A contribution to ULF activity in the Pc 3–4 range correlated with IMF radial orientation


  • E. W. Greenstadt,

  • J. V. Olson


The data of an early report tentatively linking the radial orientation of the interplanetary magnetic field (IMF), measured by Explorer 35, with ULF activity in the Pc 3–4 range observed at Calgary in September 1969 have been supplemented by data from October and November 1969. The larger sample of hourly values confirms the results discerned before: (1) there is a contribution to Pc 3 and 4 signal amplitudes that disregards the conventional separation between the bands at T = 45 s; (2) the contribution is apparent under quiet conditions when Kp ≤ 2o; (3) the contribution comes from signals whose probable amplitude is highest when the field is along the sun-earth line and lowest, essentially at instrument threshold, when the angle θXB between X(GSE) and B (the IMF) exceeds about 60°; (4) the trend of high signal amplitude with low θXB is replaced by cos θXB, suggesting a propagation property of waves in the solar wind; and (5) the trend and its properties are compatible with, although not compelled by, a model of signal origin in quasi-parallel bow shock structure.