The CHAOS-7 geomagnetic field model and observed changes in the South Atlantic Anomaly

Abstract

We present the CHAOS-7 model of the time-dependent near-Earth geomagnetic field between 1999 and 2020 based on magnetic field observations collected by the low-Earth orbit satellites Swarm, CryoSat-2, CHAMP, SAC-C and Ørsted, and on annual differences of monthly means of ground observatory measurements. The CHAOS-7 model consists of a time-dependent internal field up to spherical harmonic degree 20, a static internal field which merges to the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, estimates of Euler angles describing the alignment of satellite vector magnetometers, and magnetometer calibration parameters for CryoSat-2. Only data from dark regions satisfying strict geomagnetic quiet-time criteria (including conditions on IMF $$B_z$$ B z and $$B_y$$ B y at all latitudes) were used in the field estimation. Model parameters were estimated using an iteratively reweighted regularized least-squares procedure; regularization of the time-dependent internal field was relaxed at high spherical harmonic degree compared with previous versions of the CHAOS model. We use CHAOS-7 to investigate recent changes in the geomagnetic field, studying the evolution of the South Atlantic weak field anomaly and rapid field changes in the Pacific region since 2014. At Earth’s surface a secondary minimum of the South Atlantic Anomaly is now evident to the south west of Africa. Green’s functions relating the core–mantle boundary radial field to the surface intensity show this feature is connected with the movement and evolution of a reversed flux feature under South Africa. The continuing growth in size and weakening of the main anomaly is linked to the westward motion and gathering of reversed flux under South America. In the Pacific region at Earth’s surface between 2015 and 2018 a sign change has occurred in the second time derivative (acceleration) of the radial component of the field. This acceleration change took the form of a localized, east–west oriented, dipole. It was clearly recorded on ground, for example at the magnetic observatory at Honolulu, and was seen in Swarm observations over an extended region in the central and western Pacific. Downward continuing to the core–mantle boundary, we find this event originated in field acceleration changes at low latitudes beneath the central and western Pacific in 2017.

COV-OBS.x2: 180 years of geomagnetic field evolution from ground-based and satellite observations

“…As observed by Gillet et al. ( 2015 ) with COV-OBS.x1, their evolutions are overall coherent with that of the CHAOS-7 model (Finlay et al 2020 ), put aside an nT shift on in link with the differences of induced model. However, we notice that We do not capture some of the rapid changes (of period less than years) for the larger length-scales, due to the use of 2 years knot spacing for the splines basis; Consequence of the employed stochastic prior, our solutions tend to be less smooth towards small length-scales.…”

Observing Earth’s magnetic environment with the GRACE-FO mission

“…All other products have been interpolated to these reference time stamps using linear interpolation. At these times and respective satellite locations, we further generated predictions of the core, the crustal, and the large-scale magnetospheric field by the CHAOS-7 model (Finlay et al 2019(Finlay et al , 2020. The modelled values will later be the reference during calibrations of GRACE-FO magnetic data.…”

Decomposition of Geomagnetic Secular Acceleration Into Traveling Waves Using Complex Empirical Orthogonal Functions

“…Our results are also affected by the duration of the record, especially when the mode period is comparable to the record length. To assess the influence of regularization we repeat the analysis using CHAOS‐7 (Finlay et al, 2020). A much weaker regularization in CHAOS‐7 puts more power in the secular acceleration.…”