Response of equatorial ionosphere to the transit of interplanetary magnetic cloud of January 13–15, 1967. Transient disturbance in F region

Abstract

Quarter-hourly ionograms of Kodaikanal (10°4′N, 77°29′E, dip 3°N) are analysed to evaluate the response of equatorial ionosphere to the severe geomagnetic storm (| Dst|max, 176 nT) associated with the transit of an interplanetary magnetic cloud at Earth during January 13–15, 1967. It is shown that during the early stage of the storm initial phase on January 13, which corresponded to the local dusk-midnight sector, the entire bottomside F region over Kodaikanal experienced a sudden and spectacular downward drift for a short-duration of ∼2 h (18:45 – 20:30 L.T.). The decrease in height of F layer peak ( hm) by 223 km over the interval 18:45–20:30 L.T. implied a gross apparent downward drift of ∼35.4 m s −1. The height disturbance occured during the recovery phase of an auroral substorm (bay disturbance) that developed at the commencement of the geomagnetic storm. It also displayed an apparent temporal association with a prominent northward swing in IMF Bz and decrease in polar cap potential drop, φ, estimated from IMF parameters. The anomalous and rapid post-sunset descent of F layer was accompanied by an increase in electron density at and below the F layer peak. The evident perturbation in F region height near the dip Equator is interpreted as the signature of a transient westward electric field disturbance generated by the decrease in magnetospheric convection (decrease in polar cap potential) during the recovery phase of the substorm. The values of vertical downward drift derived from the time rate of change of h′F and corrected for chemical loss effects indicated the maximum amplitude of the westward electric fields to be ∼1.9 mV m −1. The increase in Nm is shown to be the outcome of the ionization convergence rate induced by the abnormally large downward drift exceeding the chemical loss rate ( βNe) at altitudes ⩾250 km.