Statistical studies of geomagnetic pulsations with periods between 10 and 70 sec and their relationship to the plasmapause region

Geomagnetic pulsations, in the period range 10–150 sec, have been analysed from five stations; Eskdalemuir (L = 3.1), Lerwick (L = 4.0), St. Anthony (L = 4.9), Sodankyla (L = 5.3) and Tromsø (L = 6.6). The results of 12 observatory years' worth of data are presented in the form of contour maps showing the frequency of occurrence of the pulsations as a function of K_p index and of local time. The maps show that a ground based observatory is more likely to record shorter period oscillations (pc 3) when the geomagnetic field line linking the station with the southern hemisphere passes through the plasmatrough than when the observatory field line links the plasmasphere. The peak occurrence of pc 3 for the observatories considered is at 08:45 hr ± 1 hr LT and is related to the observatory L value and the average night-time K_p index by the equation, L = 8.1 ? 1.2K_p. At Eskdalemuir, the spectrum is broader band than the other stations and tends to divide into two peaks; the pc 3 (20 sec) peak tends to occur when the plasmapause has moved in close to the observatory; while the pc 4 (60 sec) peak occurs when the K_p values have been lower and the plasmapause is further away at higher latitudes.