Equatorial Counter Electrojet & Interplanetary Magnetic Field

1-15The characteristics of equatorial counter electrojet events associated with the reversal of the Bz component of interplanetary magnetic field (IMF) are discussed and compared with those of events associated with semi-diurnal luni-solar tides. The counter electrojet events due to luni-solar tides are preceded by high forenoon peaks of ΔH and very slow and smooth depressions of ΔH in the afternoon, and these events occur on a succession of days. The counter electrojet events due to the reversal of Bz component of IMF from south to north show fast and shorter period variations in H and occur at any time on some isolated days. At times these effects may be superposed on already existing counter electrojet of tidal origin. The relative importance of the two sources can be easily estimated by examining the magnetograms of two neighbouring electrojet stations separated in longitude by a couple of hours. The counter electrojet events due to tides are seen on the H magnetograms of only the electrojet stations while the events due to the reversal of Bz have their signatures at equatorial stations over an extended longitude zone as well as at non-equatorial stations and occur at the same universal time at all stations. It is the rapid change of the latitude (θ) of the IMF from the southward to the northward direction in the presence of large and rather steady value of the scalar field B which is effective in decreasing the equatorial electric field resulting in the counter electrojet event. Thus a comparison of the electrojet current or the equatorial electric field with hourly mean values of Bz, has led to misleading conclusions by some workers. The changes in the equatorial electric fields due to the reversal of IMF latitudes are seen both in the dayside and the nightside of the earth and are associated with similar changes in the auroral electric fields