Neutral wind measurements obtained by instruments on board the DynamicsExplorer 2 (DE 2) spacecraft have been used to study the effects of geomagnetic activity on the circulation of the highlatitude neutral thermosphere for solar maximum conditions during the periods The data have been sorted and ordered according to the two geophysical indices Kp and (auroral electrojet) AE. Simple expressions have been derived which describe (1) the maximum antisunward wind speed in the geomagnetic polar cap, (2) the maximum sunward wind speeds in the dawn and dusk sectors of the auroral oval, and (3) the latitudinal extent of the polar cap antisunward neutral wind as functions of Kp and AE. The results show a positive correlation between the geomagnetic indices and the three characteristic features of the neutral circulation described above. Averaged vector wind fields in geomagnetic coordinates for Kp < 2 and Kp > 4 in both northern and southern hemispheres for the 6 months have been derived from the data. In doing this, a first-order invariance of the neutral wind circulation in geomagnetic coordinates as a function of universal time (UT) was assumed. The results show a two-cell circulation pattern in the northern winter hemisphere for both quiet and active geomagnetic periods. The cell sizes increase with increasing geomagnetic activity. The dusk cell is always dominant. The southern summer hemisphere averages show only the dusk circulation cell for both quiet and active geomagnetic periods. A diminution of this cell occurs for reduced levels of geomagnetic activity. Data from an individual DE 2 orbit are presented to illustrate how these average patterns tend to smooth out typically observed structure in the thermospheric wind.

Simultaneous measurements of global‐scale auroral luminosity distributions and neutral winds over the northern (winter) polar cap have been obtained using instrumentation on the Dynamics Explorers 1 and 2 spacecraft. Several examples of these simultaneous measurements are presented to illustrate the relationship between the circulation of neutral air in the high‐latitude F region and the spatial distribution of the aurora. The auroral images are obtained using the Spin‐Scan Auroral Imager on the high‐altitude spacecraft (DE 1), and the neutral wind vectors are obtained along the orbit of the low‐altitude spacecraft (DE 2), using the Fabry‐Perot interferometer and the Wind and Temperature Spectrometer. These measurements make it possible to appreciate more fully the pattern of neutral winds when placed in the context of global auroral activity. Supplementary DE 2 measurements of neutral composition, ion densities, and the cross‐track component of the ion drift enable the ion drag force on the neutral gas to be correlated with the aurora. Our initial study of these data indicates (1) that a definite correlation exists between boundaries in the neutral wind field and the location of the auroral oval, with large‐scale features of the neutral circulation tracking the substorm‐dependent expansion and contraction of the auroral oval, (2) that the influence of ion drag from sunward convecting ions can extend to latitudes much lower than those normally associated with the auroral oval, (3) that the sunward neutral flow associated with the auroral oval in the dusk sector is, in general, more pronounced than that associated with the dawn sector and that this asymmetry is ascribed to the different effect of the Coriolis force in the two sectors, (4) that the flow patterns for neutrals and ions within the geomagnetic polar cap are generally asymmetric with respect to the noon‐midnight meridian, an effect considered to be controlled by the orientation of the interplanetary magnetic field, (5) that the region of the polar cusp and the apparent “midday gap,” or reduction in luminosities observed in the VUV wavelength auroral images near local noon, is closely associated with a large, antisunward surge in the neutral wind, and (6) that the morphology of the ion drag force in the polar regions is considerably more complex, even for very quiet geomagnetic conditions, than that computed by the National Center for Atmospheric Research thermospheric general circulation model for the “steady state.” This last result is considered to be a consequence of the short‐term variability in the characteristics of high‐latitude ion convection and, in particular, the motion of convection boundaries with respect to boundaries in the less responsive neutral circulation pattern.

DE-2 and AE-C satellite measurements of plasma and neutral densities have been used to derive time constants for momentum transfer to the neutrals from ions in the high-latitude thermosphere. The momentum transfer time constants for solar cycle maximum (DE-2) and for solar cycle minimum (AE-C) have been averaged and binned according to geomagnetic latitude and local time to provide a quantitative measure for the tightness of ionneutral momentum coupling in the altitude range 250-350 km. During solar maximum conditions, the neutrals respond relatively rapidly to forcing from the ions. with e-folding time-constants of the order of 1-3 hours. For solar minimum conditions, however, the time constants are typically about an order of magnitude larger, implying that the neutrals are relatively insensitive to ion-drag forcing and that the winds are controlled principally by the large-scale, day-to-night pressure gradient. The measurements are compared with model calculations of the time constants using the Chiu (1975) and MSIS-83 semi-empirical models for electron (ion) density and neutral composition. respectively. Since thermospheric general circulation models (TGCMs) rely on these two semi-empirical models for their parameterizations of the ion drag momentum source. the comparisons enable an important TGCM input to be critically examined. The agreement between the derived time constants and the corresponding values obtained from the semi-empirical models is reasonable for solar maximum. At solar minimum, however, the model time constants are significantly smaller than the experimentallyderived values. The discrepancy is principally due to the overestimation of the polar ionospheric densities by the Chiu model. TGCM calculations which use a polar ionosphere based on the Chiu semi-empirical model therefore exaggerate the importance of the ion drag momentum source at solar minimum by a significant margin.

photochemical calculations for Uranus predict an extensive region of condensation of acetylene, ethane and methane in the vicinity of the temperature inversion layer. This could explain why ethane was not detected on Uranus, unlike Neptune which has a much warmer inversion layer. Subsequent snow-out of the condensibles is expected to result in reduced visibility in the troposphere. Ionospheric calculations for the equatorial region to be probed by Voyager, indicate peak electron concentrations on the order of 5x1+ cme3, if dynamical effects are important. Upper limit to the electron peak is 3~10~ cmw3. Exospheric temperatures as high as 200-250K are conceivable.

Recent observations of the density of minor ions at high altitudes in the outer plasmasphere show relative enhancements of O++ in regions of simultaneous O+ enhancements. These regions also exhibit high ion temperatures. Computer simulations of the temperature structure of the plasmasphere under conditions of electron heating in the equatorial region suggest that such heating produces large gradients in both the electron and ion temperature in the ionosphere. These gradients result in an increase in the pressure of the electrons, which increases the polarization field, and of the ions, which results in large plasma scale heights at low altitudes and increased ion densities at high altitudes. The subsequent enhanced flux of O++ from the ionosphere produced by collisional drag of O++ by O+ and the increased polarization field results in a significant increase in the O++ density above the ionosphere. At higher altitudes the O++‐O+ collisions inhibit the upward flow of O++ resulting in a high‐altitude peak in the O++ density. Above this peak, where collisions with O+ begin to become insignificant, the O++ pressure gradient pushes the O++ into the equatorial reservior. Simulations of conditions of moderate flux tube depletion result in an increase in this effect. The N+ is also affected by collisions with O+, but the increase in its density at high altitudes is primarily due to the scale height effect.