Pc 3,4 activity and interplanetary field orientation

Examination of Pc 3,4 micropulsation wave forms recorded at Calgary in the interval September 2--20, 1969, shows a tendency for signal enhancement to have occurred when the interplanetary magnetic field made a small angle theta/sub X B/ with the sun--earth line. Scatter plots of hourly Pc 3,4 amplitudes show a definite trend toward large signals when theta/sub X B/ < 500--600 and a corresponding disappearance of significant amplitudes when theta/sub X B/ > 600, but there was appreciable variability in individual cases. The trend toward the highest amplitudes when theta/sub X B/approx. = 00 was sharpened under geomagnetically quiet conditions. Power density spectrograms improved the correlation of pulsation strength with low angle in some cases. The results are interpreted as compatible with enhanced probability of Pc 3,4 excitation when quasi-parallel structure prevails at the subsolar point of the bow shock and as consistent with previous Soviet and Canadian analyses, although certain details of the Russian results are not supported