Improved Field‐Aligned Current and Radial Current Estimates at Low and Middle Latitudes Deduced by the Swarm Dual‐Spacecraft

Ionospheric currents have widely been investigated by using magnetic measurements from low-Earth orbiting satellites. However, the assumptions of deriving currents from the magnetic measurements have not always been well considered. 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 of European Space Agency (ESA's) Swarm constellation. Data considered cover a 5-year period, from 17 April 2014 to 16 April 2019. We found for most of the cases, the IRCs and IHFACs derived from both approaches show consistent latitudinal profiles. However, there are several cases with discrepancy exceeding 5 nA/m(2) between two approaches. On average, the diurnal variations of IHFACs from both approaches agree well 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 clear tidal signatures, while such signature cannot be found in the IRCs from dual-SC approach. We interpret these tidal-signature of IRCs as spurious results, caused by equatorial electrojet contributions to the increment B-y component. The dual-SC derived IRCs show notable differences between ascending and descending orbits. Such differences might be due to a violation of the assumed perfect calibration of Swarm A and C. We suggest a systematic spacecraft-fixed bias in the along-track magnetic field component (Bx) between Swarm A and C. By interpreting the IRC differences, we obtain bias values of increment Bx reaching 1 nT. Our results reveal that ionospheric currents are better characterized by the dual-SC approach. But comparison with single-satellite current estimates can help to identify weakness.