Main field control of the equatorial electrojet: a preliminary study from the Oersted data

Abstract

There are very few phenomenon in the upper atmosphere that are as sensitive to the strength and distribution of the earth's core field as the equatorial electrojet. The Oersted initial field model (OIFM) along with the earlier IGRF models have been used to provide insight into the secular changes in the global patterns of the position and strength of the equatorial electrojet (EEJ). A northward drift in the location of the dip equator (hence the position of the axis of the EEJ) to the extent of 1.0 degree per decade is seen around 300°E. In contrast, in the Pacific, east of Australia, there is practically no drift. It is also observed that the largest secular variation in the Cowling conductivity occurs in the South American sector with a possible 4.0% increase per decade for the same level of ionisation in the E-region. The Oersted main field model does not suggest any changes or reversal of these trends. A very significant result is that the magnetic field at the dip equator need not be perpendicular to the line of zero dip and this deviation from the idealized scenario of the equatorial electrojet can result in significant contribution of the equatorial electrojet to the declination component. The scalar data from 57 orbital passes from four representative quiet days, covering the forenoon to afternoon hours, on the other hand, provides a greater insight into the relative importance of geomagnetic field structure and tidal control. This is obtained through a study of the day-to-day variability of the equatorial electrojet and its longitudinal variations. The Equatorial Electrojet can be clearly identified on almost all the passes with the peak always lying on the dip equator. There is a clear indication of zonal variation in the electrojet strength at satellite height. The largest amplitude is found in the American sector where the Cowling conductivity attains its maximum value. The electrojet strength at the satellite height also exhibits other maxima at around 100 and 190°E longitude consistent with the earlier observations of POGO. Variations at these longitudes could perhaps be linked with longitude dependent forces associated with background winds or non-migrating tides. The width of the electrojet at satellite height also seems to show a maximum in the South American sector.