Analysis of CHAMP scalar magnetic data to identify ocean circulation signals: some preliminary results

Unlike tidal ocean signals, the magnetic signal of ocean circulation has not yet been identified in satellite magnetic data. In particular, the steady signal of mean ocean flow is indistinguishable from time invariant crustal signals. One option, therefore, is to predict the seasonal and annual variations in the ocean flow signal from ocean circulation models and compare them with the corresponding variations in satellite magnetic residuals. We used the 11 year ECCO-1 simulation data to derive the ocean transport. A 3D EM induction code in its low frequency limit, was used to simulate the magnetic signals at satellite altitude. We predict annual variation amplitudes in the scalar anomaly of the order of 0.3 nT. We compare these predictions with the particularly quiet CHAMP night-time scalar data, subtracting core, mantle, crustal, ocean tidal, and magnetospheric contributions to the field. The residuals were further screened with Kp and Dst indices. Comparing the seasonal and annual harmonics of the predictions and observations, we find that the amplitude of the CHAMP residuals (about 0.5 nT) is already quite close to the prediction, with stronger signals over oceanic regions than over land. However, unremoved / unmodelled contributions from the external fields and the secondary fields it induce in the ocean still dominate the observations, preventing the identification of the small ocean magnetic signal.