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

Abstract

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∥ > 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∥ − 1 = S/β∥α, 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 κ 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.