Particle-in-cell and weak turbulence simulations of plasma emission

Abstract

The plasma emission process, which is the mechanism for solar type II and type III radio burst phenomena, is studied by means of particle-in-cell (PIC) and weak turbulence (WT) simulation methods. “Plasma emission” is meant as a loose description of a series of processes, starting from the solar flare-associated electron beam exciting Langmuir and ion-acoustic turbulence, and subsequent partial conversion of beam energy into radiation energy by nonlinear processes. PIC simulation is rigorous but the method is computationally intense, and it is difficult to diagnose the results. The numerical solution of equations of WT theory, known as WT simulation, on the other hand, is efficient and naturally lends itself to diagnostics because various terms in the equation can be turned on or off. Nevertheless, WT theory is based upon a number of assumptions. It is therefore desirable to compare the two methods, which we do for the first time with numerical solutions of the complete set of equations of WT theory and a two-dimensional electromagnetic PIC simulation. Upon making quantitative comparisons, it is found that WT theory is largely valid, although some discrepancies are also found. The present study also indicates that large computational resources are required in order to accurately simulate the radiation emission processes, especially for low electron beam speeds, such that it may be more advantageous to employ the WT method in order to describe the radiation emission itself. Findings from the present paper thus imply that both methods may be useful for the study of solar radio emissions, as they are complementary.