Early‐time plasma expansion characteristics of ionized clouds in the ionosphere

Abstract

The collisionless expansion of high‐density Ba+, Li+, and Ba+‐Li+ plasma clouds into low‐density O+ background plasmas was studied using a one‐dimensional Vlasov‐Poisson model in order to elucidate the very early‐time plasma expansion characteristics. Simulations were conducted for a range of Ba+‐Li+ mixtures, cloud/background density ratios, and electron/ion temperature ratios. Simulations were also conducted for cloud expansions against both stationary and flowing O+ background plasmas to simulate gas releases from moving spacecraft. The following results were obtained from the numerical simulations: (1) The front of an expanding high‐density Ba+ cloud acts as an “electrostatic snowplow,” and both O+ density and temperature peaks are pushed ahead of the expanding Ba+ cloud. (2) The strength of the electrostatic snowplow is increased for elevated cloud electron temperatures. (3) The effect of a flowing O+ background plasma is too slow the Ba+ expansion and change the O+ response. For small O+ drift velocities the Ba+ snowplow still occurs, for moderate O+ drift velocities ion density peaks propagate into and away from the cloud, and for large O+ drift velocities the O+ plasma quickly penetrates the Ba+ cloud and there are small density perturbations. (4) The Li+ cloud expansion is faster than the Ba+ expansion by approximately the square root of the heavy‐to‐light ion mass ratio, and the Li+ electrostatic snowplow is weaker. (5) As with a Ba+ cloud, an expanding Li+ cloud pushes an O+ density enhancement ahead of it, but some of the light Li+ ions can penetrate this O+ enhancement with the result that an Li+ plateau forms and propagates ahead of the propagating O+ enhancement. (6) For Li+ plasma expansions against a rapidly drifting O+ plasma, the two plasmas quickly penetrate each other with minor density perturbations. (7) For Ba+‐Li+ cloud expansions, with Li+ minor, into an O+ plasma, the Ba+‐O+ interactions are not affected by the presence of the light minor ion. This expansion scenario is led by suprathermal forerunner Li+ ions, then a propagating Li+ density plateau, then a propagating O+ density peak, and finally the main front of the expanding Ba+ cloud. A linear instability analysis including a constant magnetic field indicates that some of the expansion scenarios with elevated electron temperatures are unstable. When the plasma is unstable, the ion‐ion acoustic wave is the most unstable mode. Depending on the conditions, waves can be excited in the expanding plasma cloud by penetrating O+ ions and in the background O+ plasma by penetrating cloud ions. The numerical simulations are limited in that the simulation domain is relatively small, the initial plasma density gradients are fairly steep, the expansions are one‐dimensional, collisions are ignored, and only very early times are simulated.