Alfvenic electron acceleration in aurora occurs in global alfven resonosphere region

Auroral emission caused by electron precipitation (Hardy et al., 1987, J. Geophys. Res. 92, 12275-12294) is powered by magnetospheric driving processes. It is not yet fully understood how the energy transfer mechanisms are responsible for the electron precipitation. It has been proposed (Hasegawa, 1976, J. Geophys. Res. 81,5083-5090) that Alfven waves coming from the magnetosphere play some role in powering the aurora (Wygant et al., 2000, J. Geophys. Res. 105, 18675-18692, Keiling et al., 2003, Science 299, 383-386). Alfven-wave-induced electron acceleration is shown to be confined in a rather narrow radial distance range of 4-5 R-E (Earth radii) and its importance, relative to other electron acceleration mechanisms, depends strongly on the magnetic disturbance level so that it represents 10% of all electron precipitation power during quiet conditions and increased to 40% during disturbed conditions. Our observations suggest that an electron Landau resonance mechanism operating in the Alfven resonosphere is responsible for the energy transfer.