The acceleration of charged particles is an ubiquitous phenomenon in photo voltaic flares. Non-thermal electrons are particularly probed by their exhausting X-ray (HXR) and radio emissions. Most flare analyses concern the impulsive flare part, the place the X-ray signatures are probably the most intense (i.e., the time vary between the beginning and peak time of the comfortable X-ray burst).The post-impulsive part 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 a regular 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 reminiscent of 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 part will also 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 latest research by Zhang et al. (2025) investigated the connection between X-rays and radio-emitting electrons within the post-impulsive part of an eruptive photo voltaic flare on 9 Could 2021. Such research have been carried out prior to now (see the primary research by Svetska et al. (1982) and e.g. White et al. (2011) for a assessment). Nevertheless, lengthy length occasions had been at all times tough to investigate 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 comfortable 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 research is predicated on the evaluation of an occasion which is a typical C4.0 class flare, however has a post-impulsive part that lasts for greater than fifty minutes from 14:10 to fifteen:00 UT and is accompanied by shifting and stationary kind IV bursts (proven in Figures 1 and a couple 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 offers us a broad spectral and imaging vary at X-ray, EUV and decimeter-to-meter wavelengths. Whereas ground-based and SDO observations supplied flare data as considered from the Earth, Photo voltaic Orbiter, and STEREO-A supplied data 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 exhibits an summary of the occasion noticed in X-ray and radio wavelengths. The GOES X-ray time profile exhibits a C4.0 class flare with an impulsive part peaking at 13:58 UT and decaying till 14:09 UT. After the decay from the primary 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 not less than 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 energetic area and is the continuation of phenomena noticed throughout the flare impulsive part. All the emission between 13:38 and (not less than) 15:00 UT is thus thought of right here as a single flare and the time interval after 14:15 UT is referred as post-impulsive part. The ORFEES radio dynamic spectrum exhibits that the post-impulsive radio emission consists of a protracted length emission that begins with the X-rays round 14:15 UT and lasts greater than 45 minutes. This kind IV continuum covers the whole ORFEES band, with some nice buildings superposed.
Determine 2. Put up-impulsive part 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 pictures.
Determine 2 exhibits the occasion noticed in X-ray and radio wavelengths throughout the post-impulsive part. The height time above 10 keV is shortly earlier than 14:25 UT (time T3). Beneath 10 keV, the time evolution of the X-ray flux is extra gradual and the utmost is slightly flat. The ORFEES dynamic spectrum exhibits radio nice buildings superposed on the long-lived continuum kind IV emission. The comparability of Figures 2a and b exhibits 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 pictures (Figures 2c to 2f) present that completely different elements within the kind IV radio burst could be distinguished throughout the post-impulsive part: 1. A shifting kind 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 kind 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 throughout the time of T3–T4. This stationary kind IV burst will not be on the place of a pre-existing radio noise storm supply noticed earlier than the C4.0 flare, which is positioned extra to the west, not less than on the highest NRH frequencies. 3. Two sources are typically noticed (particularly at excessive frequencies) throughout the stationary kind 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. second 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 pictures. The instances of every row are related, equivalent to the time of the black (or white) lengthy dashed traces in Fig. 2. The purple crosses present the identical place within the AIA and EUVI pictures. 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 exhibits an enlargement of the energetic area noticed in EUV. STIX contours rotated to the AIA (or STEREO) discipline of view are overlaid on the AIA (or STEREO) pictures.
Determine 3 exhibits the radio and X-ray contours after the tip of the shifting kind IV burst overlaid on EUV pictures. The primary and second rows present the positions of the primary supply of the stationary kind IV burst (S1) respectively at T3 (peak time of the X-ray emission above 10 keV) and T4. Whereas the radio emission progressively 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 energetic area and localized in the 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 element S2 of the stationary kind 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 primary supply of the stationary kind 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 exhibits that the brightness temperature of the stationary kind IV burst (S1) rises progressively from 14:15 to 14:25 UT with doable 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 robust correlation within the rise part 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 tendencies are noticed with GBM/Fermi.
Conclusions
- Lengthy-duration radio emission noticed throughout the post-impulsive part of the Could 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 primary supply of the stationary kind IV burst (S1) has a very good temporal correlation with the X-ray emission: the 2 varieties of emission have an analogous rise part 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 energetic area within the peak noticed above 10 keV throughout the early rise of the stationary kind 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 energetic area on the onset of the stationary kind 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 kind IV bursts.
As a closing conclusion, the observational outcomes offered right here show the significance of mixing X-ray and radio observations to get a very good understanding of the manufacturing of non-thermal electrons in decimetric and metric kind IV bursts, in addition to on the non-thermal energetics related to these phenomena.
Extra particulars on the evaluation of the occasion could be discovered within the paper by Zhang et al. (2025).
Based mostly on the latest paper by M. Zhang, N. Vilmer, Okay. Klein, A. Hamini, D. Paipa-Leon, Y. Zhang and Y. Yan, Non-thermal power launch within the post-impulsive part of the Could 9, 2021 occasion, A&A, 697, A216 (2025), DOI: 10.1051/0004-6361/202453514
References
Salas-Matamoros, C., & Klein, Okay. L.: 2020, A&A, 639, A102
Svetska,Z. Hoyng, P. van Have a tendency W. et al.: 1982, SoPh 75 305
White, S.M., Benz, A.O., Christe, S. et al.: 2011, SSRv, 159, 225
Yu, S., Chen, B., Reeves, Okay. Okay., et al.: 2020, ApJ, 900, 17
Zhang, M. Vilmer, N., Klein, Okay.L. et al: 2025, A&A, 697, A216
