Direct Reconstruction of the Magnetospheric Magnetic Field and Electric Currents from Spacecraft Data

Abstract:

A new method is proposed to derive magnetospheric magnetic field configurations from spacecraft data.
In contrast to earlier empirical models, fully or partially based on "custom-made" modules for the field sources, the
new appoach is free of any a priori assumptions on the spatial structure of electric currents. The external part of
the geomagnetic field is represented by the sum of poloidal and toroidal terms, each expanded into series of radial
basis functions (RBF) with their nodes regularly distributed over the 3D modeling domain. The feasibility of the
approach has been verified by reconstructing the inner and high-latitude magnetospheric field within geocentric
distances up to 12Re on the basis of magnetometer data of Geotail, Polar, Cluster, Themis, and Van Allen space probes,
taken during 1995-2015. Four characteristic states of the magnetosphere before and during a disturbance have been
modeled: a quiet pre-storm period, storm deepening phase with progressively decreasing Sym-H index, the storm maximum
around the negative peak of the Sym-H, and the recovery phase. Fitting the RBF model to data faithfully resolved
contributions to the total magnetic field from all principal sources, including the westward and eastward ring current,
the tail current, diamagnetic currents associated with the polar cusps, and the large-scale effect of the field-aligned
currents. For the main phase conditions, the model field reveals a strong dawn-dusk asymmetry of the low-latitude
magnetic depression, extending to low altitudes and partly spreading sunward from the terminator plane in the dusk
sector. It is also demonstrated that large tilt angles of the Earth's dipole with respect to the terminator plane
result in largely different magnitudes and spatial configurations of the diamagnetic depression in the northern and
southern cusps. Varying the spatial extent and coverage density of the RBF nodes allows one to adjust the resolution
and focus the modeling at specific regions of interest of the geospace, which opens an interesting possibility to develop the
proposed method into a powerful tool for data-based studies of the magnetospheric currents.