We present experimental evidence and modeling results which indicate that eastward thermospheric wind is the primary controlling factor of equatorial spread-F initiation in the post-sunset ionosphere. Eastward wind-driven Pedersen currents are able to polarize F-region density perturbations with westward tilting wavefronts into rapidly growing modes to trigger the formation of spread-F bubbles. The described process is so rapid that seeding requirements of spread-F initiation by external factors such as gravity waves are effectively eliminated.
Kudeki, E., Akgiray, A., Milla, M., Chau, J. L., & Hysell, D. L. (2007). Equatorial spread-F initiation: Post-sunset vortex, thermospheric winds, gravity waves.Journal of Atmospheric and Solar-Terrestrial Physics, 69(17-18), 2416-2427. https://doi.org/10.1016/j.jastp.2007.04.012