Solar Radio Bursts with Spectral Fine Structures in Preflares

TL;DR

This study analyzes microwave spectral fine structures observed before solar flares, revealing short-timescale bursts and associated phenomena that could improve understanding and prediction of flare onset.

Contribution

It presents detailed characteristics of preflare microwave spectral fine structures, including quasi-periodic pulsations and dot bursts, observed in the 0.8-2.0 GHz range.

Findings

Preflare microwave bursts occur 1-4 minutes before flares.

Spectral fine structures include short-period QPP and millisecond dot bursts.

Associated EUV and X-ray activities suggest independent energy release processes.

Abstract

A good observation of preflare activities is important for us to understand the origin and triggering mechanism of solar flares, and to predict the occurrence of solar flares. This work presents the characteristics of microwave spectral fine structures as preflare activities of four solar flares observed by Ond\v{r}ejov radio spectrograph in the frequency range of 0.8--2.0 GHz. We found that these microwave bursts which occurred 1--4 minutes before the onset of flares have spectral fine structures with relatively weak intensities and very short timescales. They include microwave quasi-periodic pulsations (QPP) with very short period of 0.1-0.3 s and dot bursts with millisecond timescales and narrow frequency bandwidths. Accompanying these microwave bursts, there are filament motions, plasma ejection or loop brightening on the EUV imaging observations and non-thermal hard X-ray emission enhancements observed by RHESSI. These facts may reveal certain independent non-thermal energy releasing processes and particle acceleration before the onset of solar flares. They may be conducive to understand the nature of solar flares and predict their occurrence.