Distinctive effects of interplanetary electric field and substorm on nighttime equatorial F layer: A case study

[1] A geomagnetic storm event is identified wherein the base of the equatorial F layer (h′F) over dip equator moved unusually upward till premidnight hours and descended thereafter. The h′F variation is linearly correlated with the variation in the smoothed auroral electrojet index (AE) which varies slowly. The fast fluctuations (residuals) with periodicity ∼42 min in the vertical plasma drift are found to be causally related with the fast fluctuations in the dawn-to-dusk component of interplanetary electric field (IEFy) and interestingly, not with the fast fluctuations in AE. However, during the interval when a substorm was triggered in association with sharp transitions in IEFy polarity, the drift fluctuations follow the AE fluctuations and are out of phase with IEFy fluctuations. The investigation demonstrates distinctive effects of IEFy and substorm-related current systems on the zonal electric field over dip equator in the premidnight hours during geomagnetic storms.