Combining LOFAR radio observations and simulations to understand solar electron beam speeds and spatial expansion

Electron beams escaping the Sun can produce type III radio bursts via coherent plasma processes. Analysing type III bursts simultaneously provides parameters of electron beams and the solar corona and solar wind plasma they travel through. Studies routinely use type III bursts to estimate the bulk velocity of escaping electron beams. However, the motion of different regions of an electron beam (front, middle and back) have never been systematically analysed before. We present results that utilise high-resolution Low Frequency Array (LOFAR) radio observations and numerical simulations of escaping solar electron beam propagation through the solar corona. We show how type III frequency drift rates have rise times larger than decay times, driven by electron beam speeds being faster at the front of the beam and slower at the back. The difference in speed naturally elongates the beam in space. The energy density of electron beams strongly dictates their speed, expansion, and type III peak brightness temperatures. Higher background plasma temperatures also increase beam speeds, particularly at the back of the beam. Our radial predictions can be tested by the upcoming in situ measurements made by Solar Orbiter and Parker Solar Probe.