Plasma expansion characteristics of ionized clouds in the ionosphere: Macroscopic formulation

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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 ionospheric 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+ 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 affected 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.