A MULTISTATION STUDY OF LONG-PERIOD GEOMAGNETIC-PULSATIONS AT CUSP AND BOUNDARY-LAYER LATITUDES

The characteristics of 1-20 mHz (Pc5) geomagnetic pulsations recorded during the daytime on the ground at cusp and boundary layer latitudes have been examined. On quiet and moderately disturbed days the major s pectral contributions are due to three different mechanisms. Sustained oscillations whose properties are consistent with the Kelvin-Helmholt z instability at the low latitude boundary layer are the dominant mech anism at -70 to -75-degrees geomagnetic latitude. Transient irregular pulsations are frequently seen at single stations at the foot of polar cap and boundary layer field lines. Occasionally similar transients o ccur essentially simultaneously at widely spaced stations accompanied by absorption spikes on riometer records. The latter signals are most likely due to solar wind pressure pulses on the magnetopause. At cusp latitudes the major spectral contribution arises from sustained irregu lar pulsations centred on magnetic noon. Although their occurrence is related to the proximity of the cusp's particle signature, it may be m ore appropriate to discuss these signals in terms of fluctuations in b oundary layer or mantle currents.