Proton Cyclotron Instability Threshold Condition of Suprathermal Protons by Kappa Distribution

摘要

Observation has clearly shown that natural space plasmas generally possess a pro-nounced non-Maxwellian high-energy tail distribution that can be well modeled by a kappa distri-bution.In this study we investigate the proton cyclotron wave instability driven by the temper-ature anisotropy (T_⊥/T_∥>1) of suprathermal protons modeled with a typical kappa distributionin the magnetosheath.It is found that as in the case for a regular bi-Maxwellian,the suprather-mal proton temperature anisotropy is subject to the threshold condition of this proton cyclotroninstability and the instability threshold condition satisfies a general form T_⊥/T_∥-1=S/β_∥~α,with a very narrow range of the fitting parameters:0.40α0.45,and a relatively sensitivevariation 0.27S0.65,over 0.01≤β_∥≤10.Furthermore,the difference in threshold condi-tions between the kappa distribution and the bi-Maxwellian distribution is found to be small fora relatively strong growth but becomes relatively obvious for a weak wave growth.The resultsmay provide a deeper insight into the physics of this instability threshold for the proton cyclotronwaves.