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. Seven years of highly accurate CHAMP satellite magnetic measurements offer an unprecedented opportunity to track the time variations of the field originating in the Earth's core. An interesting question is whether we can start to see changes in the secualar acceleration (2nd time derivative) of the geomagnetic field. Sudden changes in the acceleration are called geomagnetic jerks and have been reported from magnetic ground observatory time series. The time variation of the satellite magnetic measurements from 2000.5 to 2007.5 can be represented fairly well by assuming linear changes in the secular variation, given by a constant secular acceleration (Figure 1), as in a Taylor series around the model epoch 2004.0. This can be considered as a "conservative" way of representing the field, which we adopted for POMME-4.0 and POMME-4.1.

However, the observed changes in the field can be more accurately reproduced by a model with a changing secular acceleration. In this case, we split the secular acceleration into pre-2004 and post-2004 values. This is equivalent to a Taylor series expansion at 2004.0 using a different 2nd time derivative before and after 2004. We call this model POMME-4.0s and POMME-4.1s, where the "s" stands for a "split" secular acceleration. Plots of the secular acceleration before and after 2004.0 are given in Figure 2. The problem with this representation is that some of the apparent change in the secular acceleration may in fact be due to contamination be factors such as the change of the external magnetic field due to the progression of the solar cycle. Nevertheless, the westward displacement of the main acceleration features over Africa and the Indian ocean looks credible. Furthermore, there is little signal over the high latitudes and over the Pacific ocean, which is compatible with the behavior of the historical geomagnetic field. For this reason, we decided to offer POMME4 with a split secular acceleration as an alternative model.

POMME-4.0 and POMME-4.0s include the time varying core field, crustal field to degree 36, 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 models POMME-4.1 and POMME-4.1s include the MF5 crustal field coefficients up to degree 100, while the models POMME-4.2 and POMME-4.2s additionally include the the NGDC-720 model coefficients.

To evaluate any of these models 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-4 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 2007.5. The center of the model is at 2004.0.
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
6. A correction for the diamagnetic effect of the ambient plasma was applied
7. The simple correction for gravity driven F-Region currents, which was applied to the input data for POMME-3, was NOT applied because it was found to over-correct the signal after midnight. A more accurate correction is in development.

Fig.1: (click to enlarge) Secular variation (top) and secular acceleration (bottom) as given by the POMME-4 model.

Fig.2: (click to enlarge) Secular acceleration before 2004.0 (top) and after 2004.0 (bottom) as given by the POMME-4s model.