GP33C-1441
Ocean circulation generated magnetic signals: Application to geomagnetic field modeling
Recently, simulations have been carried out to estimate the strength of electric and magnetic signals generated by the steady motion (ocean circulation) of highly conducting oceans. The magnetic signals have been found to be on the order of several nT at the Earth's surface. The ocean circulation signals are especially pronounced in the high latitudes of the southern hemisphere, where the eastward current circulates without hitting continental landmass. In this study, we estimated the magnetic field generated by the ocean circulation and modelled it on a global scale using the spherical harmonics. We found that the energy spectrum of the expansion is dominated by rather high spherical harmonic degrees. Results of a spherical harmonic analysis are compared with recent spherical harmonic description of the lithospheric magnetic field. We seek to explain uncertainties in the lithospheric magnetic field models by oceanic signals, in order to improve current models of the geomagnetic field.
GP33C-1442
Tidal Magnetic Models: A New Field Modelling Approach
For many years it has been recognised that the passive flow of the circulating oceans through the Earth's magnetic field generates a secondary field through the process of motional induction. However, while this phenomena is known to exist, little work has been carried out to characterise these secondary fields. Global flows such as tides have received some attention, but have not been the subject of an intensive magnetic field modelling process. Here we present a new, multi-mode spherical harmonic field model derived to represent the induced magnetic field associated with global tides. By integrating data from all three of the major orbiting satellites, CHAMP, SAC-C and Oersted, in both scalar and vector forms, we are able to derive a tidal model that in preliminary testing has been found to be consistent with early work by others. We are also able to extend the latitude range of the model from mid latitudes into the previously problematic, high latitude region. This model also has the potential to be expanded to include any number of previously unstudied tidal modes, thus giving the opportunity to greatly increase our understanding of global tidal behaviour on a whole range of timescales.
GP33C-1443 INVITED
Where do we stand on identifying motional ocean induction signals in satellite magnetic data?
After the successful identification and mapping of the M2 ocean tidal signal in CHAMP magnetic field measurements in 2002, no further significant progress has been reported in inferring motional ocean induction signals from satellite data. Apart from the necessity to resolve remaining disagreements between model predictions and observations of the M2 tidal signal, the next important step is to find evidence of non-tidal ocean flow signals. Unfortunately, the steady ocean circulation signal is practically indistinguishable from the crustal magnetic field. Efforts have therefore concentrated on identifying signals of annual and semi-annual variations in ocean currents. Model simulations indicate that these signals have amplitudes of the order of only 0.1 nT at satellite altitude. In an unpublished study in 2004, these signals were found to be masked in CHAMP residuals by noise levels of around 0.5nT. With advances in field modeling and the decreasing altitude of the CHAMP satellite under solar minimum conditions, it should be worthwhile to make another attempt at identifying these small signals. http://geomag.org
GP33C-1444 INVITED
Magnetic fields due to motional induction in the oceans.
The depth integrated ocean flow directly impacts the magnetic and electric field generation associated with the sea water movement. The implications are that (1) the ocean flow generates non-trivial magnetic fields, and these need to be considered while processing geomagnetic data and (2) the measured geomagnetic data contain signatures of ocean flow and the magnetic data can possibly be used to infer the flow strength and variability. This paper describes an overview of the recent efforts to study the motional induction due to ocean circulation, tides and tsunami. In addition we will also discuss the attempts to recover/extract the ocean magnetic fields from satellite magnetic data.