Tools for sharing and evaluating the CHAOS geomagnetic field model and the shc-file format for time-dependent spherical harmonic models

Geomagnetic field models are essential in the study of the physical processes that contribute to the Earth’s magnetic field. There are several groups that build models of the Earth’s magnetic field. These models essentially differ in the magnetic data and mathematical methods used during the model estimation, and in the represented sources of the geomagnetic field. It is then the users who choose the models that are most suitable for the study of the geophysical signals of interest. However, there is currently no single platform where field models are collected in a standardised way, and that provides information which helps users to find the best models for their purposes.

Here, we present the geomagnetic field model called CHAOS that is developed and regularly updated by the Technical University of Denmark. CHAOS provides estimates of the recent time-dependent and static internal magnetic fields, and the external magnetospheric field during quiet geomagnetic conditions. It is derived from magnetic data collected by the Swarm, CHAMP, Ørsted, SAC-C, CryoSat-2 satellite missions supplemented by ground observatory data. It is updated approximately every 4 months with the latest ground and satellite data; the current version CHAOS-7.9 covers the time from 1997 to November 2021.

The model is distributed in various formats. For the time-dependent internal field, B-spline coefficients for each spherical harmonic are provided in a similar format as traditionally used for the gufm1 historical field model and the CALS7K millennial timescale models. It is also provided in the shc-file format, which was developed and adopted for distributing spherical harmonic models determined in connection with the Swarm magnetic satellite mission. This format allows reconstruction of spline-based models from a dense sampling of the time series of the spherical harmonic coefficients and is easier for non-experts to use. A piecewise polynomial Matlab version is also available. For reading and evaluating the CHAOS model, we provide Fortran, Matlab and Python software. In particular, we have recently developed the ChaosMagPy Python package, which allows the CHAOS model (and other spherical harmonic field models) to be easily evaluated and visualized.

Although the shc-file format and ChaosMagPy have been developed primarily in support of the Swarm mission and the CHAOS model, they can be used more broadly for time-dependent spherical harmonic field models or serve as a starting point for the development of new tools that enable cross-disciplinary sharing of data and models.