Main field and secular variation candidate models for the 12th IGRF generation after 10 months of Swarm measurements

Abstract

We describe the main field and secular variation candidate models for the 12th generation of the International Geomagnetic Reference Field model. These two models are derived from the same parent model, in which the main field is extrapolated to epoch 2015.0 using its associated secular variation. The parent model is exclusively based on measurements acquired by the European Space Agency Swarm mission between its launch on 11/22/2013 and 09/18/2014. It is computed up to spherical harmonic degree and order 25 for the main field, 13 for the secular variation, and 2 for the external field. A selection on local time rather than on true illumination of the spacecraft was chosen in order to keep more measurements. Data selection based on geomagnetic indices was used to minimize the external field contributions. Measurements were screened and outliers were carefully removed. The model uses magnetic field intensity measurements at all latitudes and magnetic field vector measurements equatorward of 50°a bsolute quasi-dipole magnetic latitude. A second model using only the vertical component of the measured magnetic field and the total intensity was computed. This companion model offers a slightly better fit to the measurements. These two models are compared and discussed.We discuss in particular the quality of the model which does not use the full vector measurements and underline that this approach may be used when only partial directional information is known. The candidate models and their associated companion models are retrospectively compared to the adopted IGRF which allows us to criticize our own choices.

International Geomagnetic Reference Field: the 12th generation

“…In addition, nine SV models were submitted for the predictive part covering epochs 2015.0-2020.0. Team A was from BGS, UK (Hamilton et al 2015); team B was from DTU Space, Denmark ; team C was led by ISTerre, France, with input from DTU Space (Gillet et al 2015); team D was from IZMIRAN, Russia; team E was from NGDC/NOAA (Alken et al 2015); team F was from GFZ, Germany ; team G was led by GSFC-NASA, USA, in collaboration with UMBC; team H was from IPGP (Fournier et al 2015;Vigneron et al 2015), France, in collaboration with the CEA-Léti and with input from LPG Nantes and CNES, France; team I was led by LPG Nantes, France (Saturnino et al 2015) with input from CNES; team J was from ETH Zurich, Switzerland. These teams contributed to all or parts of the three model constituents of IGRF.…”

Evaluation of candidate geomagnetic field models for IGRF-12

“…Some teams nevertheless considered a physics-based approach applying the tools of geophysical assimilation or setting a priori hypothesis on the core flow (e.g., Gillet et al 2015;Hamilton et al 2015). Other teams (e.g., Alken et al 2015;Finlay et al 2015;Lesur et al 2015;Saturnino et al 2015) relied on simple analytical extrapolation assuming that the magnetic field will evolve linearly over the next 5 years. From October to early December 2014, some members of the task force and interested parties carried out evaluations of the candidate models submitted by the different teams.…”

“…Such models co-estimate the various source fields through a grand inversion and perform a mathematical separation of the field into its internal and external parts. Other teams focused their effort on dedicated internal candidate models for each of the epochs requested by the call, thus using data within windows centered on epochs of interest Lesur et al 2015;Saturnino et al 2015;Vigneron et al 2015). This necessarily involved less complex parameterization in space and in time and sometimes more drastic data selection and pre-processing to minimize the effects of unwanted magnetic field contributions arising from external fields.…”

Special issue “International Geomagnetic Reference Field—the twelfth generation”

“…The candidate models to IGRF were subsequently estimated from the parent models by selecting the internal field contribution at the epoch and to a spatial resolution requested by the call for IGRF-12. Other teams focused their effort on deriving directly a model closely meeting the IGRF specifications (Alken et al 2015, Lesur et al 2015Saturnino et al 2015;Vigneron et al 2015). They relied on data selected within time windows centered on the epochs of interest.…”

“…Teams submitted prediction candidates derived using both "physical" and "mathematical" approaches. The "mathematical" models were built by teams relying on analytical extrapolation and who assumed that the magnetic field will evolve linearly over the next 5 years (e.g., Alken et al 2015;Finlay et al 2015;Lesur et al 2015;Saturnino et al 2015). The "physical" models were proposed by teams who considered that forecasting the chaotic geodynamo although difficult is numerically and statistically achievable.…”