Detection of weak ubiquitous impulsive nonthermal emissions from the solar corona

TL;DR

This paper reports the detection of extremely weak impulsive radio emissions from the solar corona, supporting the nanoflare hypothesis for coronal heating, using a robust separation technique on Murchison Widefield Array data.

Contribution

It introduces an independent method to detect weak impulsive solar radio emissions and characterizes their properties, providing evidence for nanoflare-related phenomena.

Findings

Detected milli-SFU level bursts across the Sun

Estimated nonthermal particle energies in the sub-pico flare range

Discovered the weakest type III radio burst and quasi-periodic pulsations

Abstract

A ubiquitous presence of weak energy releases is one of the most promising hypotheses to explain coronal heating, referred to as the nanoflare hypothesis. The accelerated electrons associated with such weak heating events are also expected to give rise to coherent impulsive emission via plasma instabilities in the meterwave radio band, making this a promising spectral window to look for their presence. Recently \citet{Mondal2020b} reported the presence of weak impulsive emissions from quiet Sun regions which seem to meet the requirements of being radio counterparts of the hypothesized nanoflares. Detection of such low-contrast weak emission from the quiet Sun is challenging and, given their implications, it is important to confirm their presence. In this work, using data from the Murchison Widefield Array, we explore the use of an independent robust approach for their detection by separating the dominant slowly varying component of emission from the weak impulsive one in the visibility domain. We detect milli-SFU level bursts taking place all over the Sun and characterize their brightness temperatures, distributions, morphologies, durations and association with features seen in EUV images. We also attempt to constraint the energies of the nonthermal particles using inputs from the FORWARD coronal model along with some reasonable assumptions and find them to lie in the sub-pico flare ($\sim 10^{19}-10^{21}$ ergs) range. In the process, we also discover perhaps the weakest type III radio burst and another one that shows clear signatures of weakest quasi-periodic pulsations.