New parameterization of external and induced fields in geomagnetic field modeling, and a candidate model for IGRF 2005

When deriving spherical harmonic models of the Earth’s magnetic field, low-degree external field contributions are traditionally considered by assuming that their expansion coefficient q10 varies linearly with the Dst-index, while induced contributions are considered assuming a constant ratio Q1 of induced to external coefficients. A value of Q1 = 0.27 was found from Magsat data and has been used by several authors when deriving recent field models from Èrsted and CHAMP data. We describe a new approach that considers external and induced field based on a separation of Dst = Est + Ist into external (Est) and induced (Ist) parts using a 1D model of mantle conductivity. The temporal behavior of q10 and of the corresponding induced coefficient are parameterized by Est and Ist, respectively. In addition, we account for baseline-instabilities of Dst by estimating a value of q10 for each of the 67 months of Èrsted and CHAMP data that have been used. We discuss the advantage of this new parameterization of external and induced field for geomagnetic field modeling, and describe the derivation of candidate models for IGRF 2005.