Generation of Filaments by Beam-Plasma Interaction

Filaments have been observed near the bow shock and in other regions of Earth's magnetospheric plasmas. In this study, we carry out two dimensional hybrid simulations to investigate the structures and generation of filaments via interaction between the background plasma and an ion beam, whose velocity is parallel to the ambient magnetic field B₀. For the case with beam density n_b=0.1 and beam velocity V_{b=10V_A}, the simulation shows that right-hand nonresonant modes are dominant in the earlier linear stage, consistent with the linear theory prediction. In the nonlinear stage, the dynamics are, however, dominanted by two Alfven modes. While one Alfven wave mode propagates along B₀, the other is obliquely propagating. In the last stage of nonlinear wave evolution, filaments corresponding to field aligned structures of magnetic field B and density N are formed. The dominant filaments have anti-phase relation between B and N, and nearly zero phase speed. Shear Alfven waves, meanwhile, remain coexisting with the filaments. The detailed mechanism for filament formation and nonlinear wave evolution will be presented.