Magnetotelluric (MT) data from two sites 150 and
300 km southeast of San Francisco, California (geomagnetic
dipole latitude: 43 degrees, L approximately 1.9) show that
the usual MT assumption of spatially uniform external magnetic
fields is violated to a significant degree in the period
range 10-30 s. Inter-station transfer functions exhibit large
systematic temporal variations which are consistent with a
combination of two distinct sources: electromagnetic noise
due to the San Francisco Bay Area Rapid Transit (BART)
DC electric railway, and Pc3 geomagnetic pulsations. There
is a suggestion in the data that some of the Pc activity may
actually be excited by BART
