Abstract:  This talk explores the high-latitude Alfvénic response of the transition region between the topside ionosphere and magnetosphere to geomagnetic storms. From superposed epoch and storm phase–dependent analyses, we find that the occurrence rate of small-scale Alfvénic field variations through this region increases by as much as a factor of 5 shortly after storm commencement relative to pre-storm levels. This increase is accompanied by order-of-magnitude enhancements in coincident energy deposition rates into the ionosphere and ion outflow rates into the magnetosphere, particularly near noon, pre-midnight and on the dawn flank. During main phase on the dayside, these waves shift to lower invariant latitudes (ILATs), expand over a larger range of ILATs and magnetic local times (MLTs) and are associated with significant enhancements in upward ion flux. On the nightside the overall storm-enhanced occurrence probability is lower than that on the dayside, as is the associated upward ion flux, but the average precipitating electron energy flux is larger. There is also a localized region of intense ion outflow pre-midnight at low latitudes during storm main phase. Wave occurrence rates subside to pre-storm levels about 20 hours after storm commencement.