Geoelectric field measurements on a planetary scale: Oceanographic and geophysical applications

Abstract

The characteristics of an initial year of electric field measurements obtained from a long (≈3900 km), unpowered submarine cable extending from Point Arena, California, to Hanauma Bay, Hawaii are reported. The power spectrum estimated from the cable time series is similar to that from seafloor point electric field sensors at frequencies above 1 cpd, but differs at lower frequencies by decreasing monotonically down to at least 0.02 cpd. The multiple squared coherence between the cable data and three component geomagnetic fluctuations measured at Fresno, Ca., is high (γ²≈0.9) from 1.0‐0.09 cpd, then declines slowly with decreasing frequency. Maps of the squared coherence between the cable voltage and the surface air pressure, wind stress, and wind stress curl derived from the Fleet Numerical Oceanography Center product over the entire eastern North Pacific show no significant relationships at frequencies above ≈0.05 cpd, but strong nonlocal coherence with the wind stress curl forcing obtains at lower frequencies. Taken together, these observations support a transition from high frequency dominance by external sources to low frequency dominance by oceanic motional sources at 0.05‐0.1 cpd, a considerably smaller value than has previously been observed with point electric field sensors on the ocean bottom. The difference is caused by horizontal averaging of the short spatial scale motional component over the long submarine cable.