Improved field-aligned current and radial current estimates at low and middle latitudes deduced by the Swarm dual-spacecraft

The Swarm satellite constellation provides an excellent opportunity to explore ionospheric current systems. In this study we performed a detailed analysis of the ionospheric radial current (IRC) and inter-hemispheric field-aligned current (IHFAC) estimates at equatorial and low latitudes derived from the single-satellite and dual-spacecraft (dual-SC) approaches. We found that the diurnal variations of average IHFACs from both approaches agree qualitatively with each other for all seasons. But the amplitudes of single-satellite results reach only about 70% of those from the dual-SC. This difference is attributed to the fact that only the magnetic field By component is utilized in the single-satellite approach, while both Bx and By components are considered in the dual-SC approach. Above the magnetic equator, the IRCs derived from single-satellite approach show spurious tidal signatures, caused by equatorial electrojet (EEJ) contributions to the dBy component. The EEJ does not contaminate dual-satellite results. Further, we improved the IHFACs from the dual-satellite approach by considering the local influence of the ambient magnetic field on current densities and normalize them to their ionospheric E-region footprints. Then we extend the analysis to ±60° MLat; the middle latitude IHFACs show features different from those at low latitudes. They are dominated by longitudinal wave-1 and wave-2 patterns. A superposition of these tidal components reflects confinement of the IHFAC modulation to daytime and the western hemisphere. The tidal signatures at middle latitudes are better organized in universal time than in local time. The strongest IHFACs appear at 18-19 UT, near noon in the American sector. This is related to the overlap with the South Atlantic Anomaly.