The (sub)millimeter radiation of solar flares, explosive events detected as brightenings across the electromagnetic spectrum, is poorly understood (Krucker et al. 2013). The major limit of the previous >100 GHz observations was spatial resolution, impeding the combination with observations at other wavelengths. The Atacama Large Millimeter-submillimeter Array (ALMA) offers sufficient resolution for the first time. Shimizu et al. (2021) reported a microflare observed in interferometric mode. However, used as an interferometer, the ALMA field of view is small, and ALMA cannot observe on demand when a flare occurs because of scheduling constraints. Therefore, we used readily available large scale full disk single-dish observations for first detections of full-scale ALMA flares. We searched for flares in archive ALMA single-dish images in times listed in the solar flare catalog (HEK). Five flares were found with enough data for our analysis, four from the HEK database search (all of GOES class B) and one added manually. Their temporal and spatial evolution is analyzed by comparing ALMA measurements with EUV (SDO AIA/HMI) and Hα filtergrams, and GOES X-ray fluxes.
Figure 1. ALMA 1.3 mm difference image of the flare with its contours overlaid on SDO/HMI magnetogram, GONG Hα and SDO/AIA 304, 171 and 94 Å images. Several solar features are indicated that are related to the millimeter emission (adapted from Skokić et al. 2022).
Spatial counterparts of the millimeter flare emission
One flare was observed in ALMA band 6 (λ≈1.3 mm), while the rest were detected in band 3 (λ≈3 mm). The comparison of the ALMA images with SDO/HMI magnetograms and SDO/AIA and Hα filtergrams shows a surprising variety of associations with spatial features (Figure 1). In both ALMA bands 3 and 6, the ALMA flare emission occurs mainly above the neutral line observed in HMI magnetograms. However, this is not always the case. Millimeter emission may correspond to different features such as Hα active filaments, hot loop tops, footpoints, post-flare loops, and occasionally to no features in other wavelengths at all.
Intensity profiles of the millimeter flare emission
Significant millimeter peaks of up to 600 K above the background preflare level were found (Figure 2). Area-averaged intensity profiles correlate well with soft X-ray and 94/304 Å EUV channels, but not in all cases. One flare does not have a corresponding soft X-ray enhancement but does show up in the EUV channels. The intensity profiles suggest millimeter flares to be mostly gradual phase phenomena with occasional impulsive phase emission.
Figure 2. Intensity profiles of five flares observed by ALMA and compared with 94, 171 and 304 Å SDO/AIA EUV channels and GOES-15 XRSB fluxes. Vertical gray bands represent ALMA full disk scans of the Sun. Intensities shown are averages over the circular area around the flaring region (adapted from Skokić et al. 2022).
Most surprising is the fact that different phenomena in both hot and cold plasma lead to enhanced brightness in millimeter waves. Although there is no evidence contradicting the assumption that the emission process is thermal free-free emission, it seems that the previous scenario of a chromospheric hot spot heated by a precipitating electron beam is too simplistic. In addition to footpoints of hot loops, millimeter flare emission was found to be associated with activated Hα filaments, impact points of plasma motions, post-flare loops, and hot loop tops. Surprisingly, we found cases where no feature in Hα, 304 Å, 171 Å, and 94 Å was visible at the position of the millimeter wave emission peak.
Based on a recent paper by I. Skokić, A. O. Benz, R. Brajša, D. Sudar, F. Matković, M. Bárta, 2022, A&A, in press (https://doi.org/10.1051/0004-6361/202244532)
Krucker, S., Giménez de Castro, C. G., Hudson, H. S., et al. 2013, A&A Rev., 21, 58
Shimizu, T., Shimojo, M., & Abe, M. 2021, ApJ, 922, 113