Effects of Superthermal Plasmas on the Linear Growth of Multiband EMIC Waves

Abstract

Abstract Observations show that particle velocity distributions in space plasmas generally exhibit a non-Maxwellian high-energy tail that can be well fitted with kappa distributions. To better understand the correlation between realistic particle velocity distributions and plasma wave excitation, we investigate the linear cyclotron instability of multiband electromagnetic ion cyclotron (EMIC) waves in a kappa plasma containing hot anisotropic protons, which provides the free energy for the wave growth. We find that the effects of superthermal plasmas on EMIC wave instability have a strong dependence on the emission band, temperature anisotropy A hp, and parallel beta β hp of hot protons. For H+ and He+ band EMIC waves, the maximum growth rates exhibit distinct behaviors with the variation of the spectral index κ of kappa distributions for different A hp values. The maximum growth rates decrease with increasing κ-value for low A hp and increase with increasing κ-value for high A hp. For O+ band waves, the effects of superthermal plasmas on the maximum growth rate are strongly controlled by β hp. For low β hp, the growth rate decreases monotonically with increasing κ-value for all A hp. For high β hp, increase of κ-value tends to enhance the wave growth for intermediate A hp and to suppress the wave growth otherwise. Our results also indicate that the presence of a high-energy tail tends to decrease the real frequency corresponding to the maximum growth rate for all three bands. While the minimum electron resonant energy for O+ band EMIC waves decreases as the κ-value increases, the minimum electron resonant energies for H+ and He+ band waves remain unaffected.