Observational features of field line resonances excited by solar wind pressure variations on 4 September 1984

The purpose of the study is to establish the most probable excitation mechanism of the magnetic storm occurred after an inverse sudden impulse on September 4, 1984. Geomagnetic pulsations in the Pc5-frequency range observed at magnetometer stations are evaluated. Attention is focused on two events of the enhanced activity: for the first one, conjugate observations on the ground are assessed and then compared with satellite-based observations on adjacent field lines; for the second event two hours later, data from an extended azimuthal range is employed. It is pointed out that the observations are consistent with the theory of filed-line resonance, and may be interpreted as excitations caused by pressure variations in the solar wind. Both magnetopause-surface waves and cavity resonances are excited; the cavity mode drives toroidal field-line oscillations at locations where its frequency matches the resonance frequency of the field lines.