Spatial Structure and Asymmetries of Magnetospheric Currents Inferred from High-Resolution Empirical Geomagnetic Field Models

Abstract:

Reconstruction of the large-scale magnetospheric current systems from data has long been based on ad hoc assumptions regarding their spatial structure. A dramatic increase of amount of data provided by space-borne magnetometers from geosynchronous satellites, IMP 8, Geotail, Polar, Cluster, THEMIS, and Van Allen Probes missions enabled the development of a new approach to empirical geomagnetic field modeling. In this approach the custom-tailored modules prescribing the configuration of magnetospheric current systems were replaced by basis function expansions making the model structure free from previous a priori constraints. The new approach reveals a complex structure of the magnetospheric current systems and, in particular, their substantial dawn-dusk asymmetry during magnetic storms. This includes the formation of the hook-shaped current in the main phase, the double partial ring current near the Sym-H minimum, ring current erosion, and near-magnetopause eastward current in the pre-noon sector. With the help of data from the Van Allen Probes mission, the highest-resolution empirical models have resolved the eastward current in the innermost magnetosphere and its local-time asymmetry, including the so-called banana-current structures. At the same time, the increase of the number of degrees of freedom of the empirical model in the description of field-aligned currents has shown that the hook-shaped equatorial current corresponds to the spiral structure of the large-scale upward Birkeland currents, which can now be resolved in detail by AMPERE.