Abstract
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.
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