Electromagnetic Wave Emission from a Dynamical Current Sheet with Pinching and the Coalescence of Magnetic Islands in Solar Flare Plasmas

By using a two-dimensional, fully electromagnetic and relativistic particle-in-cell simulation, we investigate coherent emission mechanisms of electromagnetic waves from a dynamical current sheet, in which the pinching of the current sheet as well as the coalescence of the magnetic islands are taking place. In a formation phase of the current sheet, the electron pinching dominates, and subsequently the sheet becomes unstable against the tearing instability. In this pinching phase, emissions of the electron Bernstein mode propagating across the magnetic field from the Harris instability appear. The extraordinary mode (the X -mode) with a second-harmonic plasma frequency can also be excited from the electron Bernstein mode through mode coupling. On the other hand, we found that the electromagnetic waves obliquely propagating to the magnetic field and the ordinary mode (the O -mode) propagating across the magnetic field can be emitted during the coalescence of magnetic islands, which are formed in the nonlinear stage of the tearing instability. The generation mechanism of these waves may be due to the decay of the electromagnetic fields inside the O -type magnetic islands produced by the counterstreaming instability. The quasi-periodic radio bursts observed in the impulsive phase of solar flares could be explained by this coherent wave emission process.