Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground‐based incoherent scatter radar and Fabry‐Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground‐based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of Fabry‐Perot observations.

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Simultaneous high time resolution observations of the Hall and Pedersen conductances, ΣH and ΣP, respectively, the ionospheric electric field and the ground magnetic field during a magnetospheric substorm are reported. The measurements discussed here were taken during the SUNDIAL/ATLAS 1 campaign of March 24 to April 2, 1992. The European Incoherent Scatter (EISCAT) UHF special programme SP‐UK‐ATLAS, which operated on March 27, 1992, provided continuous measurements of the electron density and the ion vector velocity from which ΣH and ΣP and the ionospheric electric field were calculated. During the substorm growth phase, ΣP and ΣH were less than 10 S and the ratio, R = ΣH/ΣP, was less than 1. Although both ΣH and ΣP increased at the onset of the expansion phase, R remained close to 1. This ratio provides information on the mean energy of the precipitating particles responsible for the enhanced conductances. A ratio of 1 implies a mean energy of the particles of 2.56 keV. Two distinct expansion phases were identified, the second of which included a number of intensifications. Both ΣH and ΣP increased toward the end of the second expansion phase with peak values of 71 S and 34 S, respectively. The ratio R also increased to values exceeding 2, equivalent to mean energies of more than 5.78 keV. The largest value of R was 3.25, which occurred during the substorm recovery phase and is equivalent to mean energies of more than 10 keV. The increase in mean energy as the substorm progresses may be interpreted in terms of changes in the acceleration processes in the magnetosphere. The maximum zonal (east‐west) current during this interval was 2.20 A m−1 and occurred toward the end of the second substorm expansion phase. At a number of intensifications, reversals or enhancements in the zonal current were observed, for which the electric field was responsible in most cases. While the peak value of the Pedersen conductance of 40 S is similar to recent published results, the Hall conductance peak of 75 S is less than recent published measurements. Furthermore, the westward current during the initial part of the expansion phase is dominated by the electric field, in contrast to a previously published model which suggested that this current would be conductivity‐dominated. It is suggested that further spatial structure of the current is necessary to explain these observations.

The incoherent scatter radar located at S0ndre Str0mfjord, Greenland (67øN, 51øW, 74.5øA) and the EISCAT incoherent scatter facility located in northern Scandinavia (69.5øN, 19øE, 66.3øA) both obtained E and F region measurements during the first campaign of the Lower Thermosphere Coupling Study (LTCS 1, September 21-25, 1987). Neutral winds deduced from these measurements have been analyzed for their mean flow and tidal components. A number of the altitude profiles for the mean winds and the diurnal and semidiurnal wave components at the two radar locations show similar variations with height, indicating that latitudinal rather than longitudinal effects are dominant in determining the observed wind field. Diurnal tidal amplitudes and phases are reasonably well represented by theoretical model results (Forbes, 1982). The semidiurnal amplitudes and phases, although somewhat consistent between the two radars, are not well represented in equinox tidal model results (Forbes and Vial, this issue). Results from both radars indicate a vertical wavelength for the zonal semidiurnal oscillation of approximately 60 km. During a period of impulsive magnetospheric forcing (September 22-23), winds deduced from measurements at both radars show enhanced eastward flows near midnight accompanied by equatorward winds at Sondrestrom. Comparison with the results of a National Center for Atmospheric Research thermosphere-ionosphere general circulation model (TIGCM) simulation of the LTCS I interval shows generally better agreement with the observations at EISCAT than at Sondrestrom. During the period of activity on September 22-23 the TIGCM is reasonably successful at simulating the eastward surge near midnight in the EISCAT (but not in the Sondrestrom) observations and the equatorward flow after midnight in the Sondrestrom results. The observed winds in magnetic latitude and magnetic local time coordinates indicate the presence of anticyclonic divergent flow near dusk and cyclonic converging flow near dawn. and therefore geographically controlled, can be expected to be similar, assuming that longitudinal effects are negligible. This paper presents a comparison of the neutral winds derived from the measurements obtained at the EISCAT and Sondrestrom radars during the period of simultaneous observations. A companion paper presents discussion of the measurements obtained at Sondrestrom alone [Johnson, this issue]. The following section briefly outlines the data acquisition modes utilized at the two facilities (see Johnson [this issue] for further details). An analysis of the deduced neutral winds follows, focusing on both the average flow and tidal structures observed during the interval as well as on the response of the neutral winds to an interval of impulsive forcing. Comparisons with appropriate theoretical models are introduced in the discussion of the radar results.

Abstract. The Global-Scale Wave Model (GSWM) is a steady-state two-dimensional linearized model capable of simulating the solar tides and planetary waves. In an e ort to understand the capabilities and limitations of the GSWM throughout the upper mesosphere and thermosphere a comparative analysis with observational data is presented. A majority of the observational data used in this study was collected during the World Day campaign which ran from 20 January to 30 January 1993. During this campaign data from 18 ground-based observational sites across the globe and two instruments located on the UARS spacecraft were analyzed. Comparisons of these data with the simulations from the GSWM indicate that the GSWM results are in reasonable agreement with the observations. However, there are a number of cases where the agreement is not particularly good. One such instance is for the semidiurnal tide in the northern hemisphere, where the GSWM estimates may exceed observations by 50% . Through a number of numerical simulations, it appears that this discrepancy may be due to the eddy di usivity pro®les used by the GSWM. Other di erences relating to the diurnal tide and the quasi-two-day wave are presented and discussed. Additionally, a discussion on the biases and aliasing di culties which may arise in the observational data is also presented.

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