Ionospheric HF pump wave triggering of local auroral activation

Experimental results from ionospheric HF pumping experiments in the nightside auroral E region above Tromso are reported. We found intriguing evidence that a modification of the ionosphere-magnetosphere coupling, due to the effects of powerful HF waves beamed into an auroral sporadic E layer, can lead to a local intensification of the auroral activity. Summarizing multi-instrument observations during two consecutive nights, one can distinguish the following peculiarities related to this auroral activation: a modification of the auroral arc and its breakup above Tromso, local changes in the horizontal currents above Tromso, a burst-like increase of the electron density and temperature, a large increase in the ion temperature in a wide altitude range and in the north-south component of the electric field, distinctive features in dynamic HF radio scatter Doppler spectra, pump-induced electron precipitation, and substorm activation exactly above Tromso. We discuss the modification of the ionosphere-magnetosphere coupling in terms of the excitation of a turbulent Alfven boundary layer between the base of the ionosphere (similar to100 km) and the level of sharp increase of the Alfven velocity (at heights up to one Earth radius), and the formation of a local magnetospheric current system. The results suggest that a possible triggering of local auroral activation requires specific geophysical conditions.