Abstract The formation process of the F3 layer at sunrise in the middle latitudes is newly investigated using data from the Mother's Day geomagnetic storm of 10–11 May 2024. Ionosonde, Fabry‐Perot Interferometer (FPI), satellite data, and model simulations of Equatorial Electric Field for India, South‐East Asia‐Australia, and Japanese sectors are utilized. The F3 layer was recorded by Ionosondes at Japanese mid‐latitude stations during 19 UT–21 UT, which coincided with the local Sunrise time. Over Perth (Australia), Wuhan (China), and Ahmedabad (India), formations were delayed at 22 UT–23 UT. The COSMIC vertical profiles showed enhanced F2 layer altitude after 19 UT, and confirmed the formation of additional ionospheric stratifications above the F2 layer. The thermospheric winds measured by FPI at northern mid‐latitude and southern low‐latitudes indicated strong equatorward winds in both hemispheres starting around 18:30 UT. The analysis revealed that dawn‐time F3 layer stratifications in middle latitude were manifestations of residual nighttime F layer, which was raised to higher altitude by the equatorward winds, even as a new weak F2 layer was formed below by photo‐ionization. The reduction in thermospheric O/N 2 caused the F3 layer at higher altitudes to survive and remain stronger than the F2 layer for a considerable time after sunrise. In contrast, the low‐latitude F3 layer was formed by vertical drift induced by the electric field, when an IMF‐Bz transition resulted in an eastward penetration electric field pulse in the dawn sector. At Perth in the southern hemisphere, both mechanisms were effective in the formation (wind) and sustenance (electric field) of the F3 layer.
Rai et al. (Sun,) studied this question.