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. The time variations of the field are represented by a Taylor series of the spherical harmonic (Gauss) coefficients of the magnetic potential. We provide the static coefficients for 2005.0 and the first and second time derivatives (secular variation and secular acceleration). The corresponding geomagnetic power spectrum is shown in Figure 1. The red curves show the actual spectra of the model, while the blue curves show the resulting spectra if no regularization (damping) were applied in the model estimation. The upward trend of the blue spectra above degree 13 indicates that the POMME-5 secular variation and acceleration coefficients are reliably determined to at least degree and order 12.

As in POMME-4, we have split the secular acceleration into pre-2005 and post-2005 values. This is equivalent to a Taylor series expansion at 2005.0 using a different 2nd time derivative before and after 2005. Plots of the secular acceleration before and after 2005.0 are given in Figure 2. It can be seen that the curvature of the geomagnetic field variation, given by the secular acceleration, changes quite dramatically over the model period.

POMME-5 includes the time varying core field, crustal field to degree 60, 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.

To evaluate the model for a given location and time, 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-5 is very similar to the one of POMME-3. The main features are

1. Only CHAMP data were used because vector data are no longer available from Oersted
2. The new input data span is from 2000.6 to 2009.3. The center of the model is at 2005.0.
3. The external field parametrization and coefficients are identical to POMME-3
4. At low latitudes, only satellite tracks verified as being undisturbed by ionospheric plasma irregularities were used (Stolle et al., 2006).
5. The input data were corrected for the magnetic signal of ocean tidal induction

Fig.1: (click to enlarge) Geomagnetic power spectrum of the POMME-5 spherical harmonic coefficients in the definition of Maus (2008).

Fig.2: (click to enlarge) Secular acceleration before 2005.0 (top) and after 2005.0 (bottom) as given by the POMME-5 model.