The collisionless expansion of high-density Ba+, Li+, Ba+-Li+ plasma clouds into low-density O+ background plasmas was studied using a one-diemnsional 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 temperature. (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 velocites 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 quare root of the heavy-to-light ion mass ratio, and the Li+ electrostatic snowplow is weaker. (5) As with a Ba+ cloud, and 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 results 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 propagting 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, collisons are ignored, and only very early times are simulated. ¿ American Geophysical Union 1988