Abstract:
The existence of an additional stratification in the daytime equatorial ionospheric F region (the
F3 layer) was known since the 1940s. However, its characteristics and the underlying physical mechanism
have been uncovered only recently. In this paper, we present and discuss the F3 layer characteristics
observed by six ionosondes distributed over equatorial and low latitudes (−20° to +25° dip latitudes) in the
Brazilian longitude sector during the strongest geomagnetic storm (DstMin = −223 nT) of solar cycle 24,
the St. Patrick's Day storm of 17 March 2015. Two eastward prompt penetration electric field (PPEF) events,
as seen in equatorial electrojet, occurred during the main phase of the storm on 17 March 2015, a strong
one (~100 nT) at around ~1200 UT and a weak one (~50 nT) at around ~1725 UT. Local time variations in the
F3 layer occurrence and ionospheric base height (h′F), peak height (hmF), and peak electron density
(Nmax) are investigated. Notably, the F3 layer occurred at all six locations, more distinctly during the
stronger PPEF event. The large latitudinal extend in the occurrence of the F3 layer in opposite hemispheres
(−20° to +25° dip latitudes) covering the equatorial ionization anomaly crests observed for the first
time is interpreted in terms of the combined effect of the super plasma fountain generated by the eastward
PPEF and storm time equatorward neutral wind.
