REDUCING THE BACKUS EFFECT GIVEN SOME KNOWLEDGE OF THE DIP-EQUATOR

Geomagnetic field models computed from intensity measurements on or ne ar the Earth's surface are plagued with strong errors known as the ''B ackus effect''. These errors are related to the mathematical non-uniqu eness of the solution which call be theoretically removed by the knowl edge of the dip-equator location. Here, we present a new; method for c omputing models using intensity data and additional information about the location of the dip-equator. We first give a numerical validation of this approach in an ideal case in which the dip-equator can be assu med to be perfectly located. We then show that in practice, updating a good past model of the field with a model of secular variation call c onstrain the location of the dip-equator sufficiently for our method t o be useful. Implications for the planning of Future satellite mission s are briefly discussed.