A novel weighting method for satellite magnetic data and a new global magnetic field model

Abstract

S U M M A R YA new data weighting scheme is introduced for satellite geomagnetic survey data. Data weights for individual satellite samples at 20-s spacing are derived from two 'noise' (or unmodelled signal) estimators for the sample. First, the standard deviation along the 20 s of satellite track, centred on each sample, is computed as a measure of local magnetic activity. Second a larger-scale noise estimator is defined in terms of a 'local area vector activity' (LAVA) index for the sample. This is derived from activity measured at the geographically nearest magnetic observatories to the sample point. Weighting of the satellite data by the inverse-sum-of-squares of these noise estimators then leads to a robust model of the field, the 'Model of Earth's Magnetic Environment 2008', or MEME08, to about spherical harmonic degree 60. Our approach allows vector samples of the field to be used at all magnetic latitudes and, for example, results in a lithospheric magnetic field model with low spectral noise, comparable with other recent global models. We also do not require a particularly complex model parametrization, regularization, or prior data correction to remove estimates of unmodelled source fields.Global geomagnetic field research and modelling has been stimulated in the past decade by the Ørsted, CHAMP and SAC-C satellite magnetic survey missions. In particular the quantity and high quality of survey data now available for studying the Earth's magnetic field has allowed greater resolution of internal and external sources of the measured field. In the recent literature, questions of satellite data selection, filtering and treatment have been major areas of study, particularly in attempts to isolate the internal field of the Earth and for revealing details of core processes and of lithospheric structure. Recently, published models range from relatively simple representations of the internal field, for example, the International Geomagnetic Reference Field, version 10 (Macmillan & Maus 2006), to the 'Comprehensive Model' of Sabaka et al. (2004), which contains representations of major internal and external field sources, including the quiet, solar-induced magnetic variation of the ionosphere and a representation of field aligned current systems. More developed magnetospheric field models have also been made possible, for example, Maus & Lühr (2005), that model the quiet-time magnetospheric field in an appropriate sun-fixed coordinate system, rather than the geocentric system widely used by the global geomagnetic modelling community. Thomson & Lesur (2007) (hereafter referred as TL)-the paper on which the work reported here is based-described the global geomagnetic field model BGS/G/L/0706. In deriving BGS/G/L/0706, TL were concerned with improving satellite magnetic data selection techniques, for example by examining the spectral characteristics of models derived according to different data selection criteria. TL experimented to find the best use of magnetic activity indices for Ørsted and CHAMP satellite data, d...

DTU candidate field models for IGRF-12 and the CHAOS-5 geomagnetic field model

“…The modelling technique and data selection closely follow those previously described by Olsen et al (2014). CHAOS-5 is similar to the IGRF parent models produced by a number of other teams (for example Maus et al 2010;Rother et al 2013;Thomson et al 2010) in not explicitly modelling the ionospheric field, in contrast to the more sophisticated comprehensive modelling approach (Sabaka et al 2015;Thébault et al 2015). Instead, data selection for CHAOS-5 is limited to dark-region data from geomagnetically quiet times (when ionospheric currents are weak, at least at non-polar latitudes), in an effort to isolate as best as possible the field of internal origin.…”

The BGS magnetic field candidate models for the 12th generation IGRF

“…The selected satellite data were individually weighted using two 'noise' estimators which we briefly outline here (for a detailed description, see Thomson et al (2010)). Firstly, we use a measure of local magnetic activity derived from the standard deviation (SD) along short segments (20 samples, approximately 150 km) of satellite track.…”

Ensemble Kalman filter analysis of magnetic field models during the CHAMP-Swarm gap

“…Since the launch of the Ørsted satellite (Olsen et al, 2000) in 1999, vector magnetic data from dedicated magnetic field missions have greatly improved models of the geomagnetic field and with it, our understanding of the behaviour of the various physical sources. A number of groups have produced main field models of the field generated by the internal sources (typically consisting of core, crust, (quasi-)steady ocean flow and the induced part from the ionosphere and magnetosphere) including the CHAOS (Olsen et al, 2006(Olsen et al, , 2009, GRIMM (Lesur et al, 2008(Lesur et al, , 2010 and MEME (Thomson et al, 2010;Hamilton et al, 2010) series of models. In addition, the quinquennial releases of the International Geomagnetic Reference Field (IGRF) (Finlay et al, 2010b;Thébault et al, 2015b) and World Magnetic Model (WMM) (Maus et al, 2010;Chulliat et al, 2015) benefited from the voluminous satellite dataset and the ground observatory network (Macmillan and Olsen, 2013), as well as advances in theoretical and numerical techniques.…”