The acceleration of charged particles is an ubiquitous phenomenon in photo voltaic flares. Non-thermal electrons are particularly probed by their onerous X-ray (HXR) and radio emissions. Most flare analyses concern the impulsive flare section, the place the X-ray signatures are essentially the most intense (i.e., the time vary between the beginning and peak time of the smooth X-ray burst).The post-impulsive section of eruptive flares with its signatures of destabilization and the eruption of large-scale coronal buildings and a re-arrangement of the corona in its wake led to an ordinary flare situation, with the build-up of a magnetic flux rope, its instability or lack of equilibrium, and magnetic reconnection in its wake that create signatures of time-extended power launch comparable to long-lasting thermal X-ray emission and rising loop programs in X-rays, EUV and Hα. Non-thermal signatures of time-extended power launch within the post-impulsive section can be noticed (Yu 2020) within the type of weak microwave and HXR emissions from electrons accelerated in magnetic reconnection occasions within the post-eruptive present sheet.
The current examine by Zhang et al. (2025) investigated the connection between X-rays and radio-emitting electrons within the post-impulsive section of an eruptive photo voltaic flare on 9 Might 2021. Such research have been carried out previously (see the primary research by Svetska et al. (1982) and e.g. White et al. (2011) for a assessment). Nevertheless, lengthy period occasions have been at all times tough to research in X-rays due to frequent occultations of the Solar to the spacecraft in a low-Earth orbit. Photo voltaic Orbiter now affords the distinctive benefit of photo voltaic observations over lengthy durations with none occultation.
Determine 1. Overview of the occasion noticed in X-ray and radio. (a) GOES smooth X-ray flux profile and GBM/Fermi X-ray rely charges. (b) STIX X-ray rely charges and NRH radio flux time profiles. The NRH radio flux profiles come from the total Solar. (c) ORFEES radio dynamic spectrum.
The current examine relies on the evaluation of an occasion which is a typical C4.0 class flare, however has a post-impulsive section that lasts for greater than fifty minutes from 14:10 to fifteen:00 UT and is accompanied by shifting and stationary sort IV bursts (proven in Figures 1 and a pair of). We used information from two ground-based radio devices at decimeter-to-meter wavelengths (ORFEES and NRH) and 5 space-borne devices (STIX/SolO, GBM/Fermi, GOES, AIA/SDO and EUVI/STEREO-A). This provides us a broad spectral and imaging vary at X-ray, EUV and decimeter-to-meter wavelengths. Whereas ground-based and SDO observations offered flare info as considered from the Earth, Photo voltaic Orbiter, and STEREO-A offered info from two completely different factors of view. The angle between Photo voltaic Orbiter and Earth is 97.5° and between STEREO-A and Earth, it’s 52.0°.
Determine 1 reveals an outline of the occasion noticed in X-ray and radio wavelengths. The GOES X-ray time profile reveals a C4.0 class flare with an impulsive section peaking at 13:58 UT and decaying till 14:09 UT. After the decay from the principle peak, the flux rises once more as much as the C2.0 degree. The X-ray rely charges within the 6–25 keV vary additionally enhance after 14:17 UT and stay on an enhanced degree till at the very least 15:00 UT. The EUV observations on the time of the brand new rise in X-rays present that the related power launch happens in the identical lively area and is the continuation of phenomena noticed through the flare impulsive section. The whole emission between 13:38 and (at the very least) 15:00 UT is thus thought-about right here as a single flare and the time interval after 14:15 UT is referred as post-impulsive section. The ORFEES radio dynamic spectrum reveals that the post-impulsive radio emission consists of a protracted period emission that begins with the X-rays round 14:15 UT and lasts greater than 45 minutes. This kind IV continuum covers your complete ORFEES band, with some high quality buildings superposed.
Determine 2. Submit-impulsive section of the C4.0 flare occasion noticed in X-ray and radio. (a) Normalized time profiles of GBM/Fermi and STIX X-ray rely charges. (b) ORFEES radio dynamic spectrum. (c) to (f) time evolution of the NRH 1D projection photographs.
Determine 2 reveals the occasion noticed in X-ray and radio wavelengths through the post-impulsive section. The height time above 10 keV is shortly earlier than 14:25 UT (time T3). Under 10 keV, the time evolution of the X-ray flux is extra gradual and the utmost is quite flat. The ORFEES dynamic spectrum reveals radio high quality buildings superposed on the long-lived continuum sort IV emission. The comparability of Figures 2a and b reveals that the long-duration radio emission is co-temporal with HXR emission above 10 keV. The strongest radio emission is noticed near the time of HXR peaks (both beneath or above 10 keV). The time evolution of the 1D projection photographs (Figures 2c to 2f) present that completely different elements within the sort IV radio burst will be distinguished through the post-impulsive section: 1. A shifting sort IV burst (the primary radio supply) between 14:15 and 14:21 UT noticed particularly at frequencies > 228 MHz, marked as ‘M’ in Fig. 2f (time interval of T1–T2). 2. A stationary sort IV burst that begins round 14:20 UT and lasts till 15:17 UT (finish of the NRH observations). It’s marked as ‘S1’ in Fig. 2c through the time of T3–T4. This stationary sort IV burst isn’t on the place of a pre-existing radio noise storm supply noticed earlier than the C4.0 flare, which is situated extra to the west, at the very least on the highest NRH frequencies. 3. Two sources are typically noticed (particularly at excessive frequencies) through the stationary sort IV burst. The north western one is the extra persistent one (S1). The southeastern one (the third radio supply, known as ‘S2’) shows a collection of bursts (e.g., at 14:36:55 UT (time T5)). It’s marked by ‘S2’ in Fig. 2nd on the time of T5.
