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

Abstract

We describe the candidate models submitted by the British Geological Survey for the 12th generation International Geomagnetic Reference Field. These models are extracted from a spherical harmonic 'parent model' derived from vector and scalar magnetic field data from satellite and observatory sources. These data cover the period 2009.0 to 2014.7 and include measurements from the recently launched European Space Agency (ESA) Swarm satellite constellation. The parent model's internal field time dependence for degrees 1 to 13 is represented by order 6 B-splines with knots at yearly intervals. The parent model's degree 1 external field time dependence is described by periodic functions for the annual and semi-annual signals and by dependence on the 20-min Vector Magnetic Disturbance index. Signals induced by these external fields are also parameterized. Satellite data are weighted by spatial density and by two different noise estimators: (a) by standard deviation along segments of the satellite track and (b) a larger-scale noise estimator defined in terms of a measure of vector activity at the geographically closest magnetic observatories to the sample point. Forecasting of the magnetic field secular variation beyond the span of data is by advection of the main field using core surface flows.

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

“…A consistent prediction would therefore require a correct mathematical description of these physical interactions which, despite important progress during the last decade, remains a frontier scientific subject. 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.…”

“…Details concerning the techniques used to derive the individual candidate models can be found in the papers appearing in this special issue. Some teams derived their candidate models from parent models describing the magnetic field over periods longer than requested by IGRF Gillet et al 2015;Hamilton et al 2015;Sabaka et al 2015). Such models involve internal temporal parameterization using splines and external field parameterization of varying complexity and differing reference frames.…”

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

“…The "physical" models were proposed by teams who considered that forecasting the chaotic geodynamo although difficult is numerically and statistically achievable. Two teams applied the tools of geophysical assimilation (Fournier et al 2015, Kuang et al unpublished), and two others explored a priori statistical hypotheses on the core flow (Gillet et al 2015;Hamilton et al 2015). These physically motivated models are not only attractive for forecasting the Earth's magnetic field but also for hindcasting it at past epochs poorly constrained by the available magnetic field measurements.…”

“…The teams adopted independent data selection, processing, and modeling procedures whose details can be found in the papers appearing in this special issue. Some teams derived their candidate models from a parent model Gillet et al 2015;Hamilton et al 2015;Sabaka et al 2015). The parent model parameterizations are more complex in space and in time than IGRF models.…”