A weak coronal heating event associated with periodic particle acceleration episodes

Abstract

Weak heating events are frequent and ubiquitous in solar corona. They derive their energy from the local magnetic field and form a major source of local heating, signatures of which are seen in EUV and X-ray bands. Their radio emission arise from various plasma instabilities that lead to coherent radiation, making even a weak flare appear very bright. The radio observations hence probe non-equilibrium dynamics providing complementary information on plasma evolution. However, a robust study of radio emission from a weak event among many simultaneous events, requires high dynamic range imaging at sub-second andsub-MHz resolutions owing to their high spectro-temporal variability. Such observations were not possible until recently.This is among the first spatially resolved studies of an active region loop hosting a transient brightening (ARTB) and dynamically linked to a metrewave type-I noise storm. It uses imaging observations at metrewave, EUV and X-ray bands, along with magnetogram data. We believe this is the first spectroscopic radio imaging study of a type-I source, the data for which was obtained using the Murchison Widefield Array. We report the discovery of 30 s quasi-periodic oscillations (QPOs) in the radio light curve, riding on a coherent baseline flux. The strength of the QPOs and the baseline flux enhanced during a mircoflare associated with the ARTB. Our observations suggest a scenario where magnetic stress builds up over an Alfv\'{e}n timescale (30s) across the typical magnetic field braiding scale and then dissipates via a cascade of weak reconnection events.