Solar activity dependence of effective winds derived from ionospheric data at Wuhan Original Research Article

The solar cycle variability of thermospheric circulation is one of the outstanding questions involving the upper atmosphere. However, it needs to be identified due to the latitude and longitude dependence of neutral winds. This article examines the solar activity trends in the vertical component of equivalent winds (VEWs) derived from ionosonde data during 1966–1985 over Wuhan (114.4° E, 30.6° N, 45.2° dip) of China. The monthly median data of routine ionosonde measurements in published tables are used to derive VEWs which are contributions from both neutral winds and zonal electric fields. The solar activity trends of VEWs over Wuhan have an obvious local time and seasonal dependence. Over Wuhan, the magnitudes of VEWs decrease with increasing solar activity around midnight for all seasons and around midday in winter and autumn. In contrast, little variation is found throughout the solar cycle around midday in summer and spring. The magnitudes of daily means and amplitudes of diurnal and semidiurnal components of VEWs over Wuhan decrease with solar activity, which also confirms that it is not a feature of a particular location. There is a more pronounced decrease in diurnal amplitudes of VEWs than other harmonic components over Wuhan. The decrease in diurnal amplitudes with increasing solar activity can be reasonably explained, if the greater ion drag at solar maximum compensates for pressure gradients due to the greater EUV input.