The possibility of supersonic plasma flow in a collapsing post-sunset ionosphere

Abstract

As a result of the rapidly decreasing pressure in the topside ionosphere during twilight hours, a rapid downward flow of hydrogen plasma from the protonosphere takes place. In the case of steady state, isothermal, frictionless flow the criterion for the existence of a critical point (transition to supersonic flow) above 1000 km is that the plasma temperature be lower than a certain limiting temperature Ti which is a function of the field line considered. In the latitude region between 40[deg] and 70[deg] this upper temperature limit varies from 963[deg]K to 1066[deg]K. Since these temperatures are considerably lower than the observed temperatures, it follows that in the case of steady state, isothermal flow the velocities will always remain subsonic. When the effect of the neglected terms is examined, the temperature gradient is shown to exert the strongest influence on the nature of the flow. For each latitude there is shown to exist a certain gradient ([part]T/[part]r)0 such that, if [part]T/[part]rT/[part]r)0 the criterion for a critical point to exist above 1000 km is again that the temperature at the critical point be less than some limiting temperature tl. If, however, [part]T/[part]r>([part]T/[part]r)O, then the criterion turns out to be that the temperature at the critical point be larger than Tl. The values of ([part]T/[part]r)0 are between 9 x 10-6[deg]K cm-1 and 6.75 x 10-6[deg]K cm-1 for latitudes between 40[deg] and 70[deg]. For values of the temperature gradient above about 4 x 10-6[deg]K cm-1 the criterion is satisfied for physically realistic temperatures (above 1500[deg]K), i.e. a critical point may exist above 1000 km. On this basis it is concluded that there is a definite possibility that supersonic downward flows in a post-sunset topside ionosphere may occur.