A synthetic view of the magnetospheric-ionospheric current system associated with substorms

Electric currents play an important role in the energy coupling between the magnetosphere and ionosphere. This paper examines in detail the distribution and dynamics of ionospheric electric currents during an isolated substorm. In particular, the relative contributions of ionospheric Pedersen and Hall currents to field-aligned currents are evaluated in hoping to shed some new light on the ionospheric closure of the substorm current wedge. It is found that the substorm-related westward electrojet is dominated by Hall current, which is 2~3 times the Pedersen current in magnitude. Under winter-time substorm conditions, a significant portion (up to more than 1 MA) of field-aligned currents is closed in the ionosphere through Hall current. Although the contribution of Hall current to field-aligned currents is about 30~50% compared to the contribution from Pedersen current, it is Hall current, not Pedersen current, that appears to be the main contributor to the substorm current wedge.