Southern hemisphere thermospheric dynamics: A review

Abstract

The dynamics of the upper thermosphere in the southern polar cap, auroral, and mid-latitude zones is reviewed. Information has been drawn from published measurements from the DE 2 and AE-C spacecraft and from ground-based stations and interpreted using the latest thermospheric general circulation models. Necessarily, frequent reference is made to the F region of the ionosphere, which permeates the upper thermosphere. Great emphasis is placed on the coupling and feedback mechanisms which exist between these interpenetrating fluids. It is shown that to obtain close approximation to observation, a complex model is required which incorporates these mechanisms in a self-consistent way. Published results from general circulation models show that there is a broad measure of agreement at both high and middle latitudes between simulation and observation over a wide range of geomagnetic activity for the sunlit southern middle and high latitudes near solar maximum. However, there are comparatively few data for a full investigation to be made, and present conclusions must be regarded as strictly interim. Strong driving forces in the auroral zones and polar caps cause a wind regime which is distinctly different from that observed at middle latitudes. Geomagnetic activity has been shown to have a strong effect on the extent of the high-latitude wind field, its strength, and its penetration to middle latitudes. The orientation of the interplanetary magnetic field has also been shown to be important. The coupling mechanisms between ions and neutrals heat the thermosphere and drive its circulation using energy from magnetospheric sources, but as yet the details of the thermosphere's variation with geomagnetic and solar activity are not clear from observations. The strong vertical winds reported in the literature are certainly indications of powerful heating, but the connection between observations of vertical winds and the production of gravity wave activity has not been established. Halley, Antarctica, is a critical station in the evaluation of model simulations of the winds in the upper thermosphere since it is a high-latitude station geographically but middle latitude geomagnetically and is subject to an unusual blend of driving forces. The observations are modeled surprisingly well considering the incomplete accounting of asynunetry caused by the large offset of the geomagnetic and geographic poles, and also of the South Atlantic anomaly. It is probable that, globally, there are detailed disagreements between Copyright 1988 by the American Geophysical Union. Paper number 8R0388. 8755-1209/88/008 R-0388515.00 the actual circulation and the simulations which have not yet come to light for the lack of observations. In the absence of new spacecraft experiments, new groundbased stations are required in the Antarctic region to enable simultaneous measurements to be made over a wide range of the southern polar cap. Introduction The thermosphere is generally of a mainly molecular composition at lower levels: molecular nitrogen and mo...

Spatial structure in the thermospheric horizontal wind above Poker Flat, Alaska, during solar minimum

“…Previous accounts of thermospheric dynamics have highlighted processes which remove complexity from thermospheric wind systems [e.g., Killeen and Robl½, 1988;Smith et al, 1988]. In particular, viscosity is known to oppose both horizontal and vertical velocity shears, so it is tempting to presume that thermospheric wind fields should appear smooth and laminar at smaller than synoptic spatial scales.…”

High-latitude thermospheric zonal winds during low solar activity period

“…During this period of solar minimum, the high latitude thermospheric zonal winds may not be coupled efficiently to the pattern of ion drift movement leading to poor response to momentum forcing (Smith et al, 1988). The geomagnetic field geometry and the sun's activity have a strong influence on the processes taking place in the earth's high-latitude thermosphere (Lühr and Marker, 2013).…”

“…Effects of solar disturbances are first observed in the high latitudes before onward propagation through mid to equatorial latitudes. The wind field and strength of earth's high latitude is significantly modified by geomagnetic activity (Smith et al, 1988). It is the neutral wind parameter that has not received the much needed attention in regard to the many issues pertaining to the forecasting of ionospheric weather (Meriwether, 2006).…”

Climatologies of nighttime upper thermospheric winds measured by ground‐based Fabry‐Perot interferometers during geomagnetically quiet conditions: 2. High‐latitude circulation and interplanetary magnetic field dependence

“…At high latitudes, thermospheric neutral winds are strongly coupled to the convecting ionosphere [e.g., Meriwether et al , 1973; Richmond and Matsushita , 1975; Mikkelsen and Larsen , 1983; McCormac and Smith , 1984; Rees et al , 1986; Smith et al , 1988; Thayer and Killeen , 1993; Richmond et al , 2003]. Other momentum sources include pressure gradients caused by Joule heating and heating from solar extreme ultraviolet (EUV) irradiation [e.g., Roble , 1995].…”