Geomagnetism

POMME is a main field model providing the geomagnetic field in the region from the Earth's surface to an altitude of a couple of thousand kilometers. POMME-3.1 includes the time varying core field, crustal field, the ring current field modulated by the Dst/Est/Ist Indices, a time averaged magnetospheric field, the penetration of the horizontal part of the Interplanetary magnetic field (IMF), and the fields induced by Earth rotation in the external fields.

The user has to provide the Est/Ist-index , which the model requires in order to track the magnetospheric disturbance field. See Est/Ist index page at NOAA. In addition, the Y component of the Interplanetary Magnetic Field (IMF) has to be supplied. In case these quantities are unknown (e.g. when the field is evaluated for a future date), then Est, Ist and IMF-By should be set to zero.

The parametrization of POMME-3 is identical to the one of POMME-2. The main differences to the previous version are

1. Only CHAMP data were used. Oersted and SAC-C data were used only to verify the accuracy of the model.
2. The new input data span is from 2000.6 to 2005.7. The center of the model is at 2003.0.
3. At low latitudes, only satellite tracks verified as being undisturbed by ionospheric plasma irregularities were used (Stolle et al., 2006).
4. A correction for gravity driven F-Region currents was applied to all input data.

Fig.1: (click to enlarge) The accuracy of POMME-3 and its prediction for the future geomagnetic field were verified by estimating an independent model from a combined data set of Oersted and SAC-C satellite measurements. The difference in the core field representations of the models (spherical harmonic degrees 1-13) were then compared at the Earth's surface for the years 1995 to 2010. In the center of the modeling period, the mean amplitude of the difference-vector is around 4 nT. The disagreement between the models increases to about 100 nT for the predicted field in 2010. This may be seen as a realistic estimate of the uncertainty in the core field representation of POMME-3.