Electron energization by Alfven waves: Freja and sounding rocket observations

Recent sounding rockets observations highlight some space-time ambiguities in phenomena taking place in the auroral ionosphere, previously observed by the Freja spacecraft. Field aligned electron populations with energies fro m a few tens of eV to a few keV streaming through cold background plasma ar e usually observed in the auroral zone in association with strongly nonline ar electromagnetic perturbations. Simultaneous observations of bursty Langm uir emissions indicate the unstable character of these plasma distributions . A detailed analysis shows that the electromagnetic perturbations take the form of quasi-stationary nonlinear spatial structures related to inertial Alfven waves. We suggest that inhomogeneous altitude distribution of the pa rallel electric field of these structures due to ionospheric density gradie ntis responsible for the observed electron distributions.