Influence of the Polar Electrojet on Field-Aligned Current Estimates From Single Satellite Magnetic Field Measurements

The high-resolution magnetic field measurements from European Space Agency's (ESA's) Swarm satellite constellation provide a good opportunity for revisiting the mean properties of ionospheric currents. Among the Swarm Level-2 data products, provided by European Space Agency, are field-aligned current (FAC) estimates based on single-spacecraft (single-SC) and dual-spacecraft (dual-SC) solutions. For the more reliable dual-SC approach only magnetic signatures from currents flowing through the integration loop are considered, while in the case of single-SC FAC estimates the magnetic effects of all remote current systems contribute to the results. A direct comparison between the two FAC products at auroral latitudes reveals that the single-SC estimates systematically overestimate the current density of region 2 (R2) FACs by about 15%, while underestimates the region 1 (R1) FACs by about 10%. The differences between the two FAC products appear closely related to the location and direction of the horizontal polar electrojet (PEJ) at auroral latitudes. A direct comparison of these two current systems suggests an influence of the PEJ induced magnetic field on the solutions from single-SC FACs. Our preferred explanation is the effect of a partial diversion of the anti-sunward flowing electrojet current into the polar cap region. The current deflection angle increases toward the pole. The B-field of such a current flow geometry can explain the single-SC differences. It is consistent with the general picture of the high-latitude Hall current vortices. As a side aspect, we found that the mean PEJ intensities are almost identical in both hemispheres, but the latitudes of the current peaks differ between the two hemispheres in the morning/afternoon sectors by about 1 degrees. The reason for this difference is not clear.