Interplanetary magnetic field and equatorial ionosphere.

Large variations in the geomagnetic field near the dip equator are examined in relation to the variations of the equatorial q-type Es, E-region drifts, and the interplanetary magnetic field. It is shown that the decrease in equatorial electrojet current is associated with a sudden increase in the northward component (Bz) of the interplanetary magnetic field. It is suggested that the ambiguities in the interpretations of DP2 events can be easily removed if the corresponding ionospheric data are also utilized in the analysis. The index delta H at an equatorial-electrojet station minus delta H at a non-electrojet station provides a very reliable index for studying the variations of the equatorial electrojet currents even during geomagnetically disturbed conditions. This index is closely correlated with the auroral indices AU and AL; a positive change in the auroral electrojet indices is associated with a decrease in the equatorial electrojet current even in minor details.