On the Impact of Quiet-Time Day-to-Day Variability of Vertical Drifts on the Generation of Postmidnight ESF

We investigated the effects of the quiet-time day-to-day variability of equatorial vertical plasma drifts on conditions favoring equatorial spread-F (ESF) development during the 2008–2009 solar minimum conditions. Of particular interest is the role of vertical drifts in the generation of postmidnight ESF and the relationship between their day-to-day variabilities during the June solstice of 2009. The investigation utilized a numerical approach motivated by the lack of adequate measurements of vertical drifts, especially during the June solstice and low solar activity. The approach evaluated the impact of simulated equatorial vertical drifts by the thermosphere and ionosphere eXtension of the NCAR Whole Atmosphere Community Climate Model (WACCM-X) on the linear stability of the low-latitude ionosphere. Vertical drifts were used to drive the SAMI2 model, and SAMI2 outputs were then used to estimate the linear growth rate of the Generalized Rayleigh-Taylor (GRT) instability, responsible for ESF. In line with previous studies, the WACCM-X drifts lead to moderate to large GRT growth rates during the post-sunset hours of the Equinox and December Solstices. The most exciting finding is that the WACCM-X model reproduces the weakening or even upward drifts around midnight and their day-to-day variability. The estimated GRT growth rates indicate that the WACCM-X vertical drifts could be associated with weak and moderate irregularity activities during the June solstice of 2009. Substantial agreement with radar observations of vertical drifts and postmidnight ESF is achieved by augmenting the climatological mean with a twofold increase in quiet-time drift variability.