Post-impulsive millimeter emission of the 2022-05-04 solar flare

The present work aims at analyzing the nature of millimeter (mm) emission observed during the post-impulsive phase of the solar flare SOL2022-05-04T08:45 (M5.7), detected by the RT-7.5 radio telescope of the Bauman Moscow State Technical University at 93 GHz. We investigate the relationship of mm and extreme ultraviolet (EUV) emission with variations in the temperature and coronal plasma emission measure obtained from SDO/AIA and GOES data. The results show that the enhanced mm emission at the post-impulsive phase of the flare coincides with the increase of EUV emission, indicating a connection with moderately hot (~1 MK) plasma. Based on the calculation of the differential emission measure, we determine the parameters of the post-impulsive flare plasma and conclude that the optically thin coronal plasma may contribute of about 20% to the mm emission.

💡 Research Summary

The paper investigates the origin of millimeter‑wave (mm) emission observed during the post‑impulsive phase of the M5.7 solar flare SOL2022‑05‑04T08:45, using the RT‑7.5 radio telescope at 93 GHz (3.2 mm) together with multi‑wavelength data from SDO/AIA, GOES, Konus‑Wind, SRH, and RSTN. The flare began at 08:45 UT, peaked in soft X‑rays at 08:59 UT, and entered a post‑impulsive stage after 09:13 UT. During this stage the 93 GHz flux rose sharply, reaching a maximum of 26 ± 3.9 sfu at 09:15:30 UT. Simultaneously, EUV intensities increased in the AIA 171, 193, 211, and 335 Å channels, which are most sensitive to plasma temperatures of roughly 0.8–2.5 MK, while channels sensitive to hotter (94, 131 Å) or cooler (304, 1600 Å) plasma showed no comparable enhancement.

To quantify the thermal plasma contribution, the authors performed a differential emission measure (DEM) analysis over the region of interest (ROI) with an area of 5.84 × 10¹⁹ cm², covering temperatures from 0.5 to 25 MK using the Hannah & Kontar (2012) regularized inversion method. The DEM‑derived average emission measure (EM) and temperature (T) showed no significant rise during the post‑impulsive interval, indicating that the bulk coronal plasma remained relatively stable. Using these DEM results, the expected thermal free‑free (bremsstrahlung) flux at 93 GHz was calculated to be about 6 sfu, assuming an optically thin source. This is roughly 20 % of the observed 26 sfu, implying that the coronal plasma contributes a modest but non‑negligible fraction of the mm emission.

The authors discuss the implications of this finding. While previous studies have shown that chromospheric and transition‑region plasma (10⁴–10⁵ K) dominate mm emission in many flares, the present results suggest that, at least in the post‑impulsive phase of a moderate‑size flare, moderately hot coronal plasma (≈1 MK) can add a measurable component. The coincidence of EUV and mm enhancements points to possible involvement of coronal rain or condensations that become optically thin at mm wavelengths. The flat spectral index between 93 GHz and lower radio frequencies (−0.06) further supports a thermal free‑free origin for the observed mm component, although the limited frequency coverage prevents a definitive spectral diagnosis.

The paper concludes that mm observations, even at a single frequency, can serve as valuable diagnostics of coronal plasma evolution during the late stages of solar flares. The ~20 % contribution from coronal plasma highlights the need for multi‑frequency mm/sub‑mm measurements (e.g., with ALMA) to disentangle chromospheric and coronal contributions and to better understand processes such as coronal condensation, cooling, and energy redistribution after the impulsive energy release. Future work should aim at higher temporal and spatial resolution, as well as broader spectral coverage, to fully exploit the diagnostic potential of mm radiation in solar flare physics.