Correction of Motional Electric Field Measurements for Galvanic Distortion*
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Interactions between motional electric fields and lateral gradients in electrical conductivity (e.g., seafloor topography) produce boundary electric charges and galvanic (i.e., noninductive) secondary electric fields that result in frequency-independent changes in the electric field direction and amplitude that are specific to a single location. In this paper, the theory of galvanic distortion of the motional electric field is developed from first principles and a procedure to correct for it is then derived. The algorithm is based on estimation of intersite transfer tensors for the horizontal electric fields at the high frequencies where external (ionospheric and magnetospheric) sources, not oceanic motionally induced electric fields, dominate. A decomposition of each measured tensor is derived that expresses it as the product of a set of distortion tensors and the underlying, undistorted transfer tensor. The algorithm may be applied simultaneously to a set of sites and assessed statistically, yielding the undistorted electric field uniquely at each site except for a single site-dependent multiplicative scalar, which must be obtained from other data. Because the distortion is frequency independent, the same tensors may be used to undistort the low-frequency, motional induction components that are of interest in oceanography. This procedure is illustrated using an electric field dataset collected in the Southern Ocean in 1995-97, which is significantly distorted by galvanic processes.