http://arxiv.org/abs/2005.08876
Type III radio bursts are radio emissions associated with solar flares. They are considered to be caused by electron beams in the solar corona. Magnetic reconnection is a possible accelerator of electron beams in the course of solar flares which causes unstable distribution functions, and density inhomogeneities. The properties of radio emission by electron beams in such environment are, however, still poorly understood. We capture the non-linear kinetic plasma processes of radio emissions in such plasmas by utilizing fully-kinetic Particle-In-Cell (PIC) code numerical simulations. Our model takes into account initial velocity distribution functions as they are supposed to be created by magnetic reconnection. These velocity distribution functions allow two distinct mechanisms of radio wave emissions: plasma emissions due to plasma-wave interactions and so called electron cyclotron maser emissions (ECME) due to wave-particle interactions. Our most important finding is that the number of harmonics of Langmuir waves increases with the density inhomogeneities. The harmonics are generated by the interaction of beam-generated Langmuir waves and their harmonics. In addition, we also find evidence for transverse harmonic electromagnetic wave emissions due to a coalescence of beam-generated and fundamental Langmuir waves with a vanishing wavevector. We investigate the effects of density inhomogeneities on the conversion process of the free energy of the electron beams to electrostatic and electromagnetic waves and the frequency shift of electron resonances caused by perpendicular gradients in the beam velocity distribution function. Our findings explain the observation of Langmuir waves and their harmonics in solar radio bursts and of the observed frequency shifts in these emissions.
X. Yao, P. Muñoz and J. Büchner
Tue, 19 May 20
73/92
Comments: 37 pages, 5 tables, 17 figures