Testing Field-Line Mapping of Electric Fields in Earth's Magnetosphere

The convection electric field plays a fundamental role in transporting plasma within the magnetosphere. A widely accepted assumption in both observational and modeling studies is that the electric fields can be instantaneously mapped along magnetic field lines. To evaluate the validity of this assumption, we conducted a global magnetohydrodynamic (MHD) simulation under a southward interplanetary magnetic field, focusing on the electric fields at midnight. Major findings are as follows: (a) The mapping between magnetospheric and ionospheric electric fields is generally imperfect, even when propagation and travel time of Alfvén waves are considered. (b) During the substorm growth phase, ionospheric electric fields are ∼1.5 times larger than those in the magnetosphere. Induction electric fields significantly reduce magnetospheric electric fields. (c) In the substorm expansion phase, the electric fields are highly variable, and the ionospheric electric fields are typically smaller than the magnetospheric electric fields by a factor of ∼6–8. (d) At low L-shells (L = 4–5), magnetic footprints are close to Alfvénic footprints. (e) At high L-shells (L = 6–10), a substantial mismatch exists between magnetic and Alfvénic footprints. The discrepancies between magnetospheric and electric fields are attributed to induction electric fields, external forces acting on plasma, and the insufficient number of interactions between the magnetosphere and the ionosphere. We conclude that field-line mapping of electric fields is marginally valid at low L-shells (L < 5) and under quasi-steady conditions, despite some differences in magnitude. Future studies incorporating dispersive and kinetic Alfvén waves are needed to obtain definitive conclusions.