Possible changes in natural Pc 1 pulsation activity caused by BART

In a previous preliminary study of the diurnal variation of Pc 1 pulsation activity at Stanford during four months in 1975, evidence was obtained for changes in the rate of occurrence of the pulsations around the times when service was started and terminated by the San Francisco Bay Area Rapid Transit (BART) system (Fraser-Smith et al., 1979). The present study extends this earlier work by analyzing Pc 1 pulsation data recorded at Stanford during 1976. Spectrograms of N--S geomagnetic activity were prepared for the complete year and 15-minute intervals containing Pc 1 pulsation activity were tabulated. The diurnal variations of the Pc 1 activity (as measured by these 15-minute intervals) for weekdays, during which BART was in operation, and weekends, during which BART was not normally in operation, were compared. Correlations of the observed differences with the BART schedule suggest once again that the ultralow-frequency electromagnetic noise produced by BART may be influencing the occurrence of Pc 1 pulsations along the Stanford geomagnetic meridian