The Pedersen current carried by electrons: effects on magnetosphere-ionosphere coupling

Buchert, S.

Swedish Institute of Space Physics, Uppsala, Sweden

Irregularities in the auroral electrojet occur when the electric field imposed by the large scale plasma convection exceeds a threshold, and the Farley-Buneman instability is excited. Then significant heating manifests itself by enhancements of the electron temperature seen in incoherent scatter radar data. The heating is powered by large scale plasma convection transfering electromagnetic energy via field-aligned currents (FACs) into the lower E region where the FACs are closed by a dissipative current. Theory shows that first order correlations between variations of charge density and electron velocity in the presence of irregularities affect the direct current, mainly its direction, turning it partially from a Hall into a Pedersen current.

The effect may be parameterized by an anomalous electron-ion collision frequency and corresponding effective conductivities. Thus Ohm's law for the lower ionosphere becomes non-linear, locally strongly, and integrated over height only slightly. Models to calculate values for the anomalous collision frequency under given conditions have been proposed, also methods to estimate them from observations. I review these works and compare the predictions and results for a selected event where a relatively sustained high electric field is seen. Birkeland currents and electric field are coupled together on large scales by a ionosphere-magnetosphere coupling equation that results from the current continuity and Ohm's law. Thus different estimates of Birkeland currents are obtained depending on whether classical or effective ionospheric conductivities are assumed. I compare estimates of the current for the high electric field event.