Observations of a Radio-quiet Solar Preflare

TL;DR

This study examines a unique solar preflare event, revealing no radio emission despite soft X-ray peaks, indicating different acceleration and heating processes compared to typical flares.

Contribution

It provides the first detailed observation of a radio-quiet preflare with multiple soft X-ray peaks, highlighting differences in energy processes from regular flares.

Findings

No radio emission detected at 17 GHz during preflare

The ratio of SXR peaks to radio upper limits is much higher than in regular flares

Lower efficiency of particle acceleration to relativistic energies in preflare phase

Abstract

The preflare phase of the flare SOL2011-08-09T03:52 is unique in its long duration, its coverage by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph, and the presence of three well-developed soft X-ray (SXR) peaks. No hard X-rays (HXR) are observed in the preflare phase. Here we report that also no associated radio emission at 17 GHz was found despite the higher sensitivity of the radio instrument. The ratio between the SXR peaks and the upper limit of the radio peaks is larger by more than one order of magnitude compared to regular flares. The result suggests that the ratio between acceleration and heating in the preflare phase was different than in regular flares. Acceleration to relativistic energies, if any, occurred with lower efficiency.