Time Evolution of the Substorm Current Wedge from Ground and Space-based Magnetic Fields

Abstract

Over the past several years, intensive efforts have resulted in a significant improvement in the ground instrumentation for auroral studies in North America. A major part of this is due to the THEMIS ground program, both in the U.S. and in Canada. The THEMIS Ground-Based Observatory network has fielded 10 additional magnetometers in Canada and Alaska. Further THEMIS magnetometers are part of the GEONS outreach effort, found in the continental U.S. and Alaska. Athabasca University initiatives and collaborations have made yet further magnetometer data available, most notably from a new network of instruments in central Alberta. Several stations of the University of Tokyo STEP network remain operational, and some have been upgraded. There is finally a dense enough set of magnetic data that techniques based on forward modeling, and most relevant to the opportunity afforded by THEMIS, Automated Regional Modeling (ARM), can be reliably used. These techniques specify where net field-aligned current (FAC) and ionospheric electric current flow are located. In some cases the Pedersen system can also be included based on data. Even when it is not, it can be considered collocated with electrojet locations given by ARM. The extension into space of the FAC (net or Pedersen) allows comparison with the perturbations observed at THEMIS. We present results from an event on March 13, 2007, during which THEMIS in its early orbital configuration was over central North America, clear weather prevailed, and a substorm took place whose perturbations were ideally suited for inversion using ARM. At about 5 UT, activations were detected from the ground with magnetic perturbations also detected from THEMIS above the affected stations. The ground perturbations are very well represented by a three-dimensional substorm current wedge (SCW) system, and perturbations in space indicate radial propagation at a time when the electrojet was expanding poleward. Little longitudinal propagation of the SCW is suggested by the ground data.