A comparison of EISCAT and HF Doppler observations of a ULF wave

Since the middle of 1995, an HF Doppler sounder has been running almost continuously in northern Norway, with the receiver at Ramfjordmoen and the transmitter at Seljelvnes. Concurrent operation of the EISCAT UHF radar in common programme (CP-1) mode has made it possible to study the ionospheric signature of a magnetospheric ULF wave. These are the ®rst results of such wave signatures observed simultaneously in both instruments. It has been demonstrated that the observed Doppler signature was mainly due to the vertical bulk motion of the ionosphere caused by the electric ®eld perturbation of the ULF wave and the ®rst direct observational con®rmation of a numerical simulation has been achieved. The wave, which was Alfve nic in nature, was detected by the instruments 8° equatorward of the broad resonance region. The implications for the deduced wave modes in the ionosphere and the mechanism producing the HF Doppler variations are discussed.