KINETIC THEORY OF EQUILIBRIUM AXISYMMETRIC COLLISIONLESS PLASMAS IN OFF-EQUATORIAL TORI AROUND COMPACT OBJECTS

Abstract

The possible occurrence of equilibrium off-equatorial tori in the\ngravitational and electromagnetic fields of astrophysical compact objects has\nbeen recently proved based on non-ideal MHD theory. These stationary structures\ncan represent plausible candidates for the modelling of coronal plasmas\nexpected to arise in association with accretion discs. However, accretion disc\ncoronae are formed by a highly diluted environment, and so the fluid\ndescription may be inappropriate. The question is posed of whether similar\noff-equatorial solutions can be determined also in the case of collisionless\nplasmas for which treatment based on kinetic theory, rather than fluid one, is\ndemanded. In this paper the issue is addressed in the framework of the\nVlasov-Maxwell description for non-relativistic multi-species axisymmetric\nplasmas subject to an external dominant spherical gravitational and dipolar\nmagnetic field. Equilibrium configurations are investigated and explicit\nsolutions for the species kinetic distribution function are constructed, which\nare expressed in terms of generalized Maxwellian functions characterized by\nisotropic temperature and non-uniform fluid fields. The conditions for the\nexistence of off-equatorial tori are investigated. It is proved that these\nlevitating systems are admitted under general conditions when both\ngravitational and magnetic fields contribute to shaping the spatial profiles of\nequilibrium plasma fluid fields. Then, specifically-kinetic effects carried by\nthe equilibrium solution are explicitly provided and identified here with\ndiamagnetic, energy-correction and electrostatic contributions. It is shown\nthat these kinetic terms characterize the plasma equation of state by\nintroducing non-vanishing deviations from the assumption of thermal pressure.\n