Harmonic ECME Excited by Energetic Electrons Travelling Inside A Coronal Loop

A complete understanding of solar radio bursts requires developing numerical techniques which can connect large-scale activities with kinetic plasma processes. As a starting point, this study presents a numerical scheme combining three different techniques: (1) extrapolation of magnetic field overlying a specific active region in order to derive the background field, (2) guiding-center simulation of dynamics of millions of particles within a selected loop to reveal the integral velocity distribution function (VDF) around certain sections of the loop, and (3) particle-in-cell (PIC) simulation of kinetic instabilities driven by energetic electrons initiated by the obtained distributions. Scattering effects at various levels (weak, moderate, and strong) due to wave/turbulence-particle interaction are considered using prescribed time scales of scattering. It was found that the obtained VDFs contain strip-like and loss-cone features with positive gradient, and both features are capable of driving electron cyclotron maser emission (ECME), which is a viable radiation mechanism for some solar radio bursts, in particular, solar radio spikes. The strip-like feature is important in driving the harmonic X mode, while the loss-cone feature can be important in driving the fundamental X mode. In the weak-scattering case, the rate of energy conversion from energetic electrons to X2 can reach up to ~2.9 * 10^-3 Ek0, where Ek0 is the initial kinetic energy of energetic electrons. The study demonstrates a novel way of exciting X2 mode in the corona during solar flares, and provides new sight into how escaping radiation can be generated within a coronal loop during solar flares.

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