Cluster Observations of Ionosphere-Directed Poynting Flux at the High Latitude Plasma Sheet Boundary

We present measurements by the Cluster satellites of Poynting flux directed toward the ionosphere on auroral field lines. During February/March, 2001, the Cluster perigee passes cut through the auroral zones in the near-midnight region, at approximately 5 RE. In several of these passes, Alfven waves, carrying Poynting flux primarily toward the ionosphere, were observed over a few degrees of invariant latitude in the high-latitude plasma sheet boundary region. Using data from the Electron Drift Instrument (EDI) and the Fluxgate Magnetometer (FGM), we calculate the Poynting flux of the waves, explore their spatial extent as constrained by the Cluster satellites, and examine their relation to convection boundaries. These waves presumably correspond to those recently reported by Wygant et al. and Keiling et al., although the flux in the Cluster events analyzed to date ( ~0.1 erg/cm²2-s) is an order of magnitude lower than the most intense events seen by Polar at similar altitudes.