Testing Conventional Assumptions for High-Latitude Electrodynamics by Using a Self- Consistent M-I Coupling Model

Two conventional assumptions have been widely used in the study of high-latitude electrodynamics of the M-I system. One is that the field-aligned currents are solely closed by the Pedersen currents in the ionosphere; the Hall currents are basically divergence-free and their contribution to the field-aligned currents can be ignored. The other is that the ground magnetic disturbances associated with the currents in the ionosphere are caused by the Hall currents only and the combined ground magnetic effects of the field-aligned currents and Pedersen currents are negligible. By using both an M-I coupling model, in which the M-I coupling electrodynamics is fully self- consistent via an Alfven wave approach, and a global magnetic inversion model, we quantitatively tested the validity of the two assumptions. It was found that in disturbed conditions, especially during substorms, these two assumptions can become invalid. Our results show that in the substorm break-up regions, the field-aligned currents are dominantly closed by the Hall currents with small-scale channel structures and sharp spatial gradients. These structured Hall current closures are associated with the rotation of convection field or a significantly distorted localized convection pattern. Our results also show that during substorms, the combined Bz magnetic effects of the Pedersen and field-aligned currents are still negligible, but the By and Bx magnetic effects of them increase significantly and become comparable with those of the Hall currents. These results indicate that care must be exercised when using the two conventional assumptions in model studies and data interpretations of the high-latitude electrodynamics.