Several authors have shown that the presence of hot plasma in the magnetosphere can severely modify an imposed convection pattern and even prevent its penetration to low latitudes. Here with the aid of simple theoretical models we argue that the important parameters governing such effects are the hot plasma pressure, its number density, and the ionospheric conductivity. A result of this is that a fluid description of the phenomenon can be given. Changes in convection naturally give rise to east-west pressure gradients near the ring current inner edge, and these give rise to east-west flows, which cause the shielding effect. We show that plasma compressional energy is lost in Joule dissipation in the ionosphere in these flows which are rapidly set up on the nightside. On the dayside the process is slower, but we conclude that the dayside conductivity is the parameter controlling final ring current penetration. In the long time limit the fluid approximation breaks down, and we qualitatively discuss the development of steady state flow patterns and the effects of collisionless plasma behavior in this limit.
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