A traveling convection vortex event study: Instantaneous ionospheric equivalent currents, estimation of field‐aligned currents, and the role of induced currents

Abstract

We analyze a traveling convection vortex (TCV) event on 31 January 1997 using ground magnetometer data of the CANOPUS, MACCS, Geological Survey of Canada, Greenland, and IMAGE networks. For the first time, spatial and instantaneous distributions of the mesoscale ionospheric equivalent currents associated with a TCV are obtained. We apply the method of spherical elementary currents (SECS) to calculate these currents, as well as to infer the part of the ground magnetic signatures that is caused by internal currents induced in the Earth. The resulting ionospheric equivalent currents consist of a leading clockwise vortex centered at 71° CGM latitude which moves westward with ∼7.3 km s−1 and a trailing anticlockwise vortex at 77 ° latitude which moves southwestward away from noon at ∼3.0 km s−1. In an area of ∼200 km around the center of the twin vortices the derived equivalent current densities are less than 70 mA m−1 but reach 100–160 mA m−1 in a broad channel of equatorward currents between the vortices. Using the assumption of a uniform Hall to Pedersen conductance ratio and the assumption that conductance gradients perpendicular to the ionospheric electric field are vanishing, we can estimate the field‐aligned current (FAC) associated with the TCV. The maxima of ∼1 μA m−2 of downward FAC in the leading western vortex, and of ∼0.4 μA m−2 of upward FAC in the trailing eastern vortex occur along the perimeter of the vortices, not in their centers, in contrast to the prediction of ionosphere‐magnetosphere coupling theories for TCVs. The integrated FAC are not fully balanced between the two vortices but show an excess of downward FAC. While the ratio between the internally generated and the total horizontal ground magnetic field for most magnetometer sites in the TCV area amounts to around 20–40%, at some stations it can reach values of 50% and larger.