A new model of field-aligned currents derived from high-precision satellite magnetic field data

A model of field-aligned currents is derived from high-precision magnetic field measurements from the Ørsted and Magsat satellites, being parameterized by the interplanetary magnetic field strength and direction for summer, winter and equinox. The high-precision data allow the model to be determined directly by a simple 2-D curl technique combined with fitting of spherical harmonic functions. New elements of the model are: (a) the FAC patterns are determined separately for both polar regions, resolving the seasonal dependence of interhemispheric asymmetries, (b) the IMF ∼ 0, ground-state patterns are also resolved; these elements are obtained for the first time. From the model, the total upward/downward currents have been determined for various IMF conditions. The ratio of the summer/winter currents is ∼1.35 and the equinox currents ∼1. The model allows FAC mapping for IMF |B| ≤ 12 nT, except during magnetic storms and substorms.