Electromagnetic ion cyclotron waves instability threshold condition of suprathermal protons by kappa distribution

Abstract

[1] The well-known generalized Lorentzian (kappa) distribution generally provides a good representation for the high-energy tail population of natural cosmic suprathermal plasmas. In this study we examine the electromagnetic ion cyclotron waves (EMIC) instability driven by the temperature anisotropy condition (T ? /T k > 1) of suprathermal protons modeled with a typical kappa distribution in a cold multispecies plasma (electron, H + , He + , and O + ). Since the EMIC wave instability is found to be significant typically above the O + band, we apply a linear theory to study the instability threshold condition for the He + and H + bands particularly around the geostationary orbit. The instability threshold condition, as in the case for a regular bi-Maxwellian, is found to follow a typical form T ? /T k À 1 = S/b k a , with higher values in the He + band than those in the H + band in the case of the strong wave instability owing to a lower maximum wave growth in the He + band. As the spectral index k increases, the instability threshold condition generally decreases and tends to the lowest limiting values of the bi-Maxwellian, since the evaluation by the bi-Maxwellian generally overestimates the maximum wave growth. The densities of the cold components (particularly protons) have impacts on the threshold condition primarily in the H + band, with a higher density of cold protons leading to a lower value of the threshold condition. The results above may further reveal the nature of this instability threshold condition for the EMIC waves in any other space plasmas where an anisotropic suprathermal ion component and cold multicomponents are present together.

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

“…This apparently disagrees with the previous results of Xiao et al (2007), who found a uniform increase of the anisotropy thresholds (at γ m /Ω p = 10 −3 , and 10 −2 ) with the increase of suprathermal protons. …”

“…An immediate conclusion is that a decrease of the powerindex κ must lower the anisotropy thresholds corresponding to the lowest EMIC growth rates (marginal conditions of instability, γ m → 0). But this evolution appears to disagree with the results of Xiao et al (2007), which are restrained only to a particular (magnetopsheric) plasma model with a fractional composition of four (dominant) cold components (electrons, H + (protons), He + , O + ) and a minor hot population of protons (relative density n h /n 0 0.15) .…”

“…More investigations are needed, given the apparent disagreement between the new anisotropy thresholds found here and the results of Xiao et al (2007), which suggest that a dominant cold ion population will temperate variations of the threshold conditions, leading to their uniform increasing. To clarify this question, one can use the model developed here, progressively decreasing the hot (suprathermal) proton fractional composition, and increasing the cold (heavier) ions composition.…”

“…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.…”

Kinetic instabilities in the solar wind driven by temperature anisotropies

“…Other extensions include inverse correlations using non-Maxwellian models by Xiao et al (2006Xiao et al ( , 2007, Lazar (2012); empirical inverse correlations for electrons and high-frequency instabilities by Gary et al (2005) and Š tverák et al (2008); modification of the inverse correlation by including binary collisional effects by ; efforts to rigorously calculate the inverse correlation by Isenberg (Isenberg 2012;Isenberg et al 2013) who sought to obtain a rigorous asymptotic solution of the plasma subjected to linear instability condition; including the effects of streaming population on the stability condition by Hadi et al (2014), , and Vafin et al (2015); the mutual dynamical influence of electrons and ions, considered by Michno et al (2014), Maneva et al (2016), and Shaaban et al (2016Shaaban et al ( , 2017, etc. Fig.…”