The CHAOS-8 geomagnetic field model

Abstract We present CHAOS-8, an extension of the CHAOS field model series that describes the time-dependent near-Earth geomagnetic field, valid from 1999 to 2025. It is estimated from magnetic measurements collected by multiple low-earth-orbit satellites, such as CHAMP and Swarm , and ground observatories. An initial version of this model, CHAOS-8.1, served as the parent model for constructing DTU’s candidate models for the 14th generation International Geomagnetic Reference Field. CHAOS-8 comprises a time-dependent internal field up to spherical harmonic degree 20, a static internal field that merges with the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, and estimates of alignment parameters for satellite vector magnetometers, along with calibration parameters for platform magnetometers. It also includes a co-estimated climatological model of the ionospheric field previously ignored within the CHAOS framework. The climatological model describes magnetic fields produced by polar ionospheric E-layer currents, which can be significant even under dark conditions. A new temporal regularization of the internal field is implemented, based on a priori statistics of the secular acceleration extracted from numerical geodynamo simulations. This enables rapid internal field changes to be better captured at small length scales. Magnetic measurements from the MSS-1 and CSES satellite missions were included for the first time in a CHAOS model. Model parameters were estimated using regularized iteratively reweighted least squares. The fit to the data was generally comparable to earlier versions of the CHAOS model. Co-estimation of an ionospheric field resulted in an improved fit in the polar regions. The new temporal regularization allowed stronger and more rapid temporal variations of the internal field at high spherical harmonic degrees. Analyzing sub-decadal variations of the internal field at the core–mantle boundary, we find westward-moving features and tentative evidence for eastward-moving features at low latitudes. The latter are of small length scales (apparent azimuthal wavenumber 13), moving at a speed of 200km/yr at the equator between $${0}^\circ$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> and $${90}^\circ$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mn>90</mml:mn> </mml:mrow> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> E after 2012. There are also indications of features moving across the geographic equator. These propagating features provide further evidence of traveling hydromagnetic waves at the core–mantle boundary. Graphical Abstract