Luis R. Gaya-Pique

New model alternatives for improving the representation of the core magnetic field of Antarctica

Use of the International Geomagnetic Reference Field Model (IGRF) to construct magnetic anomaly maps can lead to problems with the accurate determination of magnetic anomalies that are readily apparent at the edges of local or regional magnetic surveys carried out at different epochs. The situation is severe in areas like Antarctica, where ionospheric activity is intense and only a few ground magnetic observatories exist. This makes it difficult to properly separate from ionospheric variations the secular variation of the core magnetic field. We examine two alternatives to the piecewise-continuous IGRF core magnetic field in Antarctica for the last 45 years: the present global Comprehensive Model (CM4) and the new version of the Antarctic Reference Model (ARM). Both these continuous models are better at representing the secular variation in Antarctica than the IGRF. Therefore, their use is recommended for defining the crustal magnetic field of Antarctica (e.g. the next generation of the Antarctic Digital Magnetic Anomaly Map).

Information content and K-entropy of the present geomagnetic field

Abstract Concepts of information theory are applied to global models of the geomagnetic field B of the last century. The temporal behavior of information content suggests that B is in a chaotic state with characteristic times close to those of its westward drift and of the convective overturn in the outer core and the secular variation shows a comparable characteristic time of predictability. The main implication of these results is the suggestive hypothesis that a possible geomagnetic reversal or excursion is currently in progress.

Applied comparisons between SCHA and R-SCHA regional modeling techniques

Spherical cap harmonic analysis (SCHA) has become a common tool for the regional modeling of potential fields since its introduction by Haines (1985). The fact that SCHA satisfies Laplace equation and the possibility of representing high-frequency fields with a small number of coefficients (compared to the global spherical harmonic analysis) made SCHA the preferred choice for the development, for example, of magnetic field models at national scale. However, Thebault et al. (2006a) demonstrated that the traditional SCHA presented some deficiencies, in particular related to the inversion of multilevel data sets. The authors presented the R-SCHA technique as an alternative method in which the introduction of a new set of basis functions and boundary conditions solved this issue. In this paper we present some numerical comparisons between the SCHA and R-SCHA techniques applied with different synthetic vector data sets, from near-surface main field, main difference, and crustal field data simulating a World Digital Magnetic Anomaly Map subset. Other analyses are carried out with synthetic vector data set that mimics the expected data distribution from a multisatellite mission like the forthcoming European Swarm mission. No regularization, weighting, or ad hoc procedures are applied to the synthetic vector data, and a cap of 7° aperture is considered. The numerical analyses show that SCHA is a satisfying approximation in a band-limited spectral region that depends on the caps size. It does not work correctly either for main field or for the short-scale crustal field modeling. These aspects are supported by equations illustrating why SCHA may fail. On the contrary, R-SCHA converges more slowly than SCHA but is valid in all cases. It gives a consistent set of regional coefficients and fits the radial variation of the field in a realistic way. At last, the special case of data incompatibility shows that R-SCHA does not fit incompatible data while SCHA assimilates most of them. These results should help the scientific community to evaluate the level of approximation needed for the development of regional magnetic field models in the era of the European Space Agency Swarm mission.

Use of Champ Magnetic Data to Improve the Antarctic Geomagnetic Reference Model

Champ total field measurements have been used to develop the new version of the Antarctic geomagnetic Reference Model (ARM). The model was conceived as a tool to evaluate the main field in Antarctica, facilitating the merging of different magnetic surveys carried on in the region from 1960 onwards. Spherical cap harmonic analysis was used to produce the model. Together with data coming from POGO, Magsat, and Orsted satellite missions, a suitable selection of Champ data based on different criteria was performed to minimise the effect of external fields. The comparison of ARM and other global models with regard to real data demonstrates the validity of our regional model, specially for the representation of the secular variation of the geomagnetic field. Since Champ satellite tracks cover the Geographical South Pole better than other satellite missions, this fact contributed to improve the model in the central region of the cap.

Some Recent Characteristics of Geomagnetic Secular Variations in Antarctica

Some of the most interesting features of the Earth’s magnetic field and of its time variations are displayed in polar areas, where the geomagnetic field dipole poles are located. Space time models of the geomagnetic field give a mathematical description that allows generally to undertake a common epoch time reduction of magnetic surveys and to extract magnetic anomaly maps after removing the main part of the geomagnetic field; in addition in polar regions geomagnetic field models allow to follow the location of the geomagnetic dip poles in their time wandering. In this work the development of a dedicated regional magnetic reference model for Antarctica (Antarctic Reference Model, ARM) is presented and compared to the well known IGRF (International Geomagnetic Reference Field) model and it is shown that the first is more appropriate to better study the behaviour of secular variation and its unusual characteristics as observed in Antarctica.

CHAMP Magnetic Anomalies of the Antarctic Crust

Regional magnetic signals of the crust are strongly masked by the core field and its secular variation components and hence difficult to isolate in the satellite measurements. In particular, the un-modeled effects of the strong auroral external fields and the complicated behavior of the core field near the geomagnetic poles conspire to greatly reduce the crustal magnetic signal-to-noise ratio in the polar regions relative to the rest of the Earth. We can, however, use spectral correlation theory to filter the static lithospheric anomalies and core field components from the dynamic external field effects. To help isolate regional lithospheric from core field components, the correlations between CHAMP magnetic anomalies and the pseudo-magnetic effects inferred from gravity-derived crustal thickness variations can also be exploited. Employing these procedures, we processed the CHAMP magnetic observations for an improved magnetic anomaly map of the Antarctic crust. Relative to the much higher altitude Orsted and noisier Magsat observations, the CHAMP magnetic anomalies at 400 km altitude reveal new details on the effects of intracrustal magnetic features and crustal thickness variations of the Antarctic.