The Plasmapause Formation
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Convection velocities in the equatorial region of the magnetosphere have a large dawn-dusk asymmetry as well as a significant day-night asymmetry. The latter is the consequence of (i) day-night asymmetry of the ionospheric integrated conductivities and (ii) enhanced solar wind induced convection velocity in the midnight local time sector. The observations indicate that the dynamics and the shape of the plasmasphere depend on geomagnetic activity as for instance measured by the index. At each enhancement ofa new plasmapause density gradient is formed closer to the Earth in the nightside local time sector; this new density gradient corotates subsequently into the dayside local sector. Experimental evidence of peeling off of the nightside plasmasphere and of subsequent flux tube refilling processes is presented. It is described how plasma interchange motion driven by the centrifugal force detaches plasma blocks from the gravitationally bound central part of the plasmasphere. At each new enhancement of the magnetospheric convection velocity in the midnight sector, the Zero Radial Force surface (i.e., where the radial component of the centrifugal force balances the gravitational force) penetrates deeper into the plasmasphere. The portion of plasmasphere beyond this critical surface is Rayleigh-Taylor unstable; this plasma shell is then detached with a velocity of interchange whose maximum value is determined by the value of the integrated Pedersen conductivity, The kinetic theory for plasma interchange motion in the gravitational field is presented. The limiting effect of the ionospheric integrated Pedersen conductivity on interchange velocity is discussed. Plasma interchange motion driven by grad-B and curvature drift for plasma of non-zero temperature and anisotropic pitch angle distribution is also considered as an additional factor destabilizing the outer plasmasphere. A review and critical analysis of the former "ideal MHD theory" for the formation of the plasmapause has been added.
CitationLemaire, J. (1987). The Plasmapause Formation. , Physica Scripta, Vol. 18, 111-118, DOI: 10.1088/0031-8949/1987/T18/012.