“…By providing a dense and global coverage of the Earth's surface with vector measurements of the Earth's magnetic field from November 1979 until March 1980, the Magsat mission has enabled the derivation of main field models with high accuracy (e.g., Langel et al, 1982;Gubbins and Bloxham, 1985;Lowes and Martin, 1987;Bloxham and Jackson, 1992) as well as the compilation of accurate global high degree anomaly field models (e.g., Arkani-Hamed and Strangway, 1986;Cain et al, 1989;Schmitz et al, 1989;Cohen and Achache, 1990;Hamoudi et al, 1995). However, global field models based on scalar data only are plagued with a systematic error of significant amplitude known as the Backus effect (e.g., Backus, 1970;Stern and Bredekamp, 1975;Stern et al, 1980).…”