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Dynamics of the low-latitude thermosphere: Quiet and disturbed conditions

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Journal of Atmospheric and Solar-Terrestrial Physics





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Low-latitude dynamics, electrodynamics, and plasma density structure are closely linked. Dynamically driven electric fields initiate the equatorial ionization anomaly. Between the latitudes of the anomaly crests, steep gradients in ion density span more than three orders of magnitude. Zonal winds accelerate in response to the severe deficit of plasma, and reduced ion drag, at the dip equator. Zonal winds give rise to a vertical polarization field, causing plasma to drift with the neutrals and further diminish ion drag. Signatures of neutral temperature are associated with the winds; cooling appears in the zonal jet itself and there is slight warming on either side. Chemical heating is suggested as the mechanism responsible for the temperature feature, but this has yet to be confirmed. During geomagnetic disturbances, large-scale waves propagate efficiently from the remote high-latitude source region. The strength of the waves and the circulation changes depend on local time; the strongest and most penetrating waves arise on the nightside, where they are hindered least by drag from the low ion densities. The rapid arrival of waves to low latitudes may be the cause of the electrodynamic drift that has been observed to follow a rise of geomagnetic activity within four hours. Winds at low latitudes respond to sources from both polar regions. The changes are manifest by the arrival and interaction of a series of waves from high latitudes that propagate well into the opposite hemisphere. Lower altitudes, below the F-region, respond more slowly because propagation speeds are limited in the cooler, dense lower thermosphere. Finally, during solstice, bulges enriched in molecular nitrogen migrate, over a period of a day or so, from their high latitude source to low latitudes. Characteristic negative phases can result, depleting the ionosphere and further feeding electrodynamic change. The timing of low-latitude electrodynamic signatures in response to geomagnetic disturbances is, at least in part, closely connected to global dynamical time scales. Numerical models are used to illustrate the response of the upper atmosphere during quiet and magnetically disturbed conditions, and are used to elucidate the important physical processes.