HR: 0800h
AN: GP31A-0829    [Abstracts]
TI: New Parameterization of External and Induced Fields in Geomagnetic Field Modeling
AU: * Olsen, N
EM: nio@dsri.dk
AF: Danish Space Research Institute, Juliane Maries Vej 30, Copenhagen, 2100 Denmark
AU: Sabaka, T J
EM: sabaka@geomag.gsfc.nasa.gov
AF: Goddard Space Flight Center, Geodynamics Branch, Code 921, Greenbelt, MD 20771
AB: It is a common practice to consider low-degree external and secondary (induced) field contributions when deriving spherical harmonic models of the Earth's core and crustal fields. Traditionally, the external field is described by the spherical harmonic coefficient $q_1^0$ which consists of a static (or slowly varying) part and a part that varies linearly with the $D_{\rm st}$-index, while induced contributions are considered assuming a constant ratio $Q_1$ of induced to external coefficients. $Q_1=0.27$ was found from Magsat data by {\it Langel et al.}, and this value has been used by several authors when deriving recent field models from {\O}rsted and CHAMP data. An alternative approach of considering induced field contributions is based on the decomposition of $D_{\rm st}=E_{\rm st}+I_{\rm st}$ into external ($E_{\rm st}$) and induced ($I_{\rm st}$) parts by means of a realistic (radially symmetric) model of mantle conductivity. In this new approach the temporal behavior of external and corresponding induced coefficients are parameterized by $E_{\rm st}(t)$ and $I_{\rm st}(t)$, respectively. Deriving field models from {\O}rsted and CHAMP data using the old and new parameterization demonstrates the superiority of the new approach. However, the model residuals are correlated between the two satellites in both cases, indicating the existence of an unmodeled magnetospheric signal. This is probably due to baseline-instabilities of $D_{\rm st}$. We account for this by allowing the ``static'' external coefficient $q_1^0$ to change with time, which results in a significant reduction of the large-scale residuals.
UR: http://www.dsri.dk/Oersted/Field\_models/
DE: 1515 Geomagnetic induction
DE: 1532 Reference fields (regional, global)
DE: 1545 Spatial variations (all harmonics and anomalies)
DE: 1560 Time variations--secular and long term
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting