Characterization of ionospheric doppler oscillations in the Pc3–4 and Pi2 magnetic pulsation frequency range

Abstract

The conditions and mechanisms leading to the observation of perturbations in ionospheric records due to downgoing ULF hydromagnetic waves are examined. About 80 such events recorded at L = 2.1 with a vertical-incidence Doppler sounder and ground magnetometer have been analysed in detail. The dependence on magnetic activity, altitude and pulsation frequency are compared with model predictions. Doppler oscillations associated with Pc3–4 pulsations of amplitude ⩽3.0 nT on the ground are most likely due to wave-driven compression and rarefaction of the ionospheric plasma. At larger amplitudes an E × B mechanism is more important. The reverse holds for night Pi2 events. It is also possible to make estimates of the pulsation scale size in the ionosphere. These suggest the magnetic pulsations are due to field line resonances and global cavity mode oscillations. Shortcomings of the existing interaction models are also discussed. These include the assumption of purely transverse Alfvén waves incident on the ionosphere. In the real case fast and coupled wavemodes are also likely to be present. The proximity to a field line resonance thus determines the form of the interaction between the downgoing hydromagnetic waves and the ionosphere. Temporal variations in the contributions of the various interaction mechanisms, and in the relative amplitude of the o and x mode electromagnetic waves, further complicate the relationship between the magnetic pulsations and the observed Doppler oscillations.