High-latitude HF Doppler observations of ULF waves. 1. Waves with large spatial scale sizes

A quantitative study of observations of the ionospheric signatures of magnetospheric ultra low frequency (ULF) waves by a high-latitude (geographic: 69.6°N 19.2°E) high-frequency Doppler sounder has been undertaken. The signatures, which are clearly correlated with pulsations in ground magnetometer data, exhibit periods in the range 100±400 s and have azimuthal wave numbers in the range 3±8. They are interpreted here as local ®eld line resonances. Phase information provided by Oand X-mode Doppler data support the view that these are associated with ®eld line resonances having large azimuthal scale sizes. The relative phases and amplitudes of the signatures in the Doppler and ground magnetometer data are compared with a model for the generation of Doppler signatures from incident ULF waves. The outcome suggests that the dominant mechanism involved in producing the Doppler signature is the vertical component of an E ́ B bulk motion of the local plasma caused by the electric ®eld perturbation of the ULF wave.