Coordinated Coronal and Heliospheric Observations During the 2024 Total Solar Eclipse 

(Solar Orbiter Nugget #32 by Y. J. Rivera¹, J. Samra¹, S. T. Badman¹, B. Berkey², C. Bethge³, P. Bryans², J. Burkepile², E. DeLuca¹, G. de Toma², G. Del Zanna⁴, C. Downs⁵, R. French⁶, M. Janvier⁷, D. A. Lacatus², Xianyu Liu⁸, Weihao Liu⁸, C. Madsen¹, M. Molnar², W. Manchester IV⁸, A. Paraschiv⁶, K. Reardon⁶, B. Reese⁹, T. Schad⁶, D. B. Seaton¹⁰, S. Superczynski⁹, J. Szente⁸, A. Tritschler⁶)

Introduction

A total solar eclipse occurred on 2024 April 8th over North America that crossed several parts of Mexico, United States, and ended in the eastern part of Canada (Figure 1). The path of totality provides a unique opportunity to observe the solar corona with the naked eye while the fully occulted solar disk allows for detailed coronal observations in visible and near-Infrared wavelengths from the ground.

Figure 1: Flight path across Texas, USA of the NSF Gulfstream V (in blue) for the ACES eclipse experiment along the path of totality (red) during the solar eclipse on 2024 April 8. ACES observed totality from 18:37:30 - 18:44:30 UTC.

Eclipse planning

In preparation for the eclipse, as part of the Airborne Coronal Emission Surveyor (ACES) eclipse experiment, led by PI Dr. Jenna Samra at the Smithsonian Astrophysical Observatory, the science team spent many months planning and coordinating with instrument teams from several space and ground-based instruments. This included Solar Orbiter [1], U.S. National Science Foundation Daniel K. Inouye Solar Telescope (DKIST, [2]), Mauna Loa Solar Observatory (MLSO)-led K-coronagraph (K-Cor) and Upgraded COronal Multi-channel Polarimeter (UCoMP), and Hinode/Extreme-ultraviolet Imaging Spectrometer (EIS [3]), all of whose fields of view (FOVs) are shown in Figure 2. We also include the FOV of the NOAA GOES-R Solar UltraViolet Imager (SUVI) observations which were performed in a special off-pointing campaign operation during the eclipse. In particular, given the narrow slot of ACES, and the small FOV of Cryo/NIRSP and EIS in the low corona, pointing alignment was especially crucial between those instruments. The science objective of the multi-instrument eclipse coordination was to measure near-contemporaneous multi-wavelength observations in near-infrared, visible, and extreme ultraviolet (EUV) to derive detailed plasma conditions, elemental composition, and magnetic field properties over an unprecedented range of coronal heights.

Coordinated instruments

ACES is an imaging fourier transform spectrometer surveying the near-Infrared at wavelengths between 1-1.7 µm continuously across the low and middle corona. During the eclipse, the telescope was mounted onto a National Science Foundation (NSF) Gulfstream V airplane that traveled over the state of Texas along the path of totality enabling an extended view of totality while minimizing atmospheric absorption from the plane’s high altitude. In coordination, highly synergistic observations from DKIST’s Cryo/NIRSP measured high resolution spectroscopic and polarimetric emission at wavelengths matching those of ACES aligned low in the corona. Overlapping Hinode/EIS spectroscopic observations cover the EUV measured emission lines from several highly ionized Fe ions at the base of the ACES slot and within Cryo/NIRSP’s FOV. In conjunction, the MLSO team operated the UCoMP and K-Cor instruments, where UCoMP provided spectral and polarimetric coronal imaging in several visible and near-IR lines up to 2 solar radii, while K-Cor measured white light and polarized brightness (pB) of the extended corona (up to 3 solar radii), providing integral large-scale coronal morphology to ACES/CryoNIRSP/EIS observations.

Figure 2: Composite image of GOES SUVI 195Å, MLSO UCoMP 10740Å, SOHO LASCO C2 on 2024 April 9 with a summary of the coordinated fields of view of ACES, DKIST Cryo-NIRSP, Hinode/EIS, MLSO UCoMP and K-Cor, GOES-R SUVI.

Pointing the instruments using coronal predictions

Given that emission from near-infrared lines are dominated by scattered light, the ACES team aimed to align its FOV along a dense helmet streamer where more scattering can occur at larger coronal altitudes. To identify a coronal streamer target ahead of the eclipse, the eclipse team collaborated with two coronal magnetohydrodynamic (MHD) modeling groups: CORONACAST team led by students and researchers at the Climate and Space Sciences department at the University of Michigan, as well as the Predictive Science Inc. (PSI) group. The Space Weather Modeling Framework, specifically the Alfvén Wave Solar-atmosphere Model (AWSoM), from the Michigan group generated synthetic images that predicted the morphology of the corona at the time of the eclipse. Complementary forecasting from the Magnetohydrodynamic Algorithm outside a Sphere (MAS) MHD model from the PSI group provided independent predictions of the coronal structure leading up to eclipse day. The PSI team implemented novel time-dependent magnetic field boundary conditions through a surface flux transport model with continuously updated magnetogram data. In particular, coordination between PSI and the PHI instrument team from Solar Orbiter, enabled the integration of near-real time, full-disk vector magnetograph observations into the transport model to derive updated coronal topology counting down to the eclipse. The advanced approaches from the modeling groups provided cutting edge predictions of the global corona that were critical for deciding instrument pointing ahead of the eclipse.

