Westward electric field penetration to the dayside equatorial ionosphere during the main phase of the geomagnetic storm on 22 July 2009

Abstract

[1] The moderate geomagnetic storm (minimum SYM-H = −89 nT) that occurred during the period from 21 to 25 July 2009 is anomalous because the storm main phase developed during northward interplanetary magnetic field (IMF). A reduction (for ∼4 h) in the daytime E region westward drift (eastward electric field) over the magnetic equatorial location of Trivandrum (8.5°N, 77°E; dip latitude ∼0.5°N) in India is observed during the storm main phase. This reduction in the drift is also indicated by the disappearance of the equatorial sporadic E region (Esq) echoes on the ionograms. It is suggested that an additional westward prompt penetration electric field characterized by the northward IMF Bz is superposed on the ionosphere during the storm main phase which contributed to the observed reduction in the drift. These are the first observations which indicate the effects of the dusk-to-dawn penetration electric fields on the east-west drifts during northward IMF.