(Solar Orbiter nugget #24 by S. Purkhart1, A. M. Veronig1,2, E. C. M. Dickson1, A. F. Battaglia3,4, S. Krucker3,5, R. Jarolim1, B. Kliem6, K.Dissauer7, T. Podladchikova8)



We present key results of a case study [1] of an M4-class flare on March 28, 2022, seen by STIX and EUI/FSI onboard Solar Orbiter, and SDO/AIA. An interesting aspect of this flare was the change in the non-thermal hard X-ray (HXR) source configuration during a late HXR peak, changing from the classical two footpoints at the ends of the flaring loops to a dominant source at one of the anchor points of the erupting filament.

The event occurred when Solar Orbiter was at a distance of 0.33 AU from the Sun and at a longitudinal separation of 83.5° west of the Sun-Earth line, close to its first science perihelion. From Solar Orbiters perspective, the event appeared near the eastern limb, while Earth-orbiting spacecraft observed it near the disk center.


Evolution of STIX X-ray sources

Figure 1 shows the evolution of the STIX X-ray sources mapped onto Solar Orbiter/EUI and SDO/AIA images. Initially, the event appears consistent with the standard flare model, with thermal emission from the flare loops and two non-thermal footpoints at their ends. However, during the last HXR burst (number 6), the southern non-thermal footpoint changes to a different location. Reprojections to SDO/AIA reveal that this new non-thermal HXR source is located at the edge of the southern anchor point of the erupting filament.

Figure 1. Overview of STIX HXR sources and their relation to Solar Orbiter/EUI and SDO/AIA images. Top row: STIX light curves for five energy ranges from 4 to 84 keV. Imaging intervals are marked by different colors. Columns: Color-coded corresponding to the marked time intervals containing the following observations: 1) STIX clean image contours on top of the closest available EUI FSI 174 Å image. Maxima of thermal contours are marked. 2) AIA 171 Å image with reprojected STIX non-thermal contours and the line-of-sights through the maxima of STIX thermal sources. 3) same as 2) but with AIA 131 Å images.


Event overview in the EUV

A more extensive overview of the AIA observations is shown in Fig. 2. The filament eruption appears asymmetric, with a more active, upward-whipping leg in the north and a more anchored leg in the south. The main flare loop arcade starts very narrow and then rapidly expands westward. This westward motion continues throughout the flare, and after the final HXR burst, flare loops appear that connect to the region of the southern filament footpoint.

Figure 2. Event overview in the AIA 94 Å and 131 Å channels at six selected times (columns). The top row shows inverted AIA 94 Å images together with HMI line-of-sight magnetogram contours marking levels of ±200 and ±1000 G for negative (red) and positive (blue) polarity.


Chromospheric response in the UV

Following the evolution of the AIA 1600 Å enhancements during the major HXR peaks (see Fig. 3), we again observe a westward motion throughout the flare. During the final HXR peak, the southern AIA 1600 Å enhancement jumps to the edge of the erupting filament's anchor point, confirming the location of the STIX non-thermal footpoint. A time series analysis of the AIA 1600 Å intensities in the marked subregions confirmed a close correlation with the timing of the HXR bursts observed by STIX.

Figure 3. Overview of AIA 1600 Å observations and their relation to HMI. Top row: AIA 1600 Å images during the HXR peaks for six time steps. Colored rectangles mark subregions encompassing the northern footpoints of the filament and northern flare ribbon (orange), the southern flare ribbon (red), and the southern filament footpoint (yellow). Middle row: Running-difference images of the above frames relative to the AIA 1600 Å image recorded 48 s earlier. Red (blue) colors indicate emission increase (decrease). Bottom row: Contours of emission increase in the running difference images above drawn on top of the HMI line-of-sight magnetograms. An animation of Figure 3 is available at the bottom of this nugget.


Nonlinear force-free magnetic field extrapolation

We related the AIA 1600 Å footpoints to structures in a pre-flare nonlinear force-free (NLFF) magnetic field extrapolation obtained using a physics-informed neural network approach [2]. By starting magnetic field line tracers within the contours of AIA 1600 Å enhancements associated with HXR bursts, we identify structures that were likely involved in magnetic reconnection during that time.

