The propagation of low‐frequency whistler waves driven by ion beams in the magnetotail

Abstract

The existence in the Earth's magnetotail of magnetic field and plasma density inhomogeneities can influence the propagation of electromagnetic waves in the lobe and plasma sheet. This is important in understanding the origin of magnetic noise bursts that are observed near the neutral sheet since the waves may not necessarily be generated locally but can be excited in other regions of the tail and propagate towards the central plasma sheet due to the presence of the global inhomogeneities. In this paper the generation and propagation of low‐frequency whistler waves excited by ion beams in the plasma sheet boundary layer is examined. The trajectories of waves that are generated inside the plasma sheet boundary layer are followed using standard ray tracing techniques assuming a Harris‐type neutral sheet magnetic field profile and with the density chosen such that the total pressure remains constant. The wave energy is calculated along the wave path taking into account growth due to the ion beam while the wave is in the plasma sheet boundary layer, and damping due to resonant interactions with the background plasma. Results show that the electromagnetic waves are guided towards the central plasma sheet by the plasma gradients but are damped out before reaching the neutral plane itself due to Cerenkov resonance with the hot central plasma sheet background ions. The amplitudes and final positions of the waves depend on the temperature of the background plasma, the ion beam drift speed, and the ion beam thermal speed.