction. On the contrary, it is already established that the near-Sun dust is ionized and therefore it can be dragged out by the CME magnetic structure, an interaction described by Ragot & Kahler (2003) and regularly seen in CME–comet interactions (Vourlidas et al. 2007). The above arguments leave the reduction in the dust density as the most probable explanation. 5. Summary and Conclusions We have developed a methodology to identify the sources of brightness variations—specifically, brightness depletions—in WISPR images. Using EUV observations of the low corona along with a simple geometric representation of the coronal extent of equatorial CHs, we were able to quantify the intersection between the CH coronal extensions and the WISPR FOV. We find that the majority of the observed K-corona brightness depletions could be ascribed to the existence of a CH region within the FOV of the instrument. The WISPR observations from “within” the solar corona offer an unprecedented opportunity to remove the long LOS confusion that limits 1 au observations and finally allow us to study the coronal atmosphere above both polar and equatorial CHs. The techniques presented here become even more powerful when we take into account the ability of PSP to perform in situ measurements of the plasma images captured by WISPR only a few hours earlier. In particular, the geometric CH localization technique illustrated in Figure 3 allows us to locate the boundaries of the CH zone of influence in the PSP/ FIELDS (Bale et al. 2016) in situ measurements without much difficulty (Vamsee Jagarlamudi, private communication). We will embark on this joint remote/in situ analysis next. However, CHs are not the only sources of brightness depletion. CMEs are, in fact, the only sources of coronal evacuation known from 1 au (e.g., Vourlidas & Webb 2018, and references therein). WISPR observations have offered several such examples from an intracoronal viewpoint. In both E12 and E13, large CMEs caused clear brightness depletions (Figure 5). Again, our ability to identify the boundaries of the CME-driven depletion will be extremely helpful in analyzing the in situ measurements of those regions. The coronal environment in the wake of CMEs has never been studied with in situ measurements until now. In E13, however, the fast CME of 2022 September 5 appears to have created a depletion so deep that we had to invoke dust evacuation to explain it. The phenomenon of dust ionization and depletion has been postulated for twenty years (Ragot & Kahler 2003) but has never been observed conclusively. Although it is strongly supported by physical considerations and observed in comet interactions with the solar wind, the effect is restricted to the CME envelope. Thus it is very difficult to observe from 1 au given the small volume of a CME when compared to the interplanetary dust. There have never been any identifications of dust evolution from that distance. Our analysis, based on observations from within less than 20 R☉ provides the strongest evidence yet for this process. Although this interpretation is based on comparisons to an F-corona model, it is consistent with the observations and well-under- stood physical processes. To be conservative, we consider it as an indirect detection of dust evacuation for now. With the rising cycle, we expect more CMEs to cross the WISPR FOV and give us a chance to compare the brightness depletion across multiple events and coronal conditions. Such studies will establish whether CMEs can indeed lead to dust evacuation within the solar corona. Parker Solar Probe was designed, built, and is now operated by the Johns Hopkins Applied Physics Laboratory as part of NASAʼs Living with a Star (LWS) program (contract NNN06AA01C). Support from the LWS management and technical team has played a critical role in the success of the Parker Solar Probe mission. The Wide-Field Imager for Parker Solar Probe (WISPR) instrument was designed, built, and is now operated by the US Naval Research Laboratory in collaboration with Johns Hopkins University/Applied Physics Laboratory, California Institute of Technology/Jet Propulsion Laboratory, University of Goettingen, Germany, Centre Spatiale de Liege, Belgium, and the University of Toulouse/ Research Institute in Astrophysics and Planetology. G.S. and A.V. were supported by WISPR Phase-E funds to APL. E.P. acknowledges support from the NASA LWS grant No. 80NSSC19K0069. R.A.H. was supported by NASA grant No. 80NSSC19K1261. P.H. was supported by the NASA Parker Solar Probe Program Office for the WISPR program (contract NNG11EK11I) and the Office of Naval Research. ORCID iDs Guillermo Stenborg https:/ /orcid.org/0000-0001- 8480-947X Evangelos Paouris https:/ /orcid.org/0000-0002-8387-5202 Russell A. Howard https:/ /orcid.org/0000-0001-9027-8249 Angelos Vourlidas https:/ /orcid.org/0000-0002-8164-5948 Phillip Hess https:/ /orcid.org/0000-0003-1377-6353 References Altschuler, M. D., Trotter, D. E., & Orrall, F. Q. 1972, SoPh, 26, 354 Bale, S. D., Goetz, K., Harvey, P. R., et al. 2016, SSRv, 204, 49 Battams, K., Gutarra-Leon, A. J., Gallagher, B. M., et al. 2022, ApJ, 936, 81 Battams, K., Knight, M. M., Kelley, M. S. P., et al. 2020, ApJS, 246, 64 Beck, J. G. 2000, SoPh, 191, 47 Braga, C. R., & Vourlidas, A. 2021, A&A, 650, A31 Brueckner, G. E., Howard, R. A., Koomen, M. J., et al. 1995, SoPh, 162, 357 DeForest, C. E., Howard, R. A., Velli, M., Viall, N., & Vourlidas, A. 2018, ApJ, 862, 18 DeForest, C. 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