The acceleration of charged particles is an ubiquitous phenomenon in photo voltaic flares. Non-thermal electrons are particularly probed by their laborious 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 tender 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 state of affairs, 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 equivalent to long-lasting thermal X-ray emission and rising loop programs in X-rays, EUV and Hα. Non-thermal signatures of time-extended vitality launch within the post-impulsive part 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 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 Might 2021. Such research have been carried out up to now (see the primary research by Svetska et al. (1982) and e.g. White et al. (2011) for a evaluation). Nonetheless, lengthy period occasions had 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 tender 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 research relies on the evaluation of an occasion which is a ordinary 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 sort 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 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 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 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 principle peak, the flux rises once more as much as the C2.0 degree. The X-ray depend charges within the 6–25 keV vary additionally improve 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 vitality launch happens in the identical energetic area and is the continuation of phenomena noticed in the course of the flare impulsive part. Your complete 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 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 the whole ORFEES band, with some high quality 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 photographs.
Determine 2 exhibits the occasion noticed in X-ray and radio wavelengths in the course of 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 high quality buildings superposed on the long-lived continuum sort 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 under or above 10 keV). The time evolution of the 1D projection photographs (Figures 2c to 2f) present that totally different elements within the sort IV radio burst might be distinguished in the course of the post-impulsive part: 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 in the course of 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 positioned extra to the west, not less than on the highest NRH frequencies. 3. Two sources are generally noticed (particularly at excessive frequencies) in the course of 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. 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 photographs. The occasions of every row are related, equivalent to the time of the black (or white) lengthy dashed strains in Fig. 2. The crimson crosses present the identical place within the AIA and EUVI photographs. For the primary row, the radio burst occasions are at T3. For the second row, the radio burst occasions are at T4. For the final row, the radio burst occasions 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) photographs.
Determine 3 exhibits 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 regularly 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 energetic 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 price within the 6–10 and 10–14 keV vitality ranges. The underside panel is similar 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 exhibits that the brightness temperature of the stationary sort IV burst (S1) rises regularly from 14:15 to 14:25 UT with potential 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 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. Related traits are noticed with GBM/Fermi.
Conclusions
- Lengthy-duration radio emission noticed in the course of the post-impulsive part of the Might 9, 2021 occasion is related to HXR emission above 6 keV. The strongest radio emission happens at (or close to) the occasions 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 the same 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 in the course of 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 energetic 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 ultimate conclusion, the observational outcomes offered right here reveal 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 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 Might 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
