Plasmasphere formation in arbitrarily oriented magnetospheres

Abstract

We investigate the formation of plasmaspheres in planetary magnetospheres with arbitrary orientations of the rotation and magnetic dipole axes. A traditional plasmasphere with closed orbits inside the plasmapause and open trajectories outside it only occurs for the limiting case of aligned rotation and dipole axes. A time variable plasmapause exists if the rotation axis is perpendicular to the solar wind flow direction. In any other case no definite plasmapause exists. Solar wind driven convection transports plasma throughout the magnetosphere with an effectiveness which increases as the orientation becomes further from one of the two limiting cases of strict plasmapause formation. Our analysis is applied to Earth and Uranus using the actual orientations of the rotation and dipole axes. Particle trajectories at Earth deviate only slightly from those obtained with traditional models. Uranus has no plasmasphere, and plasma convects sunwards throughout the inner magnetosphere.