Stopping Frequency of Type III Solar Radio Bursts in Expanding Magnetic Flux Tubes

Abstract

Understanding the properties of type III radio bursts in the solar corona and interplanetary space is one of the best ways to remotely deduce the characteristics of solar accelerated electron beams and the solar wind plasma. One feature of all type III bursts is the lowest frequency they reach (or stopping frequency). This feature reflects the distance from the Sun that an electron beam can drive the observable plasma emission mechanism. The stopping frequency has never been systematically studied before from a theoretical perspective. Using numerical kinetic simulations, we explore the different parameters that dictate how far an electron beam can travel before it stops inducing a significant level of Langmuir waves, responsible for plasma radio emission. We use the quasilinear approach to model self-consistently the resonant interaction between electrons and Langmuir waves in inhomogeneous plasma, and take into consideration the expansion of the guiding magnetic flux tube and the turbulent density of the interplanetary medium. We find that the rate of radial expansion has a significant effect on the distance an electron beam travels before enhanced leves of Langmuir waves, and hence radio waves, cease. Radial expansion of the guiding magnetic flux tube rarefies the electron stream to the extent that the density of non-thermal electrons is too low to drive Langmuir wave production. The initial conditions of the electron beam have a significant effect, where decreasing the beam density or increasing the spectral index of injected electrons would cause higher type III stopping frequencies. We also demonstrate how the intensity of large-scale density fluctuations increases the highest frequency that Langmuir waves can be driven by the beam and how the magnetic field geometry can be the cause of type III bursts only observed at high coronal frequencies.

arXiv comment11 pages, 8 figures, accepted in Astronomy and Astrophysics

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• Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport
• Langmuir Wave Electric Fields Induced by Electron Beams in the Heliosphere
• Coronal type III radio bursts and their X-ray flare and interplanetary type III counterparts
• Low frequency radio observations of bi-directional electron beams in the solar corona
• Coronal mass ejections and type II radio emission variability during a magnetic cycle on the solar-type star $\epsilon$ Eridani
• On the Source Position and Duration of a Solar Type III Radio Burst Observed by LOFAR