Multi-scale Reconstruction of Field-Aligned Currents Using the Swarm Spacecraft

Field-aligned currents (FACs) are a primary channel for the transport of energy and momentum from the magnetosphere into the ionosphere, where they strongly influence atmospheric dynamics through Joule heating. Due to significant spatial and temporal variability, accurately determining FAC density vectors from magnetic field measurements remains challenging. Amongst available techniques, the curlometer method applied to multi-spacecraft magnetic field observations provides the most reliable means of estimating current density. A full three-dimensional reconstruction requires magnetic field measurements at four distinct locations arranged in a near-regular tetrahedral configuration. In contrast, configurations involving fewer spacecraft may be employed, though this approach relies on the assumption of magnetic field stationarity and favourable spacecraft alignment. Here, we investigate current density reconstruction from Swarm magnetic field measurements and evaluate associated quality metrics for a range of conditions for scenarios, spanning macro-, meso-, and micro-scale FAC structures during geomagnetic storms. We then apply this method to simulated trajectories of the tetrahedral configuration proposed for the ROARS F3 mission concept, quantifying the improvements in FAC estimation enabled by a dedicated four-spacecraft mission to Low Earth Orbit.