Abstract:
The effect of the polarity of the interplanetary magnetic field, IMF, on the equatorial zonal electric field is assessed through a comparative study of the diurnal patterns of equatorial electrojet strength on days of sunward and antisunward IMF during vernal and autumnal equinoxes of high sunspot activity years (1957–1959 and 1979–1981). The results showed that, on average, the electrojet strength is reduced around noon (09–15 L.T.) on sunward IMF days when compared with antisunward days in the vernal equinox and the pattern is reversed in the autumnal equinox. This modulation in electrojet strength collaborates the trends noticed earlier in the behaviour of the equatorial ionization anomaly (Sastri, 1985b, Adv. Space Res.5, 199), and strengthens the view that IMF polarity exerts a discernible influence on the equatorial electric field through its effect on the semiannual variation of geomagnetic activity. The average temporal profiles of the auroral electrojet index (AE) showed not only the difference in the overall level of AE between sunward and antisunward IMF days and the opposite changes in it from vernal to autumnal equinox, but also a preferential occurrence of substorm activity apparently 19–24 h prior to local noon on sunward (antisunward) IMF days in the vernal (autumnal) equinox. A substorm recovery phase is generally seen in the forenoon period more or less independent of IMF polarity in the two equinoxes. These characteristic features in AE are conspicuously absent during the autumnal equinox of 1979 1981 when the IMF related changes in electrojet strength are not apparent. The results strongly suggest that the response of the noon time equatorial electric field to IMF polarity receives substantial contribution from westward electric field disturbances associated with the ionospheric “disturbance dynamo” mechanism.
