Abstract
Abstract A component of space weather, electron beams are routinely accelerated in the solar atmosphere and propagate through interplanetary space. Electron beams interact with Langmuir waves resulting in type III radio bursts. They expand along the trajectory and, using kinetic simulations, we explore the expansion as the electrons propagate away from the Sun. Specifically, we investigate the front, peak, and back of the electron beam in space from derived radio brightness temperatures of fundamental type III emission. The front of the electron beam travels at speeds from 0.2c to 0.7c, significantly faster than the back of the beam, which travels at speeds between 0.12c and 0.35c. The difference in speed between the front and the back elongates the electron beam in time. The rate of beam elongation has a 0.98 correlation coefficient with the peak velocity, in line with predictions from type III observations. The inferred speeds of electron beams initially increase close to the acceleration region and then decrease through the solar corona. Larger starting densities and harder initial spectral indices result in longer and faster type III sources. Faster electron beams have higher beam energy densities, and produce type IIIs with higher peak brightness temperatures and shorter FWHM durations. Higher background plasma temperatures also increase speed, particularly at the back of the beam. We show how our predictions of electron beam evolution influences type III bandwidth and drift rates. Our radial predictions of electron beam speed and expansion can be tested by the upcoming in situ electron beam measurements made by Solar Orbiter and Parker Solar Probe.
Highlights
Solar electron beams, accelerated via magnetic instabilities in the solar atmosphere, do not propagate scatter-free through the solar corona and interplanetary space
Electron beams and Langmuir waves are detected in situ together (Gurnett & Frank 1975; Gurnett & Anderson 1976), along with type III radio bursts which are generated via wave– wave interactions by the Langmuir waves
The above equation should only be used for small type III bursts that are similar to our simulations; type III bursts with very long durations of tens to hundreds of seconds at 50 MHz are likely caused by long-duration injection profiles or multiple electron beams, which may change how the radio brightness temperature relates to beam energy density
Summary
Solar electron beams, accelerated via magnetic instabilities in the solar atmosphere, do not propagate scatter-free through the solar corona and interplanetary space. Previous numerical studies (e.g., Reid & Kontar 2013; Li & Cairns 2014; Ratcliffe et al 2014) have shown that, over large distances of a few solar radii or more, the velocity range of electrons that generates the bulk of the Langmuir waves decreases. This is consistent with the decrease in type III drift rate as a function of frequency (Fainberg et al 1972; Krupar et al 2015).
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