SoloHI’s viewpoint advantage: Tracking the first major geo-effective coronal mass ejection of the current solar cycle

(Solar Orbiter Nugget #30 by E. Paouris^1,2, A. Vourlidas², P. Hess³, M. Georgoulis², G. Stenborg²)

Overview of the event

Solar Cycle 25 (SC25) is ramping up towards peak solar activity. On April 23-24, 2023, Earth experienced its first severe geomagnetic storm in eight years, marked by the Dst index reaching -213 nT and the Kp index exceeding 8, and creating auroras as far south as Texas, USA. The last severe geomagnetic storm occurred on March 17, 2015 [1].

The storm on April 23, 2023, was triggered by the arrival of an Earth-directed Coronal Mass Ejection (CME) detected in the corona on April 21, at around 18:00 UT. Originally the CME, associated with a filament eruption and a GOES M1.7 class solar flare, did not appear to be of particular concern. The low magnetic complexity of the active region (AR), as expressed through the calculation of the effective connected magnetic field strength [2] in the photosphere, further indicated that the solar signatures were not suggesting an impending major event. The CME characteristics -neither extreme in terms of speed nor of flare levels (see e.g. [3])- did not suggest that it could trigger the severe G4 geomagnetic storm that actually occurred taking the space weather community by surprise. This was certainly an extraordinary space weather event.

The CME was tracked by numerous spacecraft and instruments, providing a rich dataset of in-situ and remote sensing information. Specifically, the CME was observed by the SOHO [4]/LASCO [5] and STEREO-A [6]/SECCHI COR2 [7] coronagraphs, STEREO-A/SECCHI HI1/HI2 heliospheric imagers [7], and Solar Orbiter [8] HI [9] heliospheric imagers, along with the Wind [10] and STEREO in-situ instruments. 

Our single-viewpoint analysis using the COR2 images showed an asymmetric expansion with the CME moving faster towards the south (1380 km/s ± 90 km/s) compared to the north (820 km/s ± 80 km/s). Closer to the ecliptic plane, the CME speed was approximately 1000 km/s, indicating a moderately fast CME. Again, these values do not indicate a strong geo-effective response and are in agreement with the in-situ observations where the interplanetary shock of the CME arrived at Wind on April 23, 2023 at 17:00 UT, indicating a transit time of approximately 47 hours and an average transit speed of 880 km/s. 

Figure 1: Spacecraft configuration on April 23, 2023, at the time of the CME onset (18:10 UT) in the Heliocentric Earth Ecliptic (HEE) coordinate system. The solid and dashed blue lines mark the propagation direction and angular width of the CME, respectively. The FOVs of the SoloHI and the STEREO-A HI1 telescopes are represented by the gray triangls bounded by the solid magenta and red dash-dot lines, respectively.

SoloHI is the only imager that images the CME in detail

Despite the plethora of observations, the CME’s impact was clearly difficult to assess. Because this was an Earth- (and STEREO-A) directed event, projection effects hindered the accurate determination of the event kinematics using the images from these missions alone. Fortunately, the Solar Orbiter spacecraft was positioned off the Sun-Earth line allowing SoloHI to capture the CME in exquisite detail (Figure 2). The SoloHI viewing perspective and high-resolution imaging provided the key information necessary to robustly reconstruct the 3D CME trajectory with the inclusion of the LASCO and SECCHI imaging. Thus, we were able to derive that the CME propagated in a direction of approximately 2 degrees west of the Sun’s central meridian and 13 degrees south relative to Earth.

The Earth is actually visible in the SoloHI images. Apparently, the CME front crosses the Earth’s position at approximately 06:00 UT on April 22, 2023 (see Figure 2, right panel). The actual time of arrival was 17:00 UT on April 23. This difference highlights the important role of projection effects in the images. It is obvious that we are not observing the actual front of the CME but its flank. To disentangle the projection effects in our calculations for the time of arrival, we used the deprojected height from the GCS [11] reconstruction model. Assuming a two-phase kinematic scenario [12] as the most realistic model for the CME's actual movement, we estimated the time of arrival of the CME at Earth to be just two hours earlier than the actual time of arrival. This accuracy highlights the value of the SoloHI data (and of heliospheric imaging away from the Sun-Earth line) in space weather forecasting. 

Figure 2: SoloHI image of the April 21, 2023 a few hours later from the onset of the CME were the shock and many details of the main part of the CME are visible. The Earth is the bright spot on the upper left tile (Tile 3) of the SoloHI field of view.
 

Solar activity continues to intensify. Several X-class solar flares and Earth-directed CMEs occurred from May 6-14, 2024 resulting in one of the most extreme geomagnetic storms ever recorded with Dst < 400 nT (Paouris et al., in prep.). Solar Orbiter was similarly positioned this spring to observe the solar activity that led to an even stronger geomagnetic response, and, while we don’t have the data yet, we expect a similar analysis may help explain the severity of these events.

A detailed analysis of this event is nearly completed and will be included in a peer-reviewed publication to be submitted soon. 
 

Acknowledgements

We thank the mission teams for the ready availability of their data. 

Affiliations

[1] George Mason University, 4400 University Dr, Fairfax, VA 22030, USA
[2] The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd, Laurel, MD 20723, USA
[3] US Naval Research Laboratory, Washington, DC, USA

References
[1] Wu, C.-C., Liou, K., Lepping, R.P.; et al.: 2016, EP&S, 68, 151

[3] Georgoulis, M. K., Rust, D. M.: 2007, ApJL, 661, L109

[2] Paouris, E., Vourlidas, A., Kouloumvakos, A.; et al.: 2023, ApJ, 956, 58

[4] Domingo, V., Fleck, B., & Poland, A. I.: 1995, SoPh, 162, 1

[5] Brueckner, G. E., Howard, R. A., Koomen, M. J.; et al.: 1995, SoPh, 162, 357

[6] Kaiser, M. L., Kucera, T. A., Davila, J. M.: et al.: 2008, SSRv, 136, 5

[7] Howard, R. A., Moses, J. D., Vourlidas, A.; et al.: 2008, SSRv, 136, 67

[8] Müller, D., St, Cyr, O. C., Zouganelis, I., et al. 2020, A&A, 642, A1

[9] Howard, R. A., Vourlidas, A., Colaninno, R.C.: et al.: 2020, A&A, 642, A13

[10] Ogilvie, K. W. and Desch, M. D.: 1997, AdSpR, 20, 559

[11] Thernisien, A. F. R., Howard, R. A., Vourlidas, A.: 2006, ApJ, 652, 763

[12] Paouris, E., Vourlidas, A.: 2022, SpWea, 20, 7