Density variation effect on multi-ions with kinetic Alfven wave around cusp region—a kinetic approach

Abstract

The kinetic Alfven waves in the presence of homogeneous magnetic field plasma with multi-ions effect are investigated. The dispersion relation and normalised damping rate are derived for low-$\beta$ plasma using kinetic theory. The effect of density variation of $\text{H}^{+}$, $\text{He}^{+}$ and $\text{O}^{+}$ ions is observed on frequency and damping rate of the wave. The variation of frequency ($\omega$) and normalised damping rate ($\gamma / \varOmega_{H^{ +}}$) of the wave are studied with respect to $k_{ \bot} \rho_{j}$, where $k_{ \bot}$ is the perpendicular wave number, $\rho_{j}$ is the ion gyroradius and $j$ denotes $\text{H}^{+}$, $\text{He}^{+}$ and $\text{O}^{+}$ ions. The variation with $k_{ \bot} \rho_{j}$ is considered over wide range. The parameters appropriate to cusp region are used for the explanation of results. It is found that with hydrogen and helium ions gyration, the frequency of wave is influenced by the density variation of $\text{H}^{+}$ and $\text{He}^{+}$ ions but remains insensitive to the change in density of $\text{O}^{+}$ ions. For oxygen ion gyration, the frequency of wave varies over a short range only for $\text{O}^{+}$ ion density variation. The wave shows damping at lower altitude due to variation in density of lighter $\text{H}^{+}$ and $\text{He}^{+}$ ions whereas at higher altitude only heavy $\text{O}^{+}$ ions contribute in wave damping. The damping of wave may be due to landau damping or energy transfer from wave to particles. The present study signifies that the both lighter and heavier ions dominate differently to change the characteristics of kinetic Alfven wave and density variation is also an important parameter to understand wave phenomena in cusp region.