Interhemispheric Currents Associated with Ionospheric and Magnetospheric Drivers (Invited)

The high mobility of low-energy electrons along magnetic-field lines throughout most of the magnetosphere and ionosphere strongly facilitates the flow of magnetic-field-aligned current (FAC). The FAC is fed by divergent currents perpendicular to the magnetic field that are driven by winds in the ionospheric dynamo region, by plasma pressure gradients in the magnetosphere, and by other forces. Magnetospheric sources drive net FAC into or out of both hemispheres of the ionosphere. In addition, interhemispheric FAC will flow whenever different ionospheric conductivities or thermospheric winds exist at the northern and southern ionospheric footpoints of closed magnetic-field lines. The magnetic-field-aligned electric field associated with the FAC is usually much smaller than the electric-field component perpendicular to the magnetic field, such that the parallel electric field can often be neglected altogether for modeling purposes. Model simulations illustrate the nature of interhemispheric FAC associated with hemispheric asymmetry of the conductivities, winds, and geomagnetic field. We review the results of modeling studies and examine the relative importance of the different factors affecting interhemispheric FAC.