Low-frequency drift instabilities of the ring current belt

The stability of the ring current belt against low-frequency electrostatic perturbations is analyzed, including the effect of the finite electric field along the magnetic lines of force. We derive the conditions for two purely growing instabilities, the interchange and the drift mode. For an isothermal plasma in a dipole field, the critical density gradients for the interchange and the drift mode are d ln n/d ln r = −7.5 and −1.5, respectively. Due to the parallel electric field associated with the instabilities, their main consequence is to precipitate charged particles into the ionosphere, thereby relaxing the pressure gradient of the ring current.