Evolution of electron beam generated waves resulting in transverse ion heating and filamentation of the plasma

Abstract

We present here a systematic simulational study on electron beam driven waves and their consequences in terms of plasma electrodynamics. The study is performed by using three-dimensional particle-in-cell code, parallelized to simulate a large volume of plasma. Our simulation shows that an initial electron beam of finite radius with beam velocity along the ambient magnetic field triggers a series of events in the evolution of the waves and the plasma. In the initial stage (t ≤ 200 ωpo−1, ωpo being the electron plasma frequency with the total electron density n0), high frequency waves near ω ∼ ωpo are driven. These waves progressively disappear giving way to the dominance of lower hybrid (LH) waves. The phase of the lower hybrid waves lasts over the time interval inline image. In this time period the LH waves stochastically accelerate ions transverse to B0, and the beam electrons are scattered outside the initial beam volume to occupy the entire volume of the simulation plasma. The ion acceleration leads to the formation of elongated tail in the perpendicular velocity distribution. The large-amplitude LH waves are seen to undergo a parametric decay into resonance cone waves at frequencies ω < Ωi, the ion cyclotron frequency. Such extremely low-frequency (ELF) waves are the electrostatic version of the inertial Alfven waves. The phase of the strong LH waves is followed by a stage in which HF waves with frequencies ω ∼ ωpo appear again, but in this stage they are strongly modulated by the already present ELF waves and other low-frequency waves in the frequency range near the ion cyclotron frequency Ωi and its harmonics. Beginning with the LH wave stage and continuing into the late stages of ELF waves, the plasma density is highly filamented and the filaments oscillate with the ELF frequencies. The relevance of our results to the observations on plasma waves from satellites is brought out.