High-resolution imaging of coronal mass ejections from SoloHI - Solar Orbiter
High-resolution imaging of coronal mass ejections from soloHI
(Solar Orbiter nugget #9 by Phil Hess1, R.C. Colaninno1, A. Vourlidas2, R.A. Howard2, G. Stenborg2 and the Solo/HI team)
The Solar Orbiter Heliospheric Imager (SoloHI; ) on board the Solar Orbiter spacecraft  observes the Sun’s corona and heliosphere in white light. SoloHI is a next-generation imager building upon the successful STEREO-SECCHI  pathfinder imagers and SOHO-LASCO . Heliospheric imagers are designed to observe the transient solar wind structures originating at the Sun, the most striking being large eruptions of magnetized plasma from the solar corona (i.e., coronal mass ejections or CMEs).
The first-generation heliospheric imagers, all observing from near 1 au, opened up a new research window into the evolution of large-scale structure in the inner heliosphere. Those observations are, however, significantly constrained in resolving fine-scale structures due to the long integration times and cadence. SoloHI, on the other hand, leverages the unique orbital characteristics of Solar Orbiter, to resolve the internal structure of CMEs in much greater detail than previously possible.
Just after the Solar Orbiter’s first science perihelion in March 2022, SoloHI captured a series of four eruptions (28 March - 2 April), all associated with M or X class flares from NOAA active region 12975. Thanks to the direction of these events and orbital configuration of the operating missions all CMEs were observed clearly by both SoloHI and 1 au imagers (Figure 1).
Since the CMEs propagate between STEREO/SOHO and Solar Orbiter, we can directly compare the observations taken from the SECCHI heliospheric imagers and SoloHI. First we have to identify the region of space that is common among the various imagers. As the telescopes have different FOV and heliocentric distances, translating the angular coverage into heliocentric heights requires the basic assumption that elongation is proportional to the ratio of height divided by the observer distance. SoloHI covered roughly 6 − 50R⊙ during that period. This range spans the outer portion of the COR2 coronagraph (∼ 4 − 15R⊙) and the inner portion of the HI-1 heliospheric imager (∼ 14 − 88R⊙). Comparisons with the LASCO C3 coronagraph are further restricted, as its FOV is approximately (4 − 30R⊙).
Figure 1: The location of Solar Orbiter in HEE coordinates is shown at the onset of the various events alongside the approximate SoloHI FOV (depicted with the dashed lines) at each time. The arrows indicate the approximate CME longitude. The Earth (where SOHO is located) and STEREO-A are also shown. The Earth is fixed in this coordinate system, and STEREO does not move significantly in the 6 day span of these events.
An example of the comparison for the 02 April 2022 CME between SoloHI, SECCHI and LASCO-C3 is shown in Figure 2. Both the CME front and the associated wave are resolved very well in SoloHI, as well as individual finer features behind the CME front. The SECCHI and C3 images, on the other hand, suffer from significant blur, especially in the C3 case. The CME intensity greatly reduces as it reaches the outer field of view. The accompanying movie files also show the other events seen during this period and a comparison between the observations further highlights the improved resolution of SoloHI, though with some noticeable jumps in time due to data gaps caused by spacecraft pointing changes. It is during these gaps that the more synoptic 1 au imaging can provide key missing information.
Figure 2: Roughly co-temporal images of the 02 April CME from SoloHI and the SECCHI in- strument suite, as well as LASCO. To highlight the common FOV, images from all SoloHI tiles, COR2/HI-1 and C3, have been projected onto identical grids, covering 6 to 30 R⊙ in the radial direction and -12 to 12 R⊙ in the y-direction. The SoloHI images have been flipped from their standard viewing direction to ease in the comparison.
The SoloHI instrument is able to provide incredibly detailed observations of the CME structure, leveraging the inner heliospheric orbit of the Solar Orbiter platform. Systems analysis with the synoptic 1 au imagers enables us to obtain vital information across the multi-scale nature of CMEs — from the large-scale evolution provide by the 1 au observations to the meso-scale structure, uniquely provided by SoloHI. As the mission progresses, SoloHI will continue gathering details on the interior of CMEs in the key coronal heights above 20 R⊙, which have historically been observed with low spatial resolution and cadence. This region corresponds to the transition between the subalfvenic corona to the superalfvenic solar wind and hold importance clues on how coronal structures become part of the heliospheric solar wind.
The detailed analysis of these observations is presented in Hess et al. (2023) (submitted to Astronomy & Astrophysics).
1 U.S. Naval Research Laboratory, Washington D.C., USA
2 Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The Solar Orbiter Heliospheric Imager (SoloHI) instrument was designed, built, and is now operated by the US Naval Research Laboratory with the support of the NASA Heliophysics Division, Solar Orbiter Collaboration Office under DPR NNG09EK11I. The SECCHI data are produced by an international consortium of the NRL, LMSAL, and NASA GSFC (USA), RAL and U. Bham (UK), MPS (Germany), CSL (Belgium), IOTA, and IAS (France). The SOHO/LASCO data used here are produced by a consortium of the Naval Research Laboratory (USA), Max-Planck-Institut fuer Aeronomie (Germany), Laboratoire d’Astronomie (France), and the University of Birmingham (UK). SOHO is a project of international cooperation between ESA and NASA.
 Brueckner, G. E., Howard, R. A., Koomen, M. J., et al. 1995, Sol. Phys., 162, 357
 Müller, D.; St. Cyr, O. C.; Zouganelis, I.; et al.: 2020, A&A, 642, 1
 Howard, R. A., Moses, J. D., Vourlidas, A., et al. 2008, Space Sci. Rev., 136, 67
 Howard, R. A., Vourlidas, A., Colaninno, R. C., et al. 2020, A&A, 642, A13
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