Science nugget: Solar flare hard X-rays from the anchor points of an eruptive filament - Solar Orbiter
Solar flare hard X-rays from the anchor points of an eruptive filament
(Solar Orbiter nugget #13 by Muriel Zoë Stiefel1,2, and the Solar Orbiter/STIX team)
The hard X-ray imaging spectrometer (STIX; ) on Solar Orbiter  observes solar flares in the hard X-ray energy range carrying on the observations of its predecessor RHESSI . Here we give an overview of the analysis of a M1.8 GOES class flare as reported in Stiefel et al. (2023) . The flare was jointly observed by many space observatories: AIA/SDO , XRT/Hinode , EIS/Hinode , IRIS  and STIX on board Solar Orbiter. Making this flare special is that we report four individual sources observed by STIX at nonthermal hard X-ray energies.
We conducted a multi-wavelength analysis of the flare in the EUV and X-ray energy range using the detectors and satellites listed above. STIX uses an indirect imaging technique providing us with the visibilities - the Fourier components - of the image of a flaring X-ray source. The reconstruction of an actual image (meaning the conversion from Fourier space back to normal [x,y] depiction of the image) is done on Earth using different imaging algorithms (e.g. Back-projection, forward-fitting, MEM_GE, see ).
We analyzed SOL2021-09-23T15:28, a M1.8 flare. This was the largest flare jointly observed by Hinode, SDO and Solar Orbiter spacecraft in the time range February to October 2021. The detailed study of this flare was motivated by the observation of four individual sources on the STIX images in the nonthermal energy range, shown from 22 to 28 keV. In the two upper images of Figure 1 the contours show the reconstructed STIX images during the peak of the nonthermal energy range. The further analysis of this flare was driven by the goal to understand the role of these four sources especially in the context of the standard model for solar flares.
Figure 1. Images showing the flaring region using the AIA 1600 Å map. The two upper images show the reprojected AIA 1600 Å map during the onset of the impulsive phase. Overlaid are the contours of the reconstructed STIX images in the thermal (red, 50-90 % contours) and nonthermal (green, 20-90 % contours) energy ranges using the forward-fitting and the MEM_GE algorithms. The yellow box indicates the field of view of the two images below. These two images are a comparison between the Earth and the Solar Orbiter vantage points during the peak time of the thermal energy. Overlaid on the AIA 1600 Å map are the 40-90 % contours of XRT (red), EIS (red) and thermal STIX (green). The black loop indicates a potential flare loop, making it easier to understand the two different viewing points. The illustration to the right shows the relative locations of the Sun, the Earth, and Solar Orbiter.
To understand the geometry of the flare, we analyzed the thermal sources observed by XRT, EIS, AIA and STIX (5-9 keV) during the peak time of the thermal energy. The two images at the bottom of Figure 1 compare the thermal sources as seen from Earth perspective with XRT, EIS and AIA 1600 Å and Solar Orbiter view points with STIX and AIA 1600 Å (reprojected). We clearly see a flare loop in the EIS and XRT filters and in the thermal energy range 5-9 keV of STIX connecting the two footpoints visible in AIA 1600 Å. These two footpoints correspond to "Fp 2" and "Fp 3" noted in the upper left image, showing the well known standard model for solar flares with the flare loop and its footpoints.
To explore further the similarities and differences among the four sources, we compared the time evolution of the intensities measured by AIA and STIX at the location of each. Figure 2 shows the comparison between the individual sources and detectors. We can observe striking differences between the outer sources (Fp 1 and Fp 4) and the inner sources (Fp 2 and Fp 3). While the outer sources increase and decrease rapidly, the inner sources have longer decay times.
Figure 2: Time evolution plots with maximum value normalised to one. The left and middle column show the intensity plots of AIA 1600 Å (black) and nonthermal STIX (red, error bars in blue) for the four sources individually. The right column shows the intensity plots of AIA 193 Å (black and red) for the four sources. As a reference, we added the GOES soft X-ray light curve (orange).
From the observations we can clearly recognise that the inner sources (Fp 2 and Fp 3) show similar behaviour and are the footpoints of a flare loop as known from the standard model for solar flares. The outer sources (Fp 1 and Fp 4) show different behaviour compared to the inner sources. Combining our observations outlined above with further studies (see ) of the CME and the measurement of the Doppler velocities, we deduce that the outer sources are likely the anchor points of an eruptive filament. The outer sources are produced by flare-accelerated electrons that precipitate down the filament and emit Bremsstrahlung in the chromosphere. Figure 3 shows a cartoon of our results, combining the standard model for solar flares with the measurements of the flare presented here. Hard X-ray sources of an eruptive filament where already predicted by .  reports similar observations for a flare in the microwave range making our observations the first report of these outer anchor points in the hard X-ray range measured by STIX on Solar Orbiter.
Figure 3: Cartoon describing the flare during the onset of the impulsive phase. The background image is the same image as shown in Figure 1 (top left). Additionally the sketch shows the standard model for solar flares.
1 Institute for Data Science, University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210 Windisch, Switzerland
2 Institute for Particle Physics and Astrophysics (IPA), Swiss Federal Institute of Technology in Zurich (ETHZ), Wolfgang-Pauli-Strasse 27, 8039 Zurich, Switzerland
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