Local Gyrokinetic Study of Electrostatic Microinstabilities in Dipole Plasmas

A linear gyrokinetic particle-in-cell scheme, which is valid for arbitrary perpendicular wavelength $k_\perp\rho_i$ 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 the most unstable mode in this system can be either electron mode or ion mode. The properties and relations of these modes are studied in detail as a function of $k_\perp\rho_i$, the density gradient $\kappa_n$, the temperature gradient $\kappa_T$, electron to ion temperature ratio $\tau=T_e/T_i$, and mass ratio $m_i/m_e$. For conventional weak gradient parameters, the mode is on ground state (with eigenstate number $l=0$) and especially $k_\parallel\sim0$ for small $k_\perp\rho_i$. Thus, bounce averaged dispersion relation is also derived for comparison. For strong gradient and large $k_\perp\rho_i$, 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 by previous studies. High order eigenstate can also easily be most unstable at weak gradient when $\tau>10$. This work can be particularly important to understand the turbulent transport in laboratory and space magnetosphere.