Latitude Dependence of Interhemispheric Field-Aligned Currents (IHFACs) as Observed by the Swarm Constellation

Abstract

The daytime ionospheric E region hosts a vortex of horizontal equivalent currents driven by the wind dynamo in each hemisphere: the solar quiet current system. Differences in the dynamo actions at conjugate points in the two hemispheres drive interhemispheric currents (interhemispheric field-aligned currents [IHFACs]). Despite the long history of investigations on this topic, only a few studies have reported a latitude dependence of the IHFACs. In this study we make use of the magnetic field observations from the European Space Agency's Swarm constellation to address the latitude dependence of IHFACs in depth. At low latitudes (<35° magnetic latitude), the statistics generally agree with previous findings from other satellite missions. However, the polarity of currents changes at magnetic latitude of about ±35°. The magnetic latitude dependence of IHFACs can be reproduced by the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model, when the lower boundary of the model (~30 km) is properly constrained with meteorological reanalysis data. As for the seasonal variations, the midlatitude (>35° magnetic latitude) IHFAC climatology during equinoxes is generally similar to that of June solstice while that of December solstice exhibits stand-alone behavior.