Supersonic neutral winds and neutral streams in the thermosphere-ionosphere-plasmasphere system

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Abstract

[1] Model studies were conducted to determine the conditions that might give rise to supersonic neutral winds in the high-latitude thermosphere and that can produce neutral streams in the polar wind and plasmasphere. For the supersonic neutral wind study, we used a time-dependent, three-dimensional, global thermosphere-ionosphere model. Simulations were conducted for different seasonal conditions, solar activity levels, cross-polar-cap potentials, and convection pattern shapes. Supersonic neutral winds were found to occur at altitudes above 200 km for cross-polar-cap potentials greater than about 150 kV, although most of the supersonic winds occurred between 300 and 600 km. The Mach number of the neutral winds was typically less than 1.5, and the supersonic winds were generated within 3 h after enhanced plasma convection. These winds were primarily located in the midnight-dawn sector of the polar cap, and their dusk-to-dawn extent was of the order of 150 km. Also, during enhanced convection, the ion outflow in the polar wind increases because of the elevated plasma temperatures, and this leads to enhanced ion-neutral charge exchange reactions, and hence, to the production of neutral streams in the polar wind (Neutral Polar Wind). Likewise, on closed magnetic field lines, the upflowing ionospheric ions can charge exchange with the background neutrals, and this produces neutral streams in the plasmasphere (Neutral Plasmasphere Wind). Coupled models of the ionosphere-polar wind-exosphere and ionosphere-plasmasphere-exosphere were used to elucidate the basic characteristics of these neutral streams. An important result is that the neutral O streams typically do not have sufficient energy to escape in both the polar wind and plasmasphere, and they rain down on the global thermosphere (Neutral Rain). The downward streaming neutral O atoms pass through the exobase at 500–600 km and provide a source of momentum and energy for the thermosphere. The results presented here correspond to a focused modeling study that was presented at the Ionospheric Effects Symposium (May 2008).

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