The Closure of the Hall Currents During Substorms and Its Ground Magnetic Effects

In the study of high-latitude electrodynamics of the M-I system, two assumptions have been widely used. One is that the field-aligned currents are solely closed by the Pedersen currents in the ionosphere and the Hall currents are divergence-free. The other is that the ground magnetic disturbances associated with the currents in the ionosphere are caused by the Hall currents only. By using an M-I coupling model, in which the electrodynamics is fully self-consistent via an Alfven wave approach, and a magnetic inversion model, we will clearly demonstrate that these two assumptions can become totally invalid when the M-I system is in disturbance conditions, especially during substorm periods. We will quantitatively show how a significant amount of the Hall currents in the substorm onset regions is closed by the field-aligned currents and how the Pedersen and field-aligned currents make significant magnetic disturbances on the ground. We conclude that the electrodynamic M-I coupling approach characterized by Alfven waves is essential for a more accurate and realistic description of the high-latitude ionospheric electrodynamics as well as the associated ground magnetic disturbances.