Thermospheric circulation, temperature, and compositional structure of the southern hemisphere polar cap during October–November 1981

Abstract

Thermospheric temperature, composition, and winds at F region heights over the southern hemisphere polar cap have been measured by the Dynamics Explorer 2 (DE 2) satellite during October–November 1981. Perigee of the DE 2 satellite during this period was over the southern hemisphere polar cap, and measurements made along 0900–2100 local time polar passes are used to determine the average universal time dependence of thermospheric properties for this local time slice. The F region wind vector is derived from the meridional wind component measured by the Fabry‐Perot interferometer (FPI) and the zonal wind component measured by the wind and temperature spectrometer (WATS) instruments on board DE 2. The neutral gas temperature is derived from FPI and WATS measurements, and the compositional structure is determined from the neutral atmosphere composition spectrometer instrument. The measured temperature, composition, and winds along the 0900–2100 local time polar passes all display a universal time dependence that is due to the displacement between geomagnetic and geographic poles. The temperature and molecular nitrogen densities are enhanced, and the atomic oxygen density is reduced in the magnetic polar cap, in comparison with lower‐latitude regions. The neutral wind pattern follows the pattern of ion drift associated with magnetospheric convection. The National Center for Atmospheric Research thermospheric general circulation model (TGCM) is used to calculate the temperature, composition, and winds over the southern hemisphere polar cap region for conditions similar to those of October 1981. The TGCM includes displaced geomagnetic poles. The temperature, composition, and winds are determined along the DE 2 satellite track for various universal times. The calculated winds are in general agreement with the observed winds, both having a two‐cell pattern of magnetospheric convection. The calculated and observed composition patterns are also in general agreement with predictions made by the mass spectrometer and incoherent scatter radar 1983 empirical model when models and observations are normalized to the same constant pressure surface. The calculated and observed maximum compositional variations in the magnetic polar cap occur near 0900 UT, when the dawn sector of the auroral oval passes over the south geographic pole. The overall pattern of the calculated temperature variation is similar to that observed except for the period between 1300 and 1500 UT. Better agreement with temperature in this time range is obtained with the inclusion of auroral particle precipitation within the magnetospheric cusp.

An analysis of the high‐latitude thermospheric wind pattern calculated by a thermospheric general circulation model: 1. Momentum forcing

“…In this paper, we present the momentum forces at high latitudes for a steady diurnally reproducible model solution that considers both solar forcing and forcing by magnetospheric convection with a 60 kV cross-tail potential. The TGCM was run for average geophysical conditions appropriate to October 21, 1981, a case that was considered previously by Hays et al [1983] and Roble et al [1984] for a comparison of model predictions with measurements made by the Dynamics Explorer over the southern hemisphere polar cap during late October and early November 1981. These previous studies have shown that the ion-drag force due to magnetospheric convection has a strong influence on the high-latitude thermospheric wind, temperature, and compositional structures.…”

“…It was used to study the global circulation, temperature, and compositional structures for equinox conditions during solar cycle minimum and also to derive a globally averaged eddy diffusion coefficient for the lower thermosphere that brings the model calculations into agreement with observed compositional structure as inferred from the MSIS empirical model of Hedin et al [1977a, b]. Roble et al [1984] furthermore used the updated TGCM to study the therm0spheric circulation, temperature, and compositional structures of the southern hemisphere polar cap during the latter part of October and early part of November 1981 and compared the model calculations with observations made by the Dynamics Explorer satellite. Their results showed that the measured temperature, composition, and winds all display a universal time dependence that is due to the displacement between geomagnetic and geographic poles.…”

A “tongue” of neutral composition

“…Many studies have shown that neutral dynamics behave similarly to the ion dynamics at high latitudes. A number of studies using Dynamics Explorer 2 (DE 2) data (e.g., Hays et al, 1984;Killeen et al, 1984McCormac et al, 1987;Thayer et al, 1987) and thermosphere general circulation models (TGCMse.g., Fuller-Rowell andRees, 1980, 1981;Dickinson et al, 1981;Roble et al, 1984) established that the highlatitude neutral circulation tends to follow that of the ions, including the changes resulting from variations in IMF and geomagnetic activity. There are some differences, such as the very weak dawn cell in the neutral circulation pattern and neutral wind speeds that are much slower than those of the ions, but the similarities are sufficiently great that neutral winds should affect the neutral gas in a similar manner to the way that ion drifts affect the ions.…”

Generation of auroral omega bands by shear instability of the neutral winds

“…Coupling between ionospheric electric fields and the neutral atmosphere is particularly strong in the F-region, where observations [e.g., Killeen et al, 1984] and numerical models [e.g., Rees et al, 1983;Roble et al, 1984] show that neutral winds can be driven to speeds • 50% of those of the largescale, electric field drifts VE. The numerical models show that large-scale, convection electric fields can also modify neutral winds in the conducting E-region of the ionosphere.…”