Statistical properties of radio flux densities of solar flares

TL;DR

This study analyzes radio flux variations in solar flares across multiple frequencies, revealing that short timescale processes are universal and often precede soft X-ray peaks, with detailed statistical properties derived from 209 observed flares.

Contribution

It provides a comprehensive statistical analysis of radio flux densities and their short timescale variations in solar flares, highlighting the universality and timing of impulsive components across frequencies.

Findings

Impulsive radio components peak at different flux levels and durations depending on frequency.

Over 60% of light curves show flux variations on 1 second or shorter timescales.

Impulsive radio emissions often peak before soft X-ray fluxes, especially at higher frequencies.

Abstract

Short timescale flux variations are closely related to the energy release process of magnetic reconnection during solar flares. Radio light curves at 1, 2, 3.75, 9.4, and 17 GHz of 209 flares observed by the Nobeyama Radio Polarimeter from 2000 to 2010 are analyzed with a running smooth technique. We find that the impulsive component (with a variation timescale shorter than 1 second) of 1 GHz emission of most flares peaks at a few tens of solar flux unit and lasts for about 1 minute and the impulsive component of 2 GHz emission lasts a shorter period and peaks at a lower flux level, while at the three high frequency channels the occurrence frequency of flares increases with the decrease of the flux density up to the noise level of the corresponding background. The gradual components of these emissions, however, have similar duration and peak flux density distributions. We also derive the power spectrum on different timescales and a normalized wavelet analysis is used to confirm features on short timescales. At a time resolution of 0.1 second, more than $\sim$ 60$\%$ of these radio light curves show significant flux variation on 1 second or shorter time scales. This fraction increases with the decrease of frequency and reaches $\sim$ 100$\%$ at 1 GHz, implying that short timescale processes are universal in solar flares. We also study the correlation between the impulsive radio flux densities and soft X-ray fluxes obtained with the GOES satellites and find that more than 65$\%$ of the flares with an impulsive component have their impulsive radio emission reach a peak value ahead of the soft X-ray fluxes and this fraction increases with the radio frequency.