J. Oksman’s research while affiliated with Loyola University Maryland and other places

The characteristics of a large-amplitude hydromagnetic wave event accompanied by cosmic noise absorption pulsations of similar frequency, that was observed by subauroral ground stations distributed in altitude and longitude, are investigated. The event occurred during the decay of a minor geomagnetic storm; the waves propagated westward over three hours of local time, near dawn. During the duration of the event, a hydromagnetic pulsation train in the Pc 5-6 emerged, with frequencies decreasing monotonically from about 3 mHz to about 1 mHz, accompanied by similar oscillations of the cosmic radio noise signal. The event is characterized by an azimuthal wave number of about 9, and a westward phase velocity varying from about 2.5 to about 7.5 km/s at 55 deg magnetic latitude. The low azimythal phase velocities make it unlikely that the waves were generated by the Kelvin-Helmholtz instability on the magnetopause. The Pc 5-6 waves appear to be odd-mode locally driven nonresonant oscillations.