The equatorial electrojet and associated currents as seen in Magsat data

Abstract

Magsat data are re-examined with regard to the presence and character of fields due to the equatorial electrojet and meridional currents at dawn and dusk local times. Dip-latitude organized field variations at dawn are: 1. (1) extremely weak, 2. (2) extremely variable with longitude, 3. (3) inconsistent with the pattern expected from a line or narrow sheet current. It is shown that the use of Magsat dusk data can ‘contaminate’ a main field model, introducing apparent equatorial electrojet effects into the dawn data. Fields due to the equatorial electrojet and (presumably) associated meridional currents are clearly present in the dusk data. They show a variation with longitude which is apparently associated with the longitudinal variation of the strength, or square of the strength, of the main field in the E-region. Also evident is a variation with time of the year, although data are available for only a six month period. The meridional currents are generally minimum during January and February and maximum either during November and December or March and April, depending upon longitude. The E-region horizontal currents are minimum in November and December and maximum in March and April, except for − 30° to −90° longitude when the maximum occurs in January and February. Assuming that field gradients in local time are considerably smaller than field gradients in dip-latitude, current densities are estimated to be 1–3.6μA/m 2 for the horizontal current at 110km and about 10–20 × 10 −9 A/m 2 for the vertical currents at 400km altitude. These results confirm and extend earlier results of Takeda and Maeda. Most models of the electrojet system in the literature disagree severely with these measurements either because their scope is inadequate or because of the wind system they assume. Those models which best describe the data invoke an eastward wind and/or an eastward electric field at dusk local time.