Figure 3: Spacecraft configuration of Parker Solar Probe and Solar Orbiter around the solar eclipse on April 8th where the solid lines cover the day of April 8th and the dashed lines are the spacecraft trajectories ± 10 days from April 8th.

Solar-heliospheric connection with Solar Orbiter and Parker Solar Probe

A highly fortuitous spacecraft alignment with Parker Solar Probe (Parker) and Solar Orbiter around the eclipse indicated the orbiting spacecraft would be located to the west of the Sun near the plane of the sky, as shown in Figure 3. To take advantage of the vantage point from the remote sensing instruments on Solar Orbiter in this configuration, and potential in situ connection, the eclipse experiments pointed instruments to the west limb of the Sun. Additionally, footpoint predictions from the Parker team indicated that instruments onboard would sample outflows from an extended, equatorial coronal hole during its 19th perihelion pass (at its closest approach on March 30th) as would be observed on the solar disk from the Sun-Earth line throughout perihelion. The same source region was predicted to rotate to the west limb at the southern hemisphere to become visible during the solar eclipse. As such, our team planned to target a large helmet streamer in the southern hemisphere, as predicted by the MHD modeling groups, where the base of the streamer was adjacent to the coronal hole source that would be sampled by Parker and Solar Orbiter and within the FOV of several instruments.

Preliminary observations taken around the solar eclipse

Operation of both DKIST and MLSO instruments were interrupted due to cloud obstruction during the day of the eclipse however both operated the following day. Figure 4 shows preliminary observations from K-Cor and UCoMP in the Fe XIII 10740Å with the superimposed GOES SUVI 171Å waveband image of the disk from April 9th. The south-west limb where ACES, CryoNIRSP, and EIS were pointed show two main coronal loop structures, associated with extended streamers, at the equator and ~45o south of the equator. The coronal loops are separated by open field that is likely coronal observations of the equatorial coronal hole predicted above. Figure 5 is an image of the SDO/AIA 211Å image (left) taken on April 1st showing the equatorial hole (outlined in cyan) connected to outflows sampled during Parker’s perihelion pass, as well as at several heliocentric distances with Solar Orbiter. On the left is an image from Solar Orbiter’s Extreme-Ultraviolet Imager / Full Sun Imager (EUI/FSI [4]) 174Å channel taken on April 8th (during the solar eclipse) while in quadrature with the Earth (Figure 3) showing an on-disk view of the same coronal hole that would appear on the limb in Figure 4. The location of the coronal hole, just south of the solar equator from the AIA and EUI/FSI images, aligns well with the open field observed in the UCoMP image of Figure 4 and partially within view of the ACES/Cryo-NIRSP observations. Ongoing work plans to characterize coronal conditions and magnetic field properties using near-synchronous observations from ACES, Cryo-NIRSP, EIS, UCoMP, K-Cor, and other ground based eclipse experiments. We also plan to trace coronal structures to the solar wind measured at the inner heliosphere with Parker and Solar Orbiter to follow the thermodynamic and magnetic field evolution of the escaping plasma.

Figure 4: Composite image of LASCO C2, MLSO K-Cor and UCoMP Fe XIII 10740Å with GOES-SUVI 171Å on 2024 April 9.

Figure 5: SDO/AIA 211Å taken from the sun-earth line on April 1st (left) and Solar Orbiter EUI/FSI 174Å waveband image (right) taken on April 8th while in quadrature with Earth-based observations, as shown in Figure 3. The cyan contour on both images indicates the outline of the coronal hole, determined from the AIA 211Å image and mapped to EUI/FSI, that rotated from disk center to the limb on April 8th during the solar eclipse and was on-disk from Solar Orbiter’s FOV.

Affiliations

1 Center for Astrophysics, Harvard & Smithsonian
2 High Altitude Observatory, NSF NCAR
3 Cooperative Institute for Research in Environmental Sciences
4 University of Cambridge
5 Predictive Science Inc.
6 National Solar Observatory
7 European Space Agency
8 University of Michigan
9 NOAA
10 Southwest Research Institute

Acknowledgements

M.J. acknowledges input from the Solar Orbiter/EUI team with EUI/FSI imagery allowing comparisons with Earth and near-Earth observations.

References

[1] D. Müller et al., The Solar Orbiter mission. Science overview

[2] T. Rimmele et al., The Daniel K. Inouye Solar Telescope - Observatory Overview

[3] J. L. Culhane et al., The Solar-B EUV Imaging Spectrometer: an Overview of the EIS instrument

[4] P. Rochus et al., The Solar Orbiter EUI instrument: The Extreme Ultraviolet Imager