Drift‐compressional modes generated by inverted plasma distributions in the magnetosphere

Abstract

The polarization and field‐aligned structure of drift‐compressional modes and the corresponding plasma instability are studied in a gyrokinetic framework in the axisymmetric model of the magnetosphere with isotropic plasma. The plasma is assumed to be composed of core cold particles and an admixture of hot protons, with an inverted distribution of hot protons. Such plasma experiences a compressional resonance when the wave frequency is equal to an eigenfrequency of the drift‐compressional mode. In this resonance, the wave is dominated by the field‐aligned and azimuthal magnetic field components and is narrowly localized along the field line at the equator, the same as the plasma to magnetic pressure ratio β. The plasma instability occurs when the temperature diamagnetic drift velocity is less than the magnetic drift velocity or opposite in direction. Furthermore, the narrower the inverted distribution, the higher the instability growth rate and the smaller the value of β required for the instability to occur. The growth rate reaches its highest values when a positive radial temperature gradient and a negative radial density gradient occur simultaneously.