Plasma Emission Induced By Ring-Beam Electrons In Plasmas With A Large Ratio Of Plasma-Electron-Gyro Frequencies

During solar eruptions great amounts of energetic electrons are produced,gen-erating enhanced electromagnetic radiations at centimeteric to kilometeric radio wavelengths,which are called as solar radio bursts.Observations of radio dynam-ic spectra and source imaging of solar radio bursts can be used to understand the radiation mechanism,to study the nature of solar eruptions,to diagnose the prop-erties of coronal plasmas,in order to understand the storage and release process of magnetic field energy in solar atmosphere.In a great number of solar radio bursts,the observed brightness temperature is much higher than the effective temperature of the radiating electrons.Such emissions cannot be produced by the classical incoherent radiation mechanism due to individual particles.Thus,plasma coherent emission mechanisms have been proposed,of which there are mainly two types,namely,electron cyclotron maser emission and plasma emis-sion.In a latest study,Ni et al.(2020)investigated the electron cyclotron resonance(maser)instability(ECMI)-plasma emission driven by energetic electrons with Dory-Guest-Harris(DGH)distribution using particle-in-cell(PIC)simulation.It was found that in plasmas with a large ratio of plasma frequency to gyrofrequency(?pe/?ce=10),DGH distribution characterized by(?)f/(?)v?>0 can excite waves in upper-hybrid(UH),Z,and whistle(W)modes,while enhanced fundamental(F)and harmonic(H)plasma emissions can also be observed in the simulation results.The excitation of Z,UH and W modes can be explained by wave-particle resonance(ECMI)as described by linear kinetic theory,which cannot account for the obtained F and H emissions.According to the PIC simulation results,Ni et al.(2020)proposed that the F emission is generated by nonlinear coalescence of the almost counterpropagating Z and W modes,and the H emission arises from coalescence of the almost counterpropagating UH mode.Note that the F and H plasma emissions obtained in the simulation are only 2 orders of magnitude above the corresponding noise level,and the emission efficiency is relatively low.To obtain a more efficient amplification of F and H plasma emissions,we use the same simulation method with similar parameters(?pe/?ce=10),but employ energetic electrons with different velocity distribution functions,including ring,ring beam and beam distribution,to investigate the amplification of the emissions and waves in corresponding modes with a lower density ratio(ne/n0=0.01,where n,e is the density of energetic electrons and n0 is the number density of total electrons)through PIC simulation.According to our results,in the case with electrons of ring distribution,similar results with Ni et al.(2020)are obtained,while UH is the strongest mode ampli-fied in the simulation,with weak excitation of W and Z modes;F and H plasma emissions are also obtained at ?pe and 2?pe.Among them,the intensity of H emission is similar to that of the F emission;the ratio of the energy of F emission to the total energy of elergetic electrons is?10-8,and the energy conversion rate of H emission is?10-7.In addition,the frequency of the obtained F emis-sion can reach up to?10.2?ce,and the corresponding group velocity can reach?0.26c.The frequency of the F emission obtained by Ni et al.(2020),together with all the simulations and theoretical studies based on the traditional version of plasma emission driven by beam electrons,are very close to ?pe,with the group velocity being almost zero,corresponding to the non-escaping quasi-electrostatic O mode.Such radiating photons can be easily influenced by the effects of re-fraction,scattering and reflection of surrounding plasmas oscillating at similar frequency,which can hardly propagate and escape from the source.Compared with that,the F emission obtained in the present work is easier to escape from the source area.In the case with beam distribution,a strong beam-UH mode is observed,which only appear in the quasi-parallel direction.Similar to the results of the case with ring distribution,UH mode is the strongest one in the simulation,while the amplification of W and Z modes are weaker;the F emission?10.2?ce is also obtained in this case.Among them,the intensity of F emission is higher than the H emission by 1 order of magnitude.In addition,the F emission intensity is basically the same as the Z mode,which is significantly higher than the intensity of F emissions that obtained in the case with ring distribution,and that obtained by Ni et al.(2020).In the case with ring beam distributions with a small pitch angle(?p=30°),where ?p is the angle between the beam velocity and the magnetic field),the temporal profiles of the electromagnetic fields obtained are very similar to the case with beam distribution.The ranges of frequency and wave number of the modes are also basically the same.As for the case of ring beam distribution with a large pitch angle(?p60°),the temporal evolutions of the electromagnetic field energies are very close to those obtained in the case with ring distribution.With different types of velocity distribution functions of energetic electrons employed in PIC simulation,though the amplified modes are the same,including UH,Z and W modes,the properties of the wave modes,including propagation range,mode intensity,frequency range,wave number range,ete.,are different with each other.As the pitch angle increases,the mode intensity of UH gradu-ally declines,and the propagation directions change from quasi-parallel direction(?=0°)to perpendicular and oblique directions(?p=90°);the intensity of W mode gradually increases,and the propagation direction changes from par-allel to anti-parallel;the Z-mode intensity presented in the dispersion diagram of Ez gradually decreases,and the propagation direction is basically unchanged.Among the cases with several different distributions,the F emission generated by ring distribution is the weakest,with the propagation direction unchanged.The intensity of F emission shown in the present thesis is very strong with a higher frequency,which is one to two orders of magnitude stronger than the obtained H emissions.Taking into account the low density ratio of energetic electrons and background electrons studied in the present thesis,the excitation efficiency of the obtained emissions is significantly h:igher than the corresponding value obtained by Ni et al.(2020).