1. Introduction
Solar flares and coronal mass ejections (CMEs) are manifestations of magnetic energy release in the Sun’s corona that some times can be coupled (see, e.g. Kahler Reference Kahler1992; Mandrini 2009; Chen Reference Chen2017). Establishing when and how a large flare is associated with a CME requires multi-instrument coverage with enough sensitivity, dynamic range, and vantage (see, e.g., Schmieder et al. Reference Schmieder, Aulanier and Vršnak2015; Temmer et al. Reference Temmer, Thalmann, Dissauer, Veronig, Tschernitz, Hinterreiter and Rodriguez2017; Georgoulis et al. Reference Georgoulis, Papaioannou, Sandberg, Anastasiadis, Daglis, Rosa Rodrguez-Gasén, Sanahuja and Nieminen2018).
On 2024 April 17, NASA’s Sounding Rocket Program Office executed its first dedicated solar flare sounding-rocket campaign, launching two Black Brant IX payloads (FOXSI-4, Buitrago-Casas et al. (2021); and Hi-C Flare, Vigil et al. (Reference Vigil, Savage, Winebarger, Golub, Kobayashi, Champey, Kobelski, Moore and Kankelborg2025)) one minute apart form each other from the Poker Flat Research Range, Alaska. The campaign was designed to obtain coordinated, multi-wavelength observations of a single large flare with complementary capabilities: FOXSI-4 provided direct-focusing hard-X-ray imaging spectroscopy, while Hi-C Flare delivered sub-arcsecond EUV/SXR imaging at high cadence (Romeo and Perez-Piel Reference Romeo and Perez-Piel2024; Barnett Reference Barnett2024). Both payloads successfully observed an M-class flare, with the rocket observations capturing the event after its peak (Vigil et al. Reference Vigil, Savage, Winebarger, Golub, Kobayashi, Champey, Kobelski, Moore and Kankelborg2025; Minami et al. 2024). Whether this flare produced a coronal mass ejection remains uncertain; the aim of this work is to examine that possibility using white-light coronagraph observations, primarily from the Solar and Heliospheric Observatory (SOHO, Domingo et al. Reference Domingo, Fleck and Poland1995), Large Angle and Spectrometric Coronagraph Experiment (LASCO, Brueckner et al. Reference Brueckner, Howard, Koomen, Korendyke, Michels, Moses, Socker, Dere, Lamy, Llebaria, Bout, Schwenn, Simnett, Bedford and Eyles1995) C2 instrument, to search for CME signatures temporally consistent with the flare.
This paper is organized as follows. Section 2 (Data and methods) details the datasets and processing steps used in this study. Section 3 (Results) presents the observational findings and measurements. Section 4 (Discussion & Conclusions) interprets these results, including limitations and implications, and recaps the main points and outlines future work.
2. Data, methods, and analysis
We used: (i) GOES XRS as a temporal reference to identify the SOL2024-04-17T22:10 M1.0 flare’s onset, peak, and decay phases; (ii) EUV images from the Atmospheric Imaging Assembly (AIA, Lemen et al. Reference Lemen, Title, Akin, Boerner, Chou, Drake, Duncan, Edwards, Friedlaender and Heyman2012) on board the Solar Dynamic Observatory (SDO, Pesnell et al. 2012) to study the evolution of the flare region before, during, and after the outburst; (iii) SOHO/LASCO C2 running difference images to identify and track a weak CME front moving North-East from the source. We applied logarithmic scaling to emphasize faint EUV loops during the post-outburst phase and measured the CME’s angular width and projected linear velocity from the cataloged height time points.
The temporal and spatial relationship between these observations was obtained using JHelioviewer (Muller et al. Reference Muller, Fleck, Dimitoglou, Caplins, Amadigwe, Ortiz, Wamsler and Alexanderian2009; Müller et al. Reference Müller, Nicula, Felix, Verstringe, Bourgoignie, Csillaghy, Berghmans, Jiggens, García-Ortiz and Ireland2017), an advanced visualization tool that allows the integration and analysis of data. Its operation is based on the collection of spatiotemporally synchronized images from different telescopes, facilitating the comparison of observations of events in several wavelength ranges, all at once. In this case, JHelioviewer allowed us to combine the AIA and LASCO/C2 sequences, which facilitated the relationship between the expansion of the arcs seen in Figure 1 with the evidence signals of the CME in the external corona of Figure 2. The upper panels of Figure 1 capture the moment of the outburst, in 171 Å and 131 Å wavelengths. AIA 171 images show the energization and growing of the arc-shaped loops that emerge from the active region during the post-flare phase. In the AIA 131 images we can observe bright structures that refer to the hottest surface of the active region, which reconnects after its release. The lower panels show the conditions after the burst. No evidence of filamentary material is observed at any time. The white arrow indicates the outer fringes of the active region, which show signs of the weak eruption that occurred after the burst. Other wavelengths of the AIA showed a fainter or nonexistent signal of the eruption, indicating its stealthy nature.
