Temporally resolved Type III solar radio bursts in the frequency range 3-13 MHz

TL;DR

This study presents high-temporal resolution observations of Type III solar radio bursts in the 3-13 MHz range, revealing decay time characteristics and scattering effects previously unresolved due to limited temporal resolution.

Contribution

The paper introduces a novel high-temporal resolution measurement technique for Type III solar radio bursts, enabling accurate decay time analysis in the 3-13 MHz range for the first time.

Findings

Decay times follow a power-law with spectral index -0.75 in 3-13 MHz range.

Discrepancies in previous decay time measurements are due to insufficient temporal resolution.

Radio-wave scattering influences decay times across a broad frequency spectrum.

Abstract

Radio observations from space allow to characterize solar radio bursts below the ionospheric cutoff, which are otherwise inaccessible, but suffer from low, insufficient temporal resolution. In this Letter we present novel, high-temporal resolution observations of type III solar radio bursts in the range $3-13$ MHz. A dedicated configuration of the Radio and Plasma Waves (RPW) High Frequency Receiver (HFR) on the Solar Orbiter mission, allowing for a temporal resolution as high as $\sim0.07$s (up to 2 orders of magnitude better than any other spacecraft measurements), provides for the very first time resolved measurements of the typical decay time values in this frequency range. The comparison of data with different time resolutions and acquired at different radial distances indicates that discrepancies with decay time values provided in previous studies are only due to the insufficient time resolution not allowing to accurately characterize decay times in this frequency range. The statistical analysis on a large sample of $\sim500$ type III radio bursts shows a power low decay time trend with a spectral index of $-0.75\pm0.03$ when the median values for each frequency are considered. When these results are combined with previous observations, referring to frequencies outside the considered range, a spectral index of $-1.00\pm0.01$ is found in the range $\sim0.05-300$ MHz, compatible with the presence of radio-wave scattering between 1 and 100 R$_{Sun}$.