Discrepancy between the Low-frequency Cutoffs of Type III Radio Bursts Based on Simultaneous Observations by WIND and PSP

Abstract

The cutoff frequency is an important characteristic parameter of type III radio bursts. Employing the radio data of the Parker Solar Probe (PSP) in the encounter phases of its first five orbits, our previous work revealed that the maximum probability distribution of the cutoff frequency f lo (∼680 kHz) is remarkably higher than that based on Ulysses and WIND (∼100 kHz) investigated by Leblanc et al. and Dulk et al. However, the main influencing factor of the discrepancy is still unknown though the possible reasons are discussed. In this study, we utilize the simultaneous observation by WIND and PSP to analyze statistically the distribution of the cutoff frequency of type III radio bursts, which had not been done before. Based on the automatic Canny edge detection and manual selection, we obtain the f lo of 491 (WIND) and 1194 (PSP) type III bursts from their simultaneous observations in the same solar activity period (from 2019 January 1 to 2020 July 31). The statistical results show that the dominant cutoff frequency measured by PSP (i.e., ∼700 kHz) is still significantly higher than that by WIND (i.e., ∼100 kHz). This implies that radiation attenuation is the main influencing factor for the difference in the statistical results of the cutoff frequency.