Radio observations of weak energy releases in the solar corona

Abstract

We report observations of weak, circularly polarized, structureless type III bursts from the solar corona in the absence of Hα/X-ray flares and other related activity, during the minimum between the sunspot cycles 23 and 24. The spectral information about the event obtained with the CALLISTO spectrograph at Mauritius revealed that the drift rate of the burst is ≈–30 MHz s–1 is in the range 50-120 MHz. Two-dimensional imaging observations of the burst at 77 MHz obtained with the Gauribidanur radioheliograph indicate that the emission region was located at a radial distance of ≈1.5 R sun in the solar atmosphere. The estimated peak brightness temperature of the burst at 77 MHz is ~108 K. We derived the average magnetic field at the aforementioned location of the burst using the one-dimensional (east-west) Gauribidanur radio polarimeter at 77 MHz, and the value is ≈2.5 ± 0.2 G. We also estimated the total energy of the non-thermal electrons responsible for the observed burst as ≈1.1 × 1024 erg. This is low compared to the energy of the weakest hard X-ray microflares reported in the literature, which is about ~1026 erg. The present result shows that non-thermal energy releases that correspond to the nanoflare category (energy ~1024 erg) are taking place in the solar corona, and the nature of such small-scale energy releases has not yet been explored.