Harmonic Maser Emission Excited By Energetic Electrons Traveling Along A Coronal Loop

Efficient radiation at second and/or higher harmonics of Ωce has been suggested to circumvent the escaping difficulty of the electron cyclotron maser emission(ECME)mechanism when it is applied to solar radio bursts,such as spikes.A complete understanding of solar radio bursts requires developing numerical techniques that 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 the magnetic field overlying a specific active region to derive the background field,(2)guiding-center simulation of the 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)particlein-cell(PIC)simulation of kinetic instabilities driven by energetic electrons initiated by the obtained distributions.It was found that the obtained VDFs contain strip-like and loss-cone features with positive gradients.These features are capable of driving 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.Chapter 1 introduces the relevant information about the solar corona and coronal plasma activities along with some observation.Chapter 2 presents an overview of relevant plasma wave theory and radio emission mechanisms.Chapter 3 investigate the emission properties by comparing different time scales of scattering at the loop-top section.Scattering effects at various levels(weak,moderate,and strong)due to wave turbulenceparticle interaction are considered using prescribed time scales of scattering.In the weakscattering case,the rate of energy conversion from energetic electrons to X2 can reach up to～2.9 ×10-3Ek0 where Ek0 is the initial kinetic energy of energetic electrons.A more substantial scattering effect disturbs the formation of multi-layer VDFs.In this chapter we merely considered the radiation from the loop top at a specific time.Chapter 4 presents the emission properties along the loop at different locations and timings considering the scattering effect.We found that few to several strip-like features can appear in all cases,and the first two strips play the major role in excitation of second harmonic X-mode(X2 for short)and Z mode(i.e.,the slow extraordinary mode)that propagate quasi-perpendicularly.The upper two sections close to the loop-top show the strongest emission of X2 compared to other parts along the loop and significant excitation of Z is observed for nearly all loop sections.The study demonstrates a novel way of exciting the X2 mode along the coronal loop during solar flares and provides new sight into how escaping radiation can be generated within a coronal loop.