The field-aligned expansion of Ba+, Li+ and Ba+-Li+ plasma clouds in the upper F region was modeled with a macroscopic hydrodynamic formulation in order to study the early-time (t≲20 s) plasma expansion characteristics. Simulations were conducted for a range of Ba+-Li+ mixtures, cloud sizes, electron/ion temperatures, and ratios of cloud/background densities and flow velocities. The expansion scenarios were chosen to be similar to previous small-scale (Debye length), short-duration (plasma period) numerical simulations of collisionless plasma clouds. Since both the spatial and temporal scales differ by four orders of magnitude in the macroscopic and small-scale simulations, the comparison of results not only elucidates a broader domain of early-time plasma expansion characteristics and their extreme sensitivity to initial conditions but sheds light on the applicability of small-scale simulations to expanding plasma clouds in the ionosphere.

The macroscopic simulations led to the following results: (1) An expanding Ba+ cloud acts as an electrostatic snowplow, creating a hole in the ionosphere (factor of 10) as it pushes O+ density bumps (factor of 1.8) ahead of it along the geomagnetic field. (2) For the same cloud half width a decrease in the cloud density leads to a weaker snowplow. (3) An initially weaker, longer lasting snowplow ultimately produces a larger hole in the ionosphere than a short-lived strong snowplow. (4) Elevated electron temperatures act both to speed the plasma cloud expansion and to strengthen the electrostatic snowplow. (5) A bulk velocity component along the magnetic field resulting from a rocket- or satellite-borne chemical release has several important effects on the plasma cloud expansion and the ionosheric response. For a Ba+ cloud with a 2 km/s release velocity, a deep asymmetric hole is created in the ambient O+ and electron densities, and a large O+ and electron densities, and a large O+ bump is pushed ahead of the moving Ba+ cloud while a small O+ bump propagates away from the back of the cloud. For a 6 km/s release velocity the Ba+ ions create an initial ionospheric perturbation but then move rapidly through the O+ plasma, leaving behind a fossil O+ hole with little-to-no Ba+ ions. (6) A Li+ cloud expansion is qualitatively similar to a Ba+ expansion but it is faster. (7) Without Li+-O+ collisions the Li+ snowplow is weaker than the comparable Ba+ snowplow, but with collisions the reverse is true at early times. (8) The presence of a minor cloud ion does not affect the cloud-ionosphere interaction. (9) Light minor Li+ ions are significantly attected by the electric fields associated with the major ion density profiles (Ba+ and O+). These and other macroscopic expansion features are in general agreement with those obtained from the small-scale numerical simulations. ¿ American Geophysical Union 1991