Climatology and storm time dependence of nighttime thermospheric neutral winds over Millstone Hill

Abstract

.[1] We use 630.0 nm nightglow Fabry-Perot measurements over Millstone Hill from 1989Hill from -1999 to study the climatology and storm time dependence of the midlatitude thermospheric winds. Our quiet time wind patterns are consistent with results from earlier studies. We determine the perturbation winds by subtracting from each measurement the corresponding quiet time averages. The climatological zonal disturbance winds are largely independent of season and solar flux and show large early night westward and small late-night eastward winds similar to disturbance ion drifts. The meridional perturbation winds vary strongly with season and solar flux. When the solar flux is low, the winter and equinox average meridional winds change from equatorward to poleward at $2200 LT, and the summer winds are equatorward throughout the night. The high solar flux meridional winds are poleward, with magnitudes increasing from dawn to dusk at all seasons. These disturbance winds patterns are in poor agreement with results from the empirical horizontal wind model, HWM-93. The zonal and meridional disturbance winds show very large variations relative to their average values. We have also studied the time-dependent response of the midlatitude thermospheric winds to enhanced magnetic activity. The early night westward winds build up to large amplitudes (about twice their climatological values) in $6 hours; the late-night eastward winds are smaller and reach their peak values $3 hours after the increase in magnetic activity. The storm time dependence of the meridional winds is considerably more complex than that of the zonal winds, and it varies with season and solar flux. Following enhanced magnetic activity, equatorward winds are observed at all local times and seasons, but the increase of their amplitudes with storm time is fastest in the late local time sector. Near midnight, and when the solar flux is low, the meridional winds reverse from equatorward to poleward $6 -10 hours after the increase in magnetic activity. This reversal is fastest (slowest) during December (June) solstice. At later local times, and for high solar flux conditions, the variation of the meridional disturbance winds is season independent. The observed storm time dependence partly explains the large variability of the disturbance winds.

Climatological modeling of horizontal winds in the mesosphere and lower thermosphere over a mid-latitude station in China

“…After midnight, the seasonal variation exhibited a predominant annual component, which was characterized by a westward wind in summer and an eastward wind in winter. This pattern is consistent with that at Millstone Hill (Fejer et al, 2002;Emmert et al, 2006) and it was generally predicted by the HWM93 model. The HWM07 model exhibited strong enhancements during the equinoxes in the post-midnight sector, which were lacking in the FPI observations.…”

The zonal E×B plasma drift effects on the low latitude ionosphere electron density at solar minimum near equinox

“…The HWM90 neutral wind velocities which determine the wind-induced plasma drift in the theoretical model described in Section 2 are known to differ from observations (Titheridge, 1995;Emmert et al, 2001;Fejer et al, 2002). To bring the modeled and measured NmF2 into better agreement over the sounders shown in Table 1, the values of the meridional and the zonal components of the neutral wind taken from the HWM90 wind model are decreased by a factor of 2 at all altitudes and latitudes of a (q,U) plane if the meridionel neutral wind is equatorward in the both hemispheres at 301 and À301 geomagnetic latitude at 300 km altitude.…”

Magnetic meridional winds in the thermosphere obtained from Global Assimilation of Ionospheric Measurements (GAIM) model

“…Note that the seasonal distribution of the data is not uniform, and the time resolution varies from station to station and from year to year, ranging from more than 30 min to less than 5 min. Also note that the FPI wind data for the three stations have previously been used in a variety of different studies [e.g., Fejer, 1993;Fejer et al, 2002;Biondi et al, 1999;Emmert et al, 2006;Brum et al, 2012].…”