An algorithm for estimating field-aligned currents from single spacecraft magnetic field measurements: a diagnostic tool applied to Freja satellite data

Introduces an algorithm which allows under certain assumptions to estimate the current density distribution along the track from single spacecraft magnetic field measurements. The assumptions are chosen that they meet at least partly the conditions encountered by the Freja satellite on its northernmost orbital segment, namely that all currents are field-aligned, ionospheric currents do not contribute significantly to the magnetic field measurements, and the velocity component perpendicular to the field direction is large (>5 km/s). Problems arise in the case of moving or temporally varying current systems. In those cases additional data like ground-based observations are needed to resolve the spatio-temporal ambiguity. With the help of simulated data the authors can show that for most of the encountered stationary current geometries the estimates fall into an uncertainty band of ±20%. Current density estimates are a local quantity with a spatial resolution of order 1 km. They are thus very suitable for use in studying the fine structure of auroral plasma processes