Response of the Neutral Thermosphere at F-Layer Heights to Interaction of a Global Wind with Anomalies of Ionization

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Abstract
Departures from a mean global-scale ionization distribution are commonly found in the ionospheric F region. Global-scale winds experience an acceleration where the ion drag is locally less than its global-scale smooth value and, they likewise, experience a deceleration where the ion drag is locally greater. Thus, a perturbation in the horizontal flow is set up in response to this ion-drag momentum source. A two-dimensional, steady-state dynamic model of the neutral thermosphere, incorporating thermal conduction, viscosity and ion drag, is used to calculate the temperature perturbation and circulation pattern caused by these ion-drag anomalies. The forcing is given by a momentum source which depends on the interaction of a basic-state neutral wind with the anomaly. For horizontal-scale anomalies of a few hundred kilometers, such as the electron density depression within the stable auroral red arc, the momentum source due to perturbation ion drag is almost completely balanced by a perturbation pressure force. The perturbation temperature and circulation responses are, therefore, negligibly small. For horizontal-scale anomalies of the order of a few thousand kilometers, such as the day-night electron density variation at sunset, the force exerted by the perturbation pressure is not able to cancel the addition of momentum by the ion-drag anomaly. Thus, such a momentum source produces a significant perturbation in the horizontal velocity, vertical motion, and temperature field.

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