Electron Cyclotron Maser Emissions from Evolving Fast Electron Beams

Abstract: Fast electron beams (FEBs) are common products of solar active phenomena. Solar radio bursts are an important diagnostic tool in the understanding of FEBs as well as the solar plasma environment in which they are propagating along solar magnetic fields. In particular, the evolutions of the energy spectrum and velocity distribution of FEBs due to the interaction with the ambient plasma and field when propagating can significantly influence the efficiency and property of their emissions. In this paper, we discuss some possible evolutions of the energy spectrum and velocity distribution of FEBs due to the energy loss processes and the pitch-angle effect caused by the magnetic field inhomogeneity, and analyze the effects of these evolutions on electron cyclotron maser (ECM) emission, which is one of the most important mechanisms of producing solar radio bursts by FEBs. The results show that the growth rates all decrease with the energy loss factor $Q$, but increase with the magnetic mirror ratio $\sigma$ as well as with the steepness index $\delta$. Moreover, the evolution of FEBs also can significantly influence the fastest growing mode and the fastest growing phase angle. This leads to the change of the polarization sense of ECM emission. In particular, our results also reveal that FEB that undergoes different evolution processes will generate different types of ECM emission. We believe the present results to be very helpful on more comprehensive understanding of dynamic spectra of solar radio bursts.