Simulation of the formation of O+ density troughs at 6000 km altitude in the polar cap ionosphere‐magnetosphere region

Abstract

Spacecraft‐based in situ observations have indicated considerable variability in O+ densities in the polar cap ionosphere‐magnetosphere region near 6000 km altitude, from above 10 O+ cm−3 to lower than 0.01 O+ cm−3. Regions with densities in the low range have been referred to as O+ density troughs or ion depletion zones at these altitudes. In this paper, we use the UT Arlington Dynamic Fluid‐Kinetic (DyFK) ionospheric plasma transport model to simulate O+ density profiles under various conditions, in order to reproduce O+ densities which were observed by the Thermal Ion Dynamics Experiment (TIDE) on the Polar spacecraft on 20 August 1998 near Polar perigee. Using solar wind parameters and IMF conditions as inputs driving the time‐varying high‐latitude electric potential model and incorporating auroral processes involving the effects of soft electron precipitation and wave‐driven transverse ion heating, we simulate the ionosphere‐magnetosphere plasma transport and the associated O+ bulk parameter profiles within four flux tubes convecting in the high‐latitude region. The principal competing processes controlling the O+ density levels at these altitudes are the auroral processes, which elevate the O+ densities in the ∼6000 km altitude region, and the effects of F region O+‐electron recombination in darkness, which decrease the O+ densities at higher altitudes through draining to the reduced F region ionosphere. Depending chiefly upon the flux tube convection trajectories and their relationship to estimated auroral oval locations, both elevated and trough‐like densities are seen at ∼6000 km altitudes in the simulations, as observed by Polar/TIDE in the polar cap magnetosphere.