DYNAMICS OF THE LOW-LATITUDE THERMOSPHERE - QUIET AND DISTURBED CONDITIONS

Low-latitude dynamics, electrodynamics, and plasma density structure a re closely linked. Dynamically driven electric fields initiate the equ atorial ionization anomaly. Between the latitudes of the anomaly crest s, steep gradients in ion density span more than three orders of magni tude. Zonal winds accelerate in response to the severe deficit of plas ma, and reduced ion drag, at the dip equator. Zonal winds give rise to a vertical polarization field, causing plasma to drift with the neutr als and further diminish ion drag. Signatures of neutral temperature a re associated with the winds; cooling appears in the zonal jet itself and there is slight warming on either side. Chemical heating is sugges ted 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 regi on. The strength of the waves and the circulation changes depend on lo cal time; the strongest and most penetrating waves arise on the nights ide, 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 e lectrodynamic drift that has been observed to follow a rise of geomagn etic activity within four hours. Winds at low latitudes respond to sou rces from both polar regions. The changes are manifest by the arrival and interaction of a series of waves from high latitudes that propagat e well into the opposite hemisphere. Lower altitudes, below the F-regi on, respond more slowly because propagation speeds are limited in the cooler, dense lower thermosphere. Finally, during solstice, bulges enr iched in molecular nitrogen migrate, over a period of a day or so, fro m their high latitude source to low latitudes. Characteristic negative phases can result, depleting the ionosphere and further feeding elect rodynamic 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 ma gnetically disturbed conditions, and are used to elucidate the importa nt physical processes. (C) 1995 Elsevier Science Ltd.