On the vertical plasma drift conditions prior to post-midnight ESF

The pre-reversal enhancement (PRE) of vertical plasma drifts around sunset is well-known to create conditions favoring the development of equatorial spread F (ESF) ionospheric irregularities during pre-midnight hours. We still seek a better understanding of F-region irregularities observed in the post-midnight sector when equatorial ionospheric drifts are expected to be predominantly downward.
Advances in our understanding of post-midnight ESF (and many other phenomena) require advances in our observational capabilities. For instance, Zhan et al. (2018) presented cases of post-midnight ESF observed over the Jicamarca Radio Observatory (JRO) during June solstice low solar flux conditions using the JULIA coherent scatter radar (CSR) mode. They explained that single-beam radar measurements could not show whether ESF developed near the observation site (i.e., locally) or drifted into the antenna beam after being generated in another longitude sector (i.e., non-locally). Previous efforts have also suggested that unusual upward drifts in the post-midnight sector can be responsible for the development of post-midnight ESF.
In this presentation, we will show and discuss results of a study using new simultaneous and collocated observations made by AMISR-14 and by the incoherent scatter radar (ISR) at the JRO. The main goals of this study are (a) to investigate the occurrence of local and non-local post-midnight ESF and (b) to determine the drift conditions under which local post-midnight ESF events occur.
For this investigation, we analyzed the AMISR-14 UHF two-dimensional (2D) coherent scatter radar observations of F-region irregularities. The 2D observations allowed us to identify local ESF events. We also analyzed observations of the vertical plasma drifts made by the Jicamarca ISR.
We will present and discuss case studies of post-midnight ESF events and vertical drifts measured during June solstice. These cases represent examples when local post-midnight ESF events were identified unambiguously. More importantly, we show that upward vertical drifts preceded the development of these events, confirming that the drifts indeed contribute to conditions favoring the development of post-midnight ESF.

Zhan, W., Rodrigues, F. S., & Milla, M. A. (2018). On the genesis of postmidnight equatorial spread F: Results for the American/Peruvian sector. Geophysical Research Letters, 45, 7354–7361. https://doi.org/10.1029/2018GL078822.

Acknowledgment: This work was supported by the NSF (AGS-1916055) and by an NDSEG fellowship. JRO is a facility of the Instituto Geofísico del Perú operated with support from NSF AGS-2213849 through Cornell University.