Abstract
We describe a new, original approach to the modelling of the Earth’s magnetic field. The overall objective of this study is to reliably render fast variations of the core field and its secular variation. This method combines a sequential modelling approach, a Kalman filter, and a correlation-based modelling step. Sources that most significantly contribute to the field measured at the surface of the Earth are modelled. Their separation is based on strong prior information on their spatial and temporal behaviours. We obtain a time series of model distributions which display behaviours similar to those of recent models based on more classic approaches, particularly at large temporal and spatial scales. Interesting new features and periodicities are visible in our models at smaller time and spatial scales. An important aspect of our method is to yield reliable error bars for all model parameters. These errors, however, are only as reliable as the description of the different sources and the prior information used are realistic. Finally, we used a slightly different version of our method to produce candidate models for the thirteenth edition of the International Geomagnetic Reference Field.
Highlights
The magnetic field surrounding the Earth is sustained by—and constantly evolving due to—the motions in the Earth’s liquid outer core
Modern models—e.g. the GRIMM (Lesur et al 2015) or, more recently, the Chaos-6 model (Finlay et al 2016), both of which use splines of order 6 for their time evolution—separate well contributions from the core and external sources. They are typically able to render short core field time scales, with a precision depending on the spatial scales—e.g. small spherical harmonics degrees have a resolution of the order of 2 years
In this paper, we have presented a sequential approach to core field modelling based on a combination of a Kalman filter and a correlation-based modelling method
Summary
The magnetic field surrounding the Earth is sustained by—and constantly evolving due to—the motions in the Earth’s liquid outer core. They evolve on time scales ranging from seconds to years and these variations induce currents in the Earth’s core, mantle, lithosphere and oceans that generate magnetic signals Separating all these contributions from the core field requires an adequate handling of data and has long been an important obstacle to the development of high-resolution core field models. Modern models—e.g. the GRIMM (Lesur et al 2015) or, more recently, the Chaos-6 model (Finlay et al 2016), both of which use splines of order 6 for their time evolution—separate well contributions from the core and external sources They are typically able to render short core field time scales, with a precision depending on the spatial scales—e.g. small spherical harmonics degrees have a resolution of the order of 2 years. The POMME model (Chulliat and Maus 2014) uses a 3-year sliding time window, but its time resolution remains of the same order
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