Determine 3. Flare seen from the Earth and STEREO-A factors of view. (First and second columns) NRH radio contours are overlaid on the AIA photographs. The instances of every row are comparable, comparable to the time of the black (or white) lengthy dashed strains in Fig. 2. The pink crosses present the identical place within the AIA and EUVI photographs. For the primary row, the radio burst instances are at T3. For the second row, the radio burst instances are at T4. For the final row, the radio burst instances are at T5. (Final column) the insert on the underside left reveals an enlargement of the lively area noticed in EUV. STIX contours rotated to the AIA (or STEREO) discipline of view are overlaid on the AIA (or STEREO) photographs.
Determine 3 reveals the radio and X-ray contours after the top of the shifting sort IV burst overlaid on EUV photographs. The primary and second rows present the positions of the principle supply of the stationary sort IV burst (S1) respectively at T3 (peak time of the X-ray emission above 10 keV) and T4. Whereas the radio emission steadily extends to decrease frequencies, the supply areas at particular person frequencies are steady from 14:24 to 14:34 UT. The radio positions are comparatively near the lively area and localized in direction of its northern half. The final row is at 14:36 UT (T5 in Fig. 2), for a while when the radio supply S2 turns into predominant. The supply of the second part S2 of the stationary sort IV burst is near the place of the southern a part of the X-ray supply, which dominates the X-ray emission at the moment.
Determine 4. Time profile of the brightness temperature of the principle supply of the stationary sort IV radio sources (S1) from 14:09 to fifteen:00 UT. The highest panel is the ORFEES spectrum. The center panel is the brightness temperature time profiles of S1 and the STIX X-ray rely price within the 6–10 and 10–14 keV power ranges. The underside panel is similar S1 brightness temperature curves as the center panel and GBM/Fermi X–ray rely charges within the 6–10 and 10–14 keV power ranges.
Determine 4 reveals that the brightness temperature of the stationary sort IV burst (S1) rises steadily from 14:15 to 14:25 UT with attainable superposed broadband bursts. A peak is clearly seen at excessive frequencies round 14:25 UT. A dip near 14:33 UT in the entire frequency band is adopted by a brand new enhancement till 15:00 UT. There’s a very sturdy correlation within the rise section of X-ray and radio emissions. The primary radio peak coincides very carefully with the X-ray peak noticed by STIX above 10 keV. The second rise of the radio emission after 14:33 UT doesn’t correspond nonetheless to a brand new episode of X-ray emission above 10 keV. The height of the 6-10 keV time profile corresponds to the dip of the radio emission. A second X-ray peak within the 6-10 keV vary noticed round 14:32 UT corresponds nonetheless to a second broad radio peak at excessive frequencies. Comparable developments are noticed with GBM/Fermi.
Conclusions
- Lengthy-duration radio emission noticed through the post-impulsive section of the Might 9, 2021 occasion is related to HXR emission above 6 keV. The strongest radio emission happens at (or close to) the instances of the HXR peaks.
- The time profile of the brightness temperature of the principle supply of the stationary sort IV burst (S1) has temporal correlation with the X-ray emission: the 2 sorts of emission have an identical rise section and there’s a good affiliation between the primary radio peak and the X-ray peak noticed above 10 keV.
- From the X-ray spectral evaluation, it’s discovered that non-thermal emission is noticed within the lively area within the peak noticed above 10 keV through the early rise of the stationary sort IV supply (S1). The electron energy regulation index is steep (δ round – 7.5). The built-in electron flux above 30 kev is on the order of 1.6 × 1032 electron s−1
That is the primary time that HXR emission is clearly detected within the lively area on the onset of the stationary sort IV burst. The current detection of deka-keV electrons on this occasion additionally confirms the earlier estimation by Salas-Matamoros & Klein (2020) of the power of the electrons radiating stationary sort IV bursts.
As a closing conclusion, the observational outcomes offered right here display the significance of mixing X-ray and radio observations to get understanding of the manufacturing of non-thermal electrons in decimetric and metric sort IV bursts, in addition to on the non-thermal energetics related to these phenomena.
Extra particulars on the evaluation of the occasion will be discovered within the paper by Zhang et al. (2025).
Based mostly on the current paper by M. Zhang, N. Vilmer, Ok. Klein, A. Hamini, D. Paipa-Leon, Y. Zhang and Y. Yan, Non-thermal power launch within the post-impulsive section of the Might 9, 2021 occasion, A&A, 697, A216 (2025), DOI: 10.1051/0004-6361/202453514
References
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White, S.M., Benz, A.O., Christe, S. et al.: 2011, SSRv, 159, 225
Yu, S., Chen, B., Reeves, Ok. Ok., et al.: 2020, ApJ, 900, 17
Zhang, M. Vilmer, N., Klein, Ok.L. et al: 2025, A&A, 697, A216
