[1] Spatial and temporal variability of the magnetic field component induced by ocean circulation is investigated on the basis of a standard thin-shell approximation of electro-and magneto-static equations. Well-known difficulties of numerical solution of the governing equations are resolved by reducing the problem to an equation for the electric field potential, F, as opposed to a more conventional approach focused on the vertical jump, y, of the magnetic field potential across a combined ocean/ marine-sediment-layer spherical shell. The present formulation permits using more realistic input data on ocean currents and ultimately yields much greater (by at least an order of magnitude) values of the magnetic field at sea surface than predicted in earlier studies. Such large values are comparable to, and in some cases exceed, magnetic field variations caused by lithospheric and ionospheric sources on monthly to interannual timescales. At the 400-km altitude (of CHAMP satellite), the field attains 6 nT. The model predictions show favorable comparisons with some in situ measurements as well as with Challenging Minisatellite Payload (CHAMP) satellite magnetometer data.Citation: Glazman, R. E., and Y. N. Golubev (2005), Variability of the ocean-induced magnetic field predicted at sea surface and at satellite altitudes,

[1] A data set of approximately 28 months of geomagnetic measurements, made by the CHAMP mission, is planned to study the ocean-generated magnetic field. An obstacle for using these data is the large statistical error associated with data averaging in each grid point of the regular grid. The error is 10 2 -10 3 times larger than the magnitude of the ocean-generated signal. Here, a technique designed to reduce this error that is based on a solution of the Laplace (Helmholtz) equation, is proposed. The main idea behind the technique is that the magnetic potential in free space obeys the Laplace equation while, in general, the statistical error does not. The approach allows a reduction of the error in the gridded data over several orders of magnitude and makes the data useable for studying the ocean-generated magnetic field. As a preliminary result for using the data for inferring the ocean-generated magnetic field, the density function is introduced and computed. The function's plot demonstrates the presence of major large-scale ocean current systems within the midlatitudes.Citation: Golubev, Y. (2011), Ocean-generated magnetic field study based on satellite geomagnetic measurements: 1. An error reduction technique for gridded data,

The continental shelf currents observed at the head of DeSoto Canyon offshore of Pensacola, FL, during Nov, 1997-Feb. 1998, are studied with the aid of a Bryan-Cox model of the entire Gulf of Mexico. The basic model circulation with no winds features a Loop Current and a weak flow in DeSoto Canyon. In contrast, the model response to northerly wind bursts produces strong canyon currents. Wind-driven southward currents on the West Florida Shelf (WFS) appear to pull a strong flow around DeSoto Canyon, giving rise to a strong up-canyon flow on the west flank The wind-enhanced flow in DeSoto Canyon is reproduced with a model run that incorporates wind stresses derived from the NCEP-NCAR reanalysis winds for March 1997-April 1998. The model flow for the winter period (Nov. 1997-Feb. 1998) is compared with observed currents at three canyon-head locations in 100 m of water available from a Science Applications International Corporation mooring experiment. Model and observed velocities in the general around-canyon direction track each other reasonably well, with peal{S synchronized roughly with but lagging slightly northerly wind bursts. The lag time appears consistent with the generation at Key West of first-mode continental shelf waves responsible for wind-driven alongshore currents on the WFS. Pressure gradient slightly in excess of that necessary for a geostrophic balance with the (eastward) alongshore flow appears to generate onshore flows, giving rise to midshelf upwelling after northerly wind bursts.

No abstract

[1] The work presented here is the second part of an ocean-generated magnetic field study and provides a procedure for inferring the ocean-generated magnetic field from satellite geomagnetic measurements. The procedure was first tested on synthetic data. The simulation employed a hypothetical satellite measuring the magnetic field at an altitude of 400 km. The "measurements" (generated by the CM4 and CHAOS models) included the core field, the lithospheric field, the ionospheric and magnetospheric fields, the secular variation, and the ocean-generated magnetic field. The search algorithm, as proposed in part 1, converts irregular measurements into fields on a regular grid. The filtration procedure is based on the Savitzky-Golay algorithm. The procedure includes four steps providing seven unknown filter parameters. Parameter values were obtained by solving the problem of minimizing the spatially averaged squared residuals between the inferred field and the model field. Then, the parameters were used in the filter to infer the ocean-generated magnetic field that was initially added to the "measurements." The inferred signal, although spatially corrupted and having a smaller magnitude (60% of the magnitude of the initial signal), indicated the presence of magnetic anomalies within the Southern Ocean. The technique was then applied to CHAMP geomagnetic measurements. The result of filtering was clear magnetic anomalies within the Southern Ocean with a spatial character that were close to what models of the ocean-generated magnetic field provided. The magnitude of the inferring signal was 5 nT, the corrected values were 7-8 nT, 1-2 nT larger than the modeled field magnitude. To compare the temporal variability of the inferred field with the variability of sea surface height, ten 10 by 10 areas were selected within the Southern Ocean and the root-mean-square of both SSH and the magnetic field were computed for each area. A comparison of the results indicated a close similarity between SSH and the magnetic field temporal variability, which allowed the identification of the inferred field as the ocean-generated magnetic field.Citation: Golubev, Y. (2012), Ocean-generated magnetic field study based on satellite geomagnetic measurements: 2. Signal inference,