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

• Published in: Journal of geophysical research. Space physics 2022-06, Vol.127 (6), p.n/a
• Main Authors: Wang, Fengjue, Lühr, Hermann, Xiong, Chao, Zhou, Yunliang
• Format: Article
• Language: English
• Subjects: Bias; Diurnal variations; Earth orbits; Electrojets; Equator; Equatorial electrojet; Estimates; inter‐hemisphere field‐aligned currents; Ionospheric currents; ionospheric radial current; Latitude; Magnetic equator; magnetic field measurement; Magnetic fields; Magnetic measurement; Satellite tracking; Satellites; single‐satellite and dual‐satellite; Spacecraft; Spacecraft tracking; Swarm
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2022JA030396

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/m2 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 ∆By 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 ∆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. Key Points The amplitudes of interhemispheric FACs from single‐satellite amount only to about 70% of those from dual‐SC due to ignored magnetic field By component We interpret the tidal‐signature of ionospheric radial currents (IRCs) from single‐satellite is spurious, as being due to contributions to the ∆By component from the equatorial electrojet The dual‐SC derived IRCs show nota