The acceleration of charged particles is an ubiquitous phenomenon in photo voltaic flares. Non-thermal electrons are particularly probed by their arduous 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 mushy 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 typical 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 vitality launch resembling long-lasting thermal X-ray emission and rising loop techniques in X-rays, EUV and Hα. Non-thermal signatures of time-extended vitality 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 examine 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 overview). Nevertheless, lengthy period occasions have been at all times troublesome to research in X-rays due to frequent occultations of the Solar to the spacecraft in a low-Earth orbit. Photo voltaic Orbiter now presents 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 mushy X-ray flux profile and GBM/Fermi X-ray depend charges. (b) STIX X-ray depend 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 is predicated on the evaluation of an occasion which is a standard 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 transferring and stationary sort IV bursts (proven in Figures 1 and a pair of). We used knowledge 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 seen from the Earth, Photo voltaic Orbiter, and STEREO-A offered info from two totally 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 summary 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 part 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 stage. The X-ray depend charges within the 6–25 keV vary additionally improve after 14:17 UT and stay on an enhanced stage 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 vitality launch happens in the identical lively area and is the continuation of phenomena noticed throughout the flare impulsive part. Your entire 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 reveals that the post-impulsive radio emission consists of an extended period emission that begins with the X-rays round 14:15 UT and lasts greater than 45 minutes. This kind IV continuum covers your entire ORFEES band, with some advantageous buildings superposed.
Determine 2. Submit-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 depend charges. (b) ORFEES radio dynamic spectrum. (c) to (f) time evolution of the NRH 1D projection pictures.
Determine 2 reveals 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). Under 10 keV, the time evolution of the X-ray flux is extra gradual and the utmost is relatively flat. The ORFEES dynamic spectrum reveals radio advantageous 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 pictures (Figures 2c to 2f) present that totally different elements within the sort IV radio burst might be distinguished throughout the post-impulsive part: 1. A transferring 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 throughout 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, not less than on the highest NRH frequencies. 3. Two sources are typically noticed (particularly at excessive frequencies) throughout 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 pictures. The instances of every row are comparable, akin to the time of the black (or white) lengthy dashed traces in Fig. 2. The pink 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 reveals an enlargement of the lively area noticed in EUV. STIX contours rotated to the AIA (or STEREO) subject of view are overlaid on the AIA (or STEREO) pictures.
Determine 3 reveals the radio and X-ray contours after the tip of the transferring sort IV burst overlaid on EUV pictures. 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 progressively extends to decrease frequencies, the supply places at particular person frequencies are secure 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 depend fee within the 6–10 and 10–14 keV vitality ranges. The underside panel is identical S1 brightness temperature curves as the center panel and GBM/Fermi X–ray depend charges within the 6–10 and 10–14 keV vitality ranges.
Determine 4 reveals that the brightness temperature of the stationary sort 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 sturdy correlation within the rise part of X-ray and radio emissions. The primary radio peak coincides very intently 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. Related 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 principle supply of the stationary sort IV burst (S1) has a very good temporal correlation with the X-ray emission: the 2 sorts 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 lively area within the peak noticed above 10 keV throughout the early rise of the stationary sort IV supply (S1). The electron energy legislation 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 vitality of the electrons radiating stationary sort IV bursts.
As a closing conclusion, the observational outcomes introduced right here display 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 sort IV bursts, in addition to on the non-thermal energetics related to these phenomena.
Extra particulars on the evaluation of the occasion might be discovered within the paper by Zhang et al. (2025).
Based mostly on the latest paper by M. Zhang, N. Vilmer, Ok. Klein, A. Hamini, D. Paipa-Leon, Y. Zhang and Y. Yan, Non-thermal vitality 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, Ok. 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, Ok. Ok., et al.: 2020, ApJ, 900, 17
Zhang, M. Vilmer, N., Klein, Ok.L. et al: 2025, A&A, 697, A216
