On the speed and acceleration of electron beams triggering interplanetary type III radio bursts

TL;DR

This study analyzes 29 interplanetary type III radio bursts to determine the speeds and deceleration of electron beams, revealing significant deceleration and providing data for improved plasma interaction models.

Contribution

It offers the first statistical analysis of electron beam speeds and accelerations in interplanetary space based on STEREO observations, highlighting their deceleration.

Findings

Electron beams decelerate noticeably in interplanetary space.

Speeds range from ~0.02c to ~0.35c, corresponding to energies from tens of eV to hundreds of keV.

Type III burst emission near the peak likely occurs at double plasma frequency.

Abstract

Type III radio bursts are intense radio emissions triggered by beams of energetic electrons often associated with solar flares. These exciter beams propagate outwards from the Sun along an open magnetic field line in the corona and in the interplanetary (IP) medium. We performed a statistical survey of 29 simple and isolated IP type III bursts observed by STEREO/Waves instruments between January 2013 and September 2014. We investigated their time-frequency profiles in order to derive the speed and acceleration of exciter electron beams. We show these beams noticeably decelerate in the IP medium. Obtained speeds range from $\sim$ 0.02c up to $\sim$ 0.35c depending on initial assumptions. It corresponds to electron energies between tens of eV and hundreds of keV, and in order to explain the characteristic energies or speeds of type III electrons ($\sim 0.1$c) observed simultaneously with Langmuir waves at 1 au, the emission of type III bursts near the peak should be predominately at double plasma frequency. Derived properties of electron beams can be used as input parameters for computer simulations of interactions between the beam and the plasma in the IP medium.