Earth Planets Space, 57, 17, 2005 Lithospheric studies using gradients: Ørsted to Swarm

Maps of the lithospheric magnetic eld made to date have used single satellite observations. In contrast, the upcoming Swarm mission will measure the magnetic eld gradient, in this case the difference in the magnetic eld between the lower two satellites. We discuss our rationale for choosing the 1:5 satellite separation, and the expected resolution achievable. We relate this gradient to underlying magnetic eld sources. In an attempt to better understand the advantages and limitations of gradient measurements, we have reviewed the Ørsted, SAC-C, and CHAMP missions for close encounters, and examined some 50; 000 of those observations in detail. The lithospheric signal over the north polar cap is reliably isolated by standard selection criteria, with about 20% of the data retained. We demonstrate that, at least in the polar caps, the selection criteria are necessary for the isolation of the lithospheric signal. There is a tendency for the predictions to underestimate the observed eld, as shown by the slopes, which average about 2. This is especially notable over stronger anomalies. These gradients have evidently captured unmodeled, probably higher frequency, signal. Comparing the gradient measurement with the eld measurement, we see a notable improvment in both the correlation coefcient and the offset, with the correlation coefcient improving from about 0.7 to in excess of 0.8. The gradient control provided by SAC-C and Ørsted is almost equally effective. Key words: gradient, magnetic eld, satellite, lithosphere 1