Comparison of the neutral wind seasonal variation from midlatitude conjugate observations

The seasonal variation of F region neutral wind from the midlatitude conjugate Fabry-Perot interferometer observations has been studied. The meridional wind at Palmer station (64°S,64°W) has a significant local time dependence with strong equatorward wind at midnight and polarward wind at dawn and dusk. The zonal wind switches from eastward to westward in the early morning section. From the June solstice (austral winter) to equinox, the maximum meridional wind increases from 90 m/s to 130 m/s, and the zonal wind switches direction at an earlier local time. The neutral winds from Palmer have been compared with those from the geomagnetic conjugate location, Millstone Hill (MH). At equinox, the local time variation of neutral wind shows a very good conjugacy between these two locations. But at June solstice, the similarity in the zonal wind becomes less clear. This seasonal dependence can be attributed to the seasonal variation of solar and geomagnetic forcings. The annual variation of daily average neutral wind from Palmer and MH has also been compared. The meridional wind shows a clear offset of season, and the magnitude at Palmer is averagely 40 m/s more equatorward than that at MH. The zonal wind is dominantly westward at Palmer and eastward at MH. The annual variation of neutral wind, especially the zonal component, is much less symmetric between the two sites than the local time variation. The empirical horizontal wind model shows a good agreement with the observations in both local time and annual variations.