Closure of the Auroral Current Loop.

Abstract:

How the magnetosphere processes electro-dynamic energy and creates quiescent auroras on closed field line regions is a central question in magnetospheric physics. In a commonly accepted simple model azimuthal pressure gradients produce radial currents in the magnetosphere and parts of these currents are shunted through field-aligned currents (FAC-s) and then Pedersen currents in the ionosphere. In an example where the field aligned currents were measured by FAST and Polar satellites we have shown that typical azimuthal pressure gradients are adequate to generate the magnetospheric B_perp currents to produce the observed FAC-s. However to power the aurora the B_perp currents from the magnetospheric generator have to turn into FAC-s. In the magnetosphere the FAC-s are produced by the divergence of the B_perp currents [Vasyliunas, 1970]. It can be shown that the B_perp divergence is caused by the non-uniformity in the magnetic field magnitude. If the generator current is radially earthward directed (dusk sector) then the gradient of the field magnitude has to be directed outward for the divergence to produce the upward Region 1 currents. The location of these Region 1 currents are thought to be where the dusk side auroral arcs occur. Such outward B field gradients would be reversed from the magnitude gradients of the Earth main field. There are several components of the tangential currents that could cause outward B magnitude gradients. Using Cluster satellite measurements we calculated field magnitude gradients, their direction and magnitude searching for regions with outward magnetic field magnitude gradients. It should be noted that the reversal of plasma pressure gradients could also occur, which would also create the correct divergence for upward FAC-s however pressure gradient reversal would also reverse the magnetospheric dynamo current and would invalidate the simple model of the radially inward currents on the duskside.