AN ALGORITHM FOR ESTIMATING FIELD-ALIGNED CURRENTS FROM SINGLE SPACECRAFT MAGNETIC-FIELD MEASUREMENTS - A DIAGNOSTIC-TOOL APPLIED TO FREJA SATELLITE DATA

We introduce an algorithm which allows under certain assumptions to es timate the current density distribution along the track from single sp acecraft magnetic field measurements, The assumptions are chosen that they meet at least partly the conditions encountered by the Freja sate llite on its northernmost orbital segment, namely that all currents ar e field-aligned, ionospheric currents do not contribute significantly to the magnetic field measurements, and the velocity component perpend icular to the field direction is large (> 5 km/s), Problems arise in t he case of moving or temporally varying current systems, In those case s additional data like ground-based observations are needed to resolve the spatio-temporal ambiguity, With the help of simulated data we can show that for most of the encountered stationary current geometries t he 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 o f amoral plasma processes.