Abstract
Abstract. The vertical equivalent winds (VEWs) at the F-layer are analyzed along the 120°-150°E longitude sector with an emphasis on their latitudinal dependence. The VEWs are derived from the monthly median data of fourteen ionosonde stations over two decades. The results show that the VEWs have considerable dependences on the magnetic latitude with an approximate symmetry about the magnetic equator. They are mostly controlled by the electric field drifts in the magnetic equatorial region, and shift to be mostly contributed by neutral winds at mid-latitudes. The relative contribution of the two dynamic factors is regulated by the magnetic dip in addition to their own magnitudes. The VEWs generally have opposite directions and different magnitudes between lower and higher latitudes. At solar minimum, the magnitudes of VEWs are only between -20 and 20m/s at lower latitudes, while at higher latitudes they tend to increase with latitudes, typically having magnitudes between 20-40m/s. At solar maximum, the VEWs are reduced by about 10-20m/s in magnitudes during some local times at higher latitudes. A tidal analysis reveals that the relative importance of major tidal components is also different between lower and higher latitudes. The VEWs also depend on local time, season and solar activity. At higher latitudes, the nighttime VEWs have larger magnitude during post-midnight hours and so do the daytime ones before midday. The VEWs tend to have an inverse relationship with solar activity not only at night, but also by day, which is different from the meridional winds predicted by the HWM93 model. The latitudinal dependence of VEWs has two prevailing trends: one is a maximum at the highest latitudes (as far as the latitudes concerned in the present work); the other is a mid-latitude maximum. These two latitudinal trends are mostly dependent on season, while they depend relatively weakly on local time and solar activity. The latitudinal gradients of VEWs also show a tendency of a mid-latitude maximum, except that there are much stronger latitudinal gradients at southern higher mid-latitudes in some seasons. The gradients during daytime are much smaller at solar maximum than minimum, whereas they are generally comparable at night under both solar activity levels.
Highlights
The climatology of thermospheric dynamics has been extensively investigated with the measurements of Fabry-Perot interferometer (FPI), incoherent scatter radar (ISR) and ionosonde (e.g. Buonsanto, 1990; Hagan, 1993; Biondi et al, 1999; Igi et al, 1999; Buonsanto and Witasse, 1999; Kawamura et al, 2000; Emmert et al, 2003; Foppiano et al, 2003)
The eastward electric fields or equatorward meridional winds contribute to the upward vertical equivalent winds (VEWs), while the westward electric field or poleward meridional winds tend to induce downward VEWs (Eq (2))
As can be seen from Eq (2), the VEWs may be more controlled by electric field at low latitudes where the magnetic inclination is small, and as expected, the VEWs can be considered as neutral winds at mid-latitude, where the contribution of zonal electric field can be neglected (Miller et al, 1997)
Summary
The climatology of thermospheric dynamics has been extensively investigated with the measurements of Fabry-Perot interferometer (FPI), incoherent scatter radar (ISR) and ionosonde (e.g. Buonsanto, 1990; Hagan, 1993; Biondi et al, 1999; Igi et al, 1999; Buonsanto and Witasse, 1999; Kawamura et al, 2000; Emmert et al, 2003; Foppiano et al, 2003). Some climatological models of the thermospheric horizontal winds Hedin et al, 1996; Miller et al, 1997) and electric field drifts (Scherliess and Fejer, 1999; Zhang et al, 2001) have been developed. The basic characteristics of thermospheric dynamics have been well known at low and higher latitudes. Electric fields play an important role in the F region dynamics. A significant feature of equatorial electric field drifts is the pre-reversal enhancement during solar maximum years, while this enhancement is either greatly reduced or completely absent during solar minimum years (Scherliess and Fejer, 1999). The meridional winds at mid-latitude are consistent with the solar EUV forcing and the consequent latitudinal pressure gradients, namely poleward by day and
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