Coherent Wave Excitation by Energetic Ring-beam Electrons in Inhomogeneous Solar Corona

Coherent radio emission mechanism of solar radio bursts is one of the most complicated and controversial topics in the solar physics. To clarify mechanism of different types of solar radio bursts, (radio) wave excitation by energetic electrons in homogeneous plasmas has been widely studied via particle-in-cell (PIC) code numerical simulations. In this study, we, however, investigate effects of inhomogeneity in plasmas of the solar coronal on wave excitation by ring-beam distributed energetic electrons utilizing 2.5-dimensional PIC simulations. Disequilibrium introduced by the inhomogeneous magnetic field is balanced by either inhomogeneous density or inhomogeneous temperature of the background plasma. Both the beam and electron cyclotron maser (ECM) instabilities could be triggered with the presence of the energetic ring-beam electrons. Onset of the ECM instability is, however, later than the beam instability to excite waves in this study. The resultant spectrum of the excited electromagnetic waves presents a zebra-stripe pattern in the frequency space. The inhomogeneous density or temperature in plasmas would, however, influence the frequency bandwidth, excitation location of these excited waves. This study will, hence, help diagnose the plasma properties at the generation sites of solar radio bursts. Applications of this study to solar radio bursts (e.g., solar type V, zebra-pattern radio burst) will be discussed.