Solar Flare: What the Radio Telescope Saw express
A solar flare is like an explosion in a star's atmosphere: in minutes, it releases energy comparable to billions of nuclear bombs. On May 4, 2022, the RT-7.5 radio telescope in Moscow was watching an M5.7-class flare. The instrument was catching millimeter radio waves: those shorter than a centimeter and invisible to the eye.
After the flare's main pulse, when brightness had already begun to fade, the telescope spotted a new rise in emission at 93 GHz. At the same moment, satellites saw a brightening of ultraviolet glow from the corona — the Sun's outer atmosphere. Such a coincidence suggests that the millimeter waves are born in plasma heated to about a million degrees.
To figure out how much matter is emitting, scientists built a "temperature map" of the plasma using data from the SDO and GOES satellites. It turned out that the tenuous hot corona, the one visible during eclipses, accounts for a fifth of the millimeter signal. The rest likely comes from denser, cooler layers.
This breakdown helps us understand how flare energy flows from magnetic fields into heating the plasma. After all, millimeter waves pass through dense layers where ordinary light gets stuck, so we get a kind of "X-ray" of the solar atmosphere.
Original abstract
as written by the authors arXiv:2602.03552
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.
The RT-7.5 radio telescope is an antenna with a diameter of 7.5 meters, mounted on the roof of an academic building at Bauman Moscow State Technical University in Moscow. It picks up signals from the Sun even in cloudy weather, because radio waves pass through clouds.