Ionosphere-thermosphere momentum coupling at solar maximum and solar minimum from DE-2 and AE-C data

Abstract

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 ion-neutral 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 experimentally-derived 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.