In Fig. 4 we show the pre-flare configuration of magnetic field structures likely involved in reconnection during HXR burst 1 (panels 1a to 1c) and HXR burst 6 (panels 2a to 2c). The first HXR burst resulted primarily from reconnection between a weakly sheared arcade (blue) and strongly sheared field lines (red/purple) near the erupting filament structure. This reconnection probably gave rise to the narrow AIA 131 Å flare loop arcade, which expanded westward during subsequent HXR bursts. During the final HXR burst, field lines (green) near the filament structure, which were initially strongly sheared and connected to the primary AIA 1600 Å emission kernel during the first HXR peak, were again involved.

Figure 4. Comparison of pre-flare magnetic field structures involved in the first and last HXR peaks. 1a and 2a: HMI LOS magnetic field with contours of AIA 1600 Å enhancements during the first (left) and last (right) HXR peaks. 1b and 2b: AIA 131 Å images taken during the two HXR peaks. 1c and 2c: Selection of the most important field lines.


Discussion and conclusion

The AIA EUV and 1600 Å observations indicate a continuous westward drift of the reconnection site during the flare and suggest that the HXR footpoints likely drifted as well. STIX probably only captured the sudden change in footpoint location during the final HXR peak, but missed the gradual westward drift in earlier peaks, due to the side-on view of the event from Solar Orbiter.

Such parallel motions of the X-ray footpoints along the inversion line have been reported previously, for example in statistical X-ray footpoint studies [3,4]. It has been argued that such motions can result from asymmetric filament eruptions in response to asymmetric external magnetic confinement [5]. In a whipping-like eruption, subsequent reconnection of the loops of the overlying arcade would shift the HXR footpoints toward the anchored filament footpoint, consistent with the observations of the flare under study.

However, our NLFF extrapolations suggest that the shift of the HXR footpoints cannot be fully explained by reconnection along a pre-existing overlying arcade. Instead, reconnection of field lines with very different shear in the early phase of the flare may play a crucial role in understanding the motion of the HXR footpoints during later parts of the flare. This generalizes simpler models, such as whipping reconnection, which only consider reconnection propagating along uniformly sheared arcades.





















Movie 1. Animation for Figure 3.


This study has been published in Stefan Purkhart et al., 2023, A&A 679, A99 https://doi.org/10.1051/0004-6361/202346354



The authors thank Dr. Cooper Downs for insightful discussions on the magnetic connectivities in this event during the workshop of the International Space Science Institute (ISSI) team no. 516 on “Coronal Dimmings and their Relevance to the Physics of Solar and Stellar Coronal Mass Ejections". Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The STIX instrument is an international collaboration between Switzerland, Poland, France, Czech Republic, Germany, Austria, Ireland, and Italy. The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA 4000134088); the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luft- und Raumfahrt (DLR); and the Swiss Space Office (SSO). S.P., E.C.M.D, and A.M.V. acknowledge the Austrian Science Fund: project no. I 4555. A.F.B. and S.K. acknowledge the Swiss National Science Foundation Grant 200021L_189180 for STIX. B.K. acknowledges support by the DFG and by NASA through Grants No. 80NSSC19K0082 and 80NSSC20K1274.



Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria

Kanzelhöhe Observatory for Solar and Environmental Research, University of Graz, Kanzelhöhe 19, 9521 Treffen, Austria

Institute for Data Science, University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210 Windisch, Switzerland

Institute for Particle Physics and Astrophysics (IPA), Swiss Federal Institute of Technology in Zurich (ETHZ), Wolfgang-Pauli-Strasse 27, 8039 Zurich, Switzerland

Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA

Institute of Physics and Astronomy, University of Potsdam, Potsdam 14476, Germany

NorthWest Research Associates, 3380 Mitchell Ln, Boulder, CO 80301, USA

Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, Moscow 121205, Russia



[1] Purkhart, S., Veronig, A. M., Dickson, E. C. M., et al. 2023, A&A, 679, A99, doi: https://doi.org/10.1051/0004-6361/202346354

[2] Jarolim, R., Thalmann, J. K., Veronig, A. M., & Podladchikova, T. 2023, Nature Astronomy, 7, 1171, doi: https://doi.org/10.1038/s41550-023-02030-9

[3] Bogachev, S. A., Somov, B. V., Kosugi, T., & Sakao, T. 2005, ApJ, 630, 561, doi: https://doi.org/10.1086/431918

[4] Gan, W. Q., Li, Y. P., & Miroshnichenko, L. I. 2008, Advances in Space Research, 41, 908, doi: https://doi.org/10.1016/j.asr.2007.05.001

[5] Liu, R., Alexander, D., & Gilbert, H. R. 2009, ApJ, 691, 1079, doi: https://doi.org/10.1088/0004-637X/691/2/1079

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