Local gyrokinetic study of electrostatic microinstabilities in dipole plasmas

Abstract

A linear gyrokinetic particle-in-cell scheme, which is valid for an arbitrary perpendicular wavelength k⊥ρi and includes the parallel dynamic along the field line, is developed to study the local electrostatic drift modes in point and ring dipole plasmas. We find that the most unstable mode in this system can be either the electron mode or the ion mode. The properties and relations of these modes are studied in detail as a function of k⊥ρi, the density gradient κn, the temperature gradient κT, electron to ion temperature ratio τ=Te/Ti, and mass ratio mi/me. For conventional weak gradient parameters, the mode is on the ground state (with eigenstate number l = 0) and especially k∥∼0 for small k⊥ρi. Thus, the bounce averaged dispersion relation is also derived for comparison. For strong gradient and large k⊥ρi, most interestingly, higher order eigenstate modes with even (e.g., l = 2, 4) or odd (e.g., l = 1) parity can be most unstable, which is not expected in the previous studies. High order eigenstate can also easily be most unstable at weak gradient when τ>10. This work can be particularly important to understand the turbulent transport in laboratory and space magnetosphere.