The L mode in electromagnetic proton-cyclotron waves in plasmas modelled by a Lorentzian distribution function

Abstract

The L mode in electromagnetic proton-cyclotron waves (EPCWs) propagating parallel to a uniform ambient magnetic field is studied here analytically. A generalized Lorentzian distribution function is used to model the plasma. Analytical expressions for the wavenumber and for both the temporal and convective growth rates for a multi-ion plasma are obtained within the linear theory. This analytical approach is appropiate for β∥<1, which is the ratio of plasma kinetic pressure to magnetic field pressure. The characteristics of the unstable spectrum are found to be independent of high-energy particles. For a plasma composed of electrons plus hot and cold protons, it is shown that the maximum growth rates as functions of cold-proton concentration δ can always decrease, or can increase until δ reaches a certain peak value and decrease thereafter, or can always increase, depending on the thermal anisotropy of the hot protons. This behaviour is similar to that in Maxwellian plasmas. However, for the convective growth rate, the expression for the optimum cold-proton concentration shows a significant dependence on the spectral index κ. Therefore, when cold protons are injected, it is more difficult to obtain optimum amplification in a Lorentzian plasma than in a Maxwellian plasma. It is also shown that the influence of the high-energy tail on the generation and amplification processes of the EPCWs is controlled by thermal anisotropy and cold-ion population. As a consequence of the latter, temporal and convective growth rates can be larger than, equal to or smaller than those of Maxwellian plasmas, depending on the anisotropy of the hot-proton distribution and on the cold-proton concentration.

The electromagnetic ion-cyclotron instability in bi-Kappa distributed plasmas

“…Studies are in general focused on the impact of suprathermal ion populations on the EMIC wave generation in magnetospheric plasma systems (Xue et al 1993(Xue et al , 1996aChaston et al 1997;Vega et al 1998;Xiao et al 2007;Mace et al 2011), even though the observational data are poor (Christon et al 1991) and not necessarily favorable to the existence of suprathermal ions in these environments. The growth rates Im(ω) ≡ γ (and their maximum values, γ max ≡ γ m ) exhibit two opposite behaviors with the increase of suprathermal population (i.e., decrease of the κ index), as they are either enhanced for plasmas with relatively low temperature anisotropies, or they are suppressed for higher anisotropies.…”

Effects of superthermal ring current ion tails on the electromagnetic ion cyclotron instability in multi-ion magnetospheric plasmas

“…To our knowledge, there have been only a few studies of EMIC wave generation in plasmas whose particle velocity distributions are modeled by kappa distributions [ Xue et al , 1996a, 1996b; Chaston et al , 1997; Vega et al , 1998; Xiao et al , 2007]. Most of these studies chose relatively high plasma parallel beta values, leading the authors to conclude that the presence of power law tails on the particle velocity distributions, as provided by a kappa model, was generally counteractive to the instability of thermal anisotropy driven EMIC waves, both for parallel propagation [ Xue et al , 1996a; Chaston et al , 1997] and for oblique propagation [ Xue et al , 1996b] with respect to the ambient magnetic field.…”

Effects of ion abundances on electromagnetic ion cyclotron wave growth rate in the vicinity of the plasmapause

“…[26][27][28][29][30] Recently Zhou et al 31 used THEMIS simultaneous measurements of fields and particle velocity distributions to model some EMIC wave events observed in the outer magnetosphere (L $ 6-7). They fitted the observed overall ion velocity distributions with (i) a sum of bi-Maxwellian distributions and (ii) a sum of bi-kappa distributions, fixing j ¼ 2 for each.…”