Self‐induced motions of thermal plasma in the magnetosphere and the stability of the plasmapause

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Abstract
Significant small‐scale convective motions can be generated by the presence of thermal plasma in the magnetosphere. An isolated plasma‐filled flux tube at L = 4, of density 3 ×108 m−3 and temperature 3000 K, can convect away from the earth at a velocity on the order of 0.7 RE hr−1 at night, due to the electric field it creates. An interchange instability occurs at the plasmapause, with a growth rate on the order of 1.9 × 10−3 sec−1 at night, creating turbulence which tends to destroy the steep plasma density gradient. This instability can explain the gradual decrease of the density gradient observed during periods of decreasing magnetic activity. In order to maintain a steep density gradient, three possibilities are discussed: (a) reduction of the instability growth rate due to enhanced ionospheric conductance; (b) reduction of the growth rate due to the presence of radiation belt plasma and/or a sharp temperature increase at the plasmapause; and (c) rapid radial convergence of large‐scale flow due to the presence of an Alfvén layer at the plasmapause. The self‐induced convective motions may also be important in the production and/or redistribution of small‐scale irregularities of thermal plasma observed both inside and outside the plasmapause.