Plasma Dynamics in Comets I. Plasma Instability.

Abstract

Biermann has suggested that ionized gas clouds in comet tails are accelerated by collision with a stream of solar particles In Paper I we treat the field-free case, in order to establish conditions for which cometary plasma acceleration may be enhanced by plasma instability. We derive a general criterion for instability of plasmas of arbitrary ionic composition and velocity distribution. This result is applied to determine the stability of a number of different combinations of cometary gas densities and solar particle densities and temperatures Electron-ion instability is able to grow for a brief period if the solar electrons are cool enough. However, this is a transient effect. As soon as the electrons lose their translational velocity, their temperature becomes high enough to restore plasma stability, even for succeeding generations of inflowing electrons. The total momentum transfer is negligibly small compared with the observed momenta of cometarytail ions We conclude that plasma instability does not appreciably contribute to tail plasma acceleration. In Paper II we examine the effect of transverse magnetic fields carried by a solar stream. We find that many features of comets can be satisfactorily explained if magnetic fields are present