Temporally Resolved Type III Solar Radio Bursts in the Frequency Range 3–13 MHz

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 ∼0.07 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 ∼500 type III radio bursts shows a power low decay time trend with a spectral index of −0.75 ± 0.03 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 ± 0.01 is found in the range ∼0.05–300 MHz, compatible with the presence of radio-wave scattering between 1 and 100 R ☉.