Magnetic activity dependence of high‐latitude thermospheric winds and densities below 200 km

Abstract

Satellite‐based measurements are utilized to elucidate the latitudinal, local time, and magnetic activity dependence of winds and densities in the scantily observed atmospheric region between 170 and 220 km above 45° magnetic latitude. One data set consists of atmospheric densities from high‐accuracy (time resolution ≈ 5‐6 hours) orbital analyses of three Doppler Beacon satellites in orbit during 1973. The perigees of these satellites are generally restricted to 160‐180 km, 1200‐1400 LST, and geographic latitudes greater than about 45°. Statistical relationships are derived between the density changes and the planetary magnetic index, Kp, and the 5‐hour mean of the auroral electro jet index, . The former relationships are compared with those derived from the MSISE90 empirical model (Hedin, 1991), which is found to overestimate the rate of increase of density with respect to Kp. Another data set consists of densities and cross‐track winds from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment for the March 21 to April 9, 1979, period, which includes several intervals of elevated magnetic activity. Besides comparing various time series, the data are also binned according to 10° latitude increments and unit increments of Kp to derive trends. Some typical results include the following, corresponding to average changes in the 45° to 65° magnetic latitude band as Kp is increased from 1 to 6: (1) for the nightside (≈ 2230 LT), a change in cross‐track (nearly zonal) wind from 25 ± 25 m s−1 (eastward) to −125 ± 25 m s−1 (westward), and an increase of about 20% in density; and (2) for dayside (≈ 1030 LT), a change in cross‐track wind from 25 ± 25 m s−1 (eastward) to 125 ± 25 m s−1 (eastward), and a density increase of 25%. For some individual sudden enhancements in magnetic activity, changes in winds and densities can be more than double the above average values. Comparisons are also made with the NCAR TIGCM (National Center for Atmospheric Research thermosphere‐ionosphere general circulation model) simulation for the complete 20‐day interval, and with recent empirical models of densities (Hedin, 1991) and winds (Hedin et al., 1991), with data points in each case derived for the satellite paths, instrument orientations, and sampling rates identical to that of the SETA experiment.