Evolution of EUV loops before and after the SOL2024-04-17T22:10 M1.0 flare. Left panels: SDO/AIA 171 Å running-difference images highlight the expanding coronal fronts/loop systems (white arrows) at 22:05 UT (top) and 22:42 UT (bottom). Right panels: co-temporal SDO/AIA 131 Å images show the hot flare arcade and overlying structures. The paired views emphasize the outward propagation seen in 171 Å and the simultaneous high-temperature response in 131 Å.
Figure 1 Long description
The image contains four separate images. Panel A: A grayscale image showing the expanding coronal fronts and loop systems at 22:05 UT with a white arrow indicating the direction of expansion. Panel B: A teal-colored image showing the hot flare arcade and overlying structures at 22:05 UT. Panel C: A grayscale image showing the expanding coronal fronts and loop systems at 22:42 UT with a white arrow indicating the direction of expansion. Panel D: A teal-colored image showing the hot flare arcade and overlying structures at 22:42 UT.
Evolution of the coronal mass ejection (CME) associated with the event of 04/18/2024. The images show observations from the SOHO/LASCO C2 coronagraph, where the progressive expansion of the CME can be seen over the course of approximately 1 hour and 20 minutes. In the first panel (03:12:07 UT), the beginning of the expansion front can be seen, emerging from the solar edge. In the second panel (04:48:07 UT), the final face of the CME is more evident.
We also examined the outer corona with SOHO/LASCO C2. Running-difference frames reveal the first clear front between 03:12:07 and 03:24:07 UT on 2024-04-18, propagating toward the same quadrant as observed in Figure 1. During the time of the flare, the only other activity observed on the AIA disk was a recurrent jet in the southeastern quadrant. The LASCO CME catalog indicates a projected plane-of-sky speed of
$\sim 246~\mathrm{km\,s^{-1}}$
, a partial-halo morphology with position angle
$\approx 238^\circ$
, and an angular width of
$\approx 248^\circ$
(see Fig. 2). When co-registered with the SDO/AIA sequences, the timing and geometry of this front are consistent with the late expansion of the 171 Å loop system seen in Fig. 1 and the contemporaneous high-temperature response in 131 Å. Although no filamentary material is detected, the combined EUV-white-light evolution supports the interpretation of a faint and slow CME that is temporally and spatially associated with the SOL2024-04-17T22:10 event.
3. Results
The source region (AR13645) shows compact EUV brightenings at flare onset and faint overlying loops that brighten after the peak. In white light, a weak CME front is visible in LASCO C2 within hours of the flare, with an angular width of
$\sim$
248
$^\circ$
and a projected plane-of-sky speed of
$\sim$
246 km s
$^{-1}$
based on the LASCO CME catalog classification of a partial-halo event. No clear filament eruption is observed in the EUV channels, suggesting a low-mass ejection or a stealth-like, low-contrast eruption.
To assess temporal correlations, we analyzed the flare region in multiple EUV passbands, focusing on the AIA 131 and 171 Å filters. The 131 Å images (bottom-right panel of Fig. 1) show a compact, high-temperature core and a coherent hot structure consistent with a confined flux-rope-like feature, suggesting that part of the released energy remained magnetically trapped. In parallel, the 171 Å sequence (left panels of Fig. 1) reveals the progressive activation and outward expansion of an overlying loop system situated above the inferred reconnection site. The loop expansion is slow but persistent, indicative of a gentle, large-scale reconfiguration rather than a rapid, filament-driven eruption.
We then examined the outer corona using SOHO/LASCO C2 white-light images. To identify the most plausible CME counterpart to the EUV evolution, we cross-related coronagraph activity with low-corona signatures in space and time. Considering the onset window, position angle, and the absence of competing EUV eruptions elsewhere on the disk, we associate the faint, partial-halo front in Fig. 2 with the SOL2024-04-17T22:10 event. This linkage is supported by the timing and geometry of the LASCO front relative to the post-flare loop expansion traced in AIA.
4. Discussion & Conclusions
The absence of filamentary signatures in EUV and the modest coronographic kinematics point to a relatively low-energy ejection, likely shaped by the surrounding magnetic topology. The AIA sequences constrain the timing of the high-temperature response (131 Å) relative to the expansion of overlying loop systems (171 Å), providing boundary conditions that could potentially be used for future data-driven modeling of the eruption onset. Co-registration with LASCO/C2 running-difference images shows a faint, partial-halo front temporally and geometrically consistent with the EUV evolution, supporting the interpretation of a low-contrast CME associated with the SOL2024-04-17T22:10 event.
This preliminary study illustrates how coordinated EUV-white-light observations can connect the flare’s impulsive energy release to the earliest formation of a faint CME. The results motivate a fuller, quantitative analysis (possibly combining AIA DEMs, CME mass and energy estimates from coronagraph data, and refined kinematic tracking) to better constrain energy partition and test whether similar partial-halo, moderate-speed CMEs accompanying large class flares share common source-region properties.
Acknowledgements
We acknowledge the SOHO/LASCO and SDO/AIA teams for making their data publicly available, and the GOES program for XRS observations. J.C.B.C. acknowledges support from NASA and UC Berkeley’s Space Sciences Laboratory (NASA LCAS grant 80NSSC22K0577 and 80NSSC25M7045).
