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.