Ionospheric F region currents at middle and low latitudes estimated from Magsat data

Abstract

Two methods for determining the F region current density at satellite heights by means of a decomposition of the magnetic field into toroidal and poloidal parts are presented. The first method allows for determination of the radial current component only by neglecting the height variation of the satellite orbit. Application to Magsat data yields the well‐known field‐aligned currents of polar latitudes (average current density Jr ≲ 200 nA/m2) and the meridional current system of the equatorial electrojet (Jr ≈ 10 – 20 nA/m2) during dusk. In addition, a midlatitude interhemispheric current system could be observed for the first time. Current direction is from the winter to the summer hemisphere during dusk and in opposite direction during dawn, as predicted by three‐dimensional models of the ionospheric dynamo. Presumably, for the first time a lunar contribution to the meridional current system of the equatorial electrojet was found, too. Amplitude at the dip equator is Jr ≈ 1.8 nA/m2; time of maximum is in agreement with magnetic ground observations, ionospheric electric field measurements, and ionospheric dynamo theory. The second method allows for estimation of both the radial and the horizontal current density by expanding the radial dependence of the magnetic field in Taylor series. First results confirm the current direction of the low‐latitude meridional current system as suggested by dynamo calculations: upward currents at the dip equator and field‐aligned downward currents at low latitudes.