Interplanetary Type III Bursts and Electron Density Fluctuations in the Solar Wind

V Krupar,O Kruparova,M Maksimovic,E P Kontar,O Santolik,J Soucek,A Szabo,J P Eastwood,A Zaslavsky

doi:10.3847/1538-4357/aab60f

Abstract

Abstract Type III bursts are generated by fast electron beams originated from magnetic reconnection sites of solar flares. As propagation of radio waves in the interplanetary medium is strongly affected by random electron density fluctuations, type III bursts provide us with a unique diagnostic tool for solar wind remote plasma measurements. Here, we performed a statistical survey of 152 simple and isolated type III bursts observed by the twin-spacecraft Solar TErrestrial RElations Observatory mission. We investigated their time–frequency profiles in order to retrieve decay times as a function of frequency. Next, we performed Monte Carlo simulations to study the role of scattering due to random electron density fluctuations on time–frequency profiles of radio emissions generated in the interplanetary medium. For simplification, we assumed the presence of isotropic electron density fluctuations described by a power law with the Kolmogorov spectral index. Decay times obtained from observations and simulations were compared. We found that the characteristic exponential decay profile of type III bursts can be explained by the scattering of the fundamental component between the source and the observer despite restrictive assumptions included in the Monte Carlo simulation algorithm. Our results suggest that relative electron density fluctuations in the solar wind are 0.06–0.07 over wide range of heliospheric distances.

Highlights

Solar flares are associated with intense radio signals in a wide range of frequencies, in particular with fast-drifting type III bursts (Bastian et al 1998; Miteva et al 2017)
We present a statistical survey of type III burst decay times that can be used to estimate relative electron density fluctuations in the solar wind
Interplanetary type III bursts have been regularly measured for decades, we still do not have a proper model to explain their extremely large apparent source sizes, widespread visibility, nor to distinguish between fundamental and harmonic components

Summary

Introduction

Solar flares are associated with intense radio signals in a wide range of frequencies, in particular with fast-drifting type III bursts (Bastian et al 1998; Miteva et al 2017) They are produced by beams of suprathermal electrons accelerated at reconnection sites of solar flares traveling outward along open magnetic filed lines through the corona and the interplanetary medium (Wild 1950). Observed source sizes of interplanetary type III bursts are apparently so extended that they sometimes spread over the entire inner heliosphere (Krupar et al 2014a) These features are often attributed to scattering of radio beams by electron density inhomogeneities as they propagate from the source to the spacecraft (Steinberg et al 1984, 1985; Bastian 1994, 1995).

Observation and Analysis

Monte Carlo Simulations

Findings

Discussion and Summary