ELECTROMAGNETIC ION-CYCLOTRON INSTABILITY IN-SPACE PLASMAS

Natural space plasmas generally exhibit a pronounced high-energy tail distribution that can best be modeled by a generalized Lorentzian (kap pa) distribution. We employ the recently introduced modified plasma di spersion function [Summers and Thorne, 1991] to obtain the dispersion relation for field aligned electromagnetic waves in such a plasma, and use this to study the instability properties of R mode and L mode wav es in the solar wind and in planetary magnetospheres. We demonstrate f or a wide range of plasma parameters that the growth of R mode waves i n the solar wind can be significantly enhanced by the presence of a pr onounced high-energy tail; previous studies based on a Maxwellian dist ribution could therefore be seriously in error. The corresponding enha ncement in the growth rate of L mode waves in planetary magnetospheres is less dramatic, but the kappa distribution tends to produce signifi cant wave amplification over a broader range of frequency than a Maxwe llian distribution with comparable bulk properties At frequencies comp arable to the ion gyrofrequency wave growth is primarily caused by cyc lotron resonance with ions. Hot anisotropic electrons can nevertheless influence the instability as a result of changes in the wave phase ve locity. This modulating effect is most important for a Maxwellian plas ma and becomes less significant as the spectral index of the kappa dis tribution is reduced.