  Experimental results from Tromso HF pumping experiments in the nightside
auroral E region 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
ground-based observations and observations from the IMP 8 and IMP 9 satellites,
one can distinguish the following peculiarities related to this auroral
activation: modification of the auroral arc and its break-up above Tromso;
local changes of the horizontal currents in the vicinity of Tromso; increase of
the electron temperature and ion velocities at altitudes above the HF pump
reflection level; distinctive features in dynamic HF radio scatter Doppler
spectra; pump-induced electron precipitation; substorm activation exactly above
Tromso. The mechanisms of the modification of the ionosphere-magnetosphere
coupling through the excitation of the turbulent Alfven boundary layer between
the base of the ionosphere (~100 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 are discussed. The results suggest that a
possible triggering of local auroral activation requires specific geophysical
conditions.
