Effects of He⁺ and O⁺ ions on kinetic Alfvén waves: application to PSBL region

Abstract

Kinetic Alfvén waves (KAWs) are investigated considering existence of multi-ions (H⁺, He⁺ and O⁺) in plasma sheet boundary layer (PSBL) region. The dispersion relation and damping rate of wave are derived by kinetic approach. The loss-cone index (for $J=1$ and $J = 2$) and densities of multi-ions are varied to study the frequency and damping rate of wave over wide range of $k_{\bot} \rho_{\mathrm{H}^{+}}$ (where $k_{\bot}$ is perpendicular wave vector and $\rho_{\mathrm{H}^{+}}$ is Larmor radius of H⁺ ion). The presence of multi-ions in plasma is assumed for four cases: (a) H⁺ only, (b) H⁺ and He⁺, (c) H⁺ and O⁺, (d) H⁺, He⁺ and O⁺ ions. The results of the cases (b), (c) and (d) are compared with (a) to understand the effects of He⁺ and O⁺ ions on KAW. It is observed that the frequency of the wave lies in range 0.1–4 Hz for each case. He⁺ enhances wave frequency with increase in steepness of loss-cone indices. O⁺ is more effective in Maxwellian plasma resulting maximum frequency for $J=0$. Increasing densities of He⁺ and O⁺ result in reduction of wave frequency at $k_{\bot} \rho_{\mathrm{H}^{+}} <1$ and enhancement in frequency at higher $k_{\bot} \rho_{\mathrm{H}^{+}}$. Presence of He⁺ and O⁺ induce fluctuations in wave frequency. Reduction in damping rate due to He⁺ and O⁺ ions in loss-cone distribution signifies propagation of wave over long distances from PSBL towards auroral ionosphere. The parameters relevant to PSBL region are used in calculation of theoretical results. The results predict that the multi-ions possessing loss-cone distribution with varying densities significantly affect nature of KAW propagation.