Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport

Abstract

Context. Solar type III radio bursts contain a wealth of information about the dynamics of electron beams in the solar corona and the inner heliosphere; currently unobtainable through other means. However, the motion of different regions of an electron beam (front, middle and back) have never been systematically analysed before. Aims. We characterise the type III burst frequency-time evolution using the enhanced resolution of LOFAR in the frequency range 30-70 MHz and use this to probe electron beam dynamics. Methods. The rise, peak and decay times with a ∼ 0.2 MHz spectral resolution were defined for a collection of 31 type III bursts. The frequency evolution is used to ascertain the apparent velocities of the front, middle and back of the type III sources and the trends are interpreted using theoretical and numerical treatments. Results. The type III time profile was better approximated by an asymmetric Gaussian profile, not an exponential as previously used. Rise and decay times increased with decreasing frequency and showed a strong correlation. Durations were smaller than previously observed. Drift rates from the rise times were faster than from the decay times, corresponding to inferred mean electron beam speeds for the front, middle and back of 0.2, 0.17, 0, 15 c, respectively. Faster beam speeds correlate with smaller type III durations. We also find type III frequency bandwidth decreases as frequency decreases. Conclusions. The different speeds naturally explain the elongation of an electron beam in space as it propagates through the heliosphere. The rate of expansion is proportional to the mean speed of the exciter; faster beams expand faster. Beam speeds are attributed to varying ensembles of electron energies at the front, middle and back of the beam.

On Solar Recurrent Coronal Jets: Coronal Geysers as Sources of Electron Beams and Interplanetary Type-III Radio Bursts

“…Their result is very close to our v/c = 0.16 estimation. Beam travel speeds in the order of v/c = 0.2 are typically reported (Meléndez et al 1999;Reid & Kontar 2018). We calculated a σ = 0.06 uncertainty in the beam propagation speed, though this may be skewed by the limited data cadence and event sample.…”

On the Efficiency of the Linear-mode Conversion for Generation of Solar Type III Radio Bursts

“…An important characteristic of the type III radio bursts is the fast frequency drift rate (Wild & McCready 1950). Type III bursts can start at a frequency of several hundreds of MHz and then go down to tens of kHz within a few minutes with the increasing duration at a given frequency (Alvarez 1973;Krupar et al 2018;Reid & Kontar 2018). To explain the frequency drift, the beams should have near relativistic speeds 0.1-0.5c (Melrose 1990), and generate radio emission near local plasma frequency, f pe , and double plasma frequency (harmonic; Ginzburg & Zhelezniakov 1958).…”

Forward Modeling of the Type III Radio Burst Exciter

“…The study of Type-III bursts can help us understand the acceleration process of energetic electrons during solar activity and electron transport in the solar atmosphere and interplanetary space (Reid and Kontar, 2018;Chen et al, 2013Chen et al, , 2018. According to the classic plasma-emission mechanism, the non-thermal electron beams first excite Langmuir waves, and then part of the Langmuir waves convert into electromagnetic waves at the fundamental and harmonic of the local plasma frequency through a non-linear process (Reid and Ratcliffe, 2014).…”