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Understanding the properties of type III radio bursts in the solar corona andinterplanetary space is one of the best ways to remotely deduce thecharacteristics of solar accelerated electron beams and the solar wind plasma.One feature of all type III bursts is the lowest frequency they reach (orstopping frequency). This feature reflects the distance from the Sun that anelectron beam can drive the observable plasma emission mechanism. The stoppingfrequency has never been systematically studied before from a theoreticalperspective. Using numerical kinetic simulations, we explore the differentparameters that dictate how far an electron beam can travel before it stopsinducing a significant level of Langmuir waves, responsible for plasma radioemission. We use the quasilinear approach to model self-consistently theresonant interaction between electrons and Langmuir waves in inhomogeneousplasma, and take into consideration the expansion of the guiding magnetic fluxtube and the turbulent density of the interplanetary medium. We find that therate of radial expansion has a significant effect on the distance an electronbeam travels before enhanced leves of Langmuir waves, and hence radio waves,cease. Radial expansion of the guiding magnetic flux tube rarefies the electronstream to the extent that the density of non-thermal electrons is too low todrive Langmuir wave production. The initial conditions of the electron beamhave a significant effect, where decreasing the beam density or increasing thespectral index of injected electrons would cause higher type III stoppingfrequencies. We also demonstrate how the intensity of large-scale densityfluctuations increases the highest frequency that Langmuir waves can be drivenby the beam and how the magnetic field geometry can be the cause of type IIIbursts only observed at high coronal frequencies.

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