Longitudinal averaged diurnal variation, drift asymmetry, and altitudinal and latitudinal variations of the relative difference. The top plots show the longitudinal averaged diurnal variation of the vertical drift at dip equator at the height of 320 km (a) and the vertical drift asymmetry driven by DE3 tide (b). The solid and dashed lines represent low and high solar flux levels, respectively. The bottom plots show the altitudinal and latitudinal variations of the relative difference of the zonal mean electron densities for low solar activity level (c) and for high solar activity level (d). The solid lines in (c, d) represent the zero lines.