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

• Main Authors: Finlay, Christopher C, Kloss, Clemens, Olsen, Nils, Hammer, Magnus D, Tøffner-Clausen, Lars, Grayver, Alexander, Kuvshinov, Alexey
• Format: Journal Article
• Language: English
• Published: 21-10-2020
• Subjects: Physics - Geophysics
• DOI: 10.48550/arxiv.2010.11313

Earth Planets and Space 72 (2020) 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 {\it Swarm}, CryoSat-2, CHAMP, SAC-C and
{\O}rsted, 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 $B_z$ and $B_y$ 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.