The internal and external Gauss coefficients model the field generated inside and outside the Earth, respectively. The core field coefficients change with time as the core-generated field changes, and describe this magnetic field component and its variation commonly the spherical harmonic analysis is used. I have been involved in elaborating the International Geomagnetic Reference Field (IGRF), as well as in more precise internal models needed to better describe the main and the crustal parts. Improving the Earth's magnetic field models, with a better description of internal and external sources, is a continuous effort in the community, as indicated by the need to produced series of models as CHAOS and GRIMM, models in which I was involved at different stages.
Taking advantage of the of MAGSAT and CHAMP satellites data, it was possible to identify and interpret main field variations over 30 years, going down to length scales previously inaccessible. At the core surface, the core field changes are weak below the Pacific Ocean and strong at polar latitudes and in a region centred below South Africa.
To cover the global and the regional pattern of the magnetic field and to eliminate the limitations of classical methods, new methods to model the Earth's magnetic field can be developed. One of them, in which I have been involved, wavelet technique plays a role in the solution of this challenging problem.
The rate of change of declination, for example, shows that there have been a number of abrupt changes in the general trend of secular variation in the past, the most remarkable being around 1925, 1969, 1978 and 1992, 1999. These sudden changes are known as geomagnetic jerks or impulses; and, at the present time, are not well understood and are certainly not predictable. A very important part of my activities concern detection, analysis and interpretation of these events, by using magnetic observatory and satellite measurements.
The field arising from magnetic materials in the Earth's crust varies on all spatial scales and is often referred to as the anomaly field. A knowledge of the crustal magnetic field is very valuable as a geophysical exploration tool for determining the local geology. Recent models are used to better characterize the crustal magnetic field, and some well-known magnetic anomalies.
The World Digital Magnetic Anomaly Map (WDMAM) aims to provide the scientific community a world-wide 5km magnetic anomaly grid. The map helps model and interpret not only the shallow geological features, but also large tectonic provinces. I have participated in this project actively, from gathering magnetic data to developing methods to analyze them.
As well as the regular daily variation, the Earth's magnetic field also exhibits irregular disturbances, and the largest ones being the magnetic storms. The prevailing conditions in the solar-terrestrial environment have also been a topic of my studies. Once more, I showed that in the modelling of strongly heterogeneous fields, such as from external currents, wavelets may play an additional role, since they allow a more geometric description of the fields.