Radio, X-ray, and extreme-ultraviolet observations of weak energy releases in the "Quiet" sun

Abstract

We analyzed ground-based low frequency (<100 MHz) radio spectral and imaging data of the solar corona obtained with the facilities in the Gauribidanur observatory during the same time as the very weak soft X-ray flares (sub-A-class, flux <10−7Wm−2 in the 1–8 Å wavelength range) from the quiet Sun observed with the X-ray Solar Monitor (XSM) on board Chandrayaan-2 during the recent solar minimum. Nonthermal type I radio burst activity was noticed in close temporal association with the X-ray events. The estimated brightness temperature (Tb) of the bursts at a typical frequency like 80 MHz is ≈3 × 105 K. Extreme-ultraviolet (EUV) observations at 94 Å with the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) revealed a brightening close to the same location and time as the type I radio bursts. As far as we know reports of simultaneous observations of X-ray and/or EUV counterpart to weak transient radio emission at low frequencies from the quiet Sun in particular are rare. Considering this and the fact that low frequency radio observations are sensitive to weak energy releases in the solar atmosphere, the results indicate that coordinated observations of similar events would be useful to understand transient activities in the quiet Sun.