Preferred locations of weak surface field in numerical dynamos with heterogeneous core–mantle boundary heat flux: consequences for the South Atlantic Anomaly

Terra-Nova, Filipe; Amit, Hagay; Choblet, G.

doi:10.1093/gji/ggy519

Cited by 28 publications

(31 citation statements)

References 80 publications

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“…Identifying the cause of the SAA structure in terms of field at the CMB and its evolution is no trivial matter. Terra-Nova et al (2019) shows that intensity anomalies at the surface such as the SAA are due to equatorial asymmetries of both reverse as well normal magnetic flux patches at the CMB. A sequence of snapshots of flow and field at the CMB and surface intensity during our longterm forecast are available at the Appendix (Figs.…”

Section: Long-term Predictionsmentioning

confidence: 97%

Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

Sanchez

Bärenzung

2020

Earth Planets Space

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The IGRF offers an important incentive for testing algorithms predicting the Earth’s magnetic field changes, known as secular variation (SV), in a 5-year range. Here, we present a SV candidate model for the 13th IGRF that stems from a sequential ensemble data assimilation approach (EnKF). The ensemble consists of a number of parallel-running 3D-dynamo simulations. The assimilated data are geomagnetic field snapshots covering the years 1840 to 2000 from the COV-OBS.x1 model and for 2001 to 2020 from the Kalmag model. A spectral covariance localization method, considering the couplings between spherical harmonics of the same equatorial symmetry and same azimuthal wave number, allows decreasing the ensemble size to about a 100 while maintaining the stability of the assimilation. The quality of 5-year predictions is tested for the past two decades. These tests show that the assimilation scheme is able to reconstruct the overall SV evolution. They also suggest that a better 5-year forecast is obtained keeping the SV constant compared to the dynamically evolving SV. However, the quality of the dynamical forecast steadily improves over the full assimilation window (180 years). We therefore propose the instantaneous SV estimate for 2020 from our assimilation as a candidate model for the IGRF-13. The ensemble approach provides uncertainty estimates, which closely match the residual differences with respect to the IGRF-13. Longer term predictions for the evolution of the main magnetic field features over a 50-year range are also presented. We observe the further decrease of the axial dipole at a mean rate of 8 nT/year as well as a deepening and broadening of the South Atlantic Anomaly. The magnetic dip poles are seen to approach an eccentric dipole configuration.

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Section: Long-term Predictionsmentioning

confidence: 97%

Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

Sanchez

Bärenzung

2020

Earth Planets Space

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“…The region of weakest magnetic field over central South America has expanded during these years, and a distinct secondary minimum has developed around 40° South on the Greenwich meridian (Terra-Nova et al. 2019 ; Rother et al. 2020 ).…”

Section: Evolution Of the South Atlantic Anomaly As Seen By mentioning

confidence: 99%

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

Finlay

Kloss

Olsen

et al. 2020

Earth Planets Space

207

206

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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.

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“…These results support the idea that the hemispherical asymmetry is a long‐term pattern of the field behavior. Large‐scale heterogeneities in conditions at the top or the bottom of the outer core that are considered responsible for the asymmetry observed in direct and historical observations (e.g., Aubert et al, ) and also found in numerical dynamo simulations with heterogeneous CMB heat flow (e.g., Terra‐Nova et al, ), may also play an important role at multimillennial timescales.…”

Section: Global Magnetic Field Characteristics For the Past 100 Kyrmentioning

confidence: 93%

“…Terra-Nova et al (2017) showed that the SAA clearly results from a combination of reversed and normal Reviews of Geophysics 10.1029/2019RG000656 flux regions below the South Atlantic, and the axial dipolarity of the field influences the position of the minimum field intensity at the Earth's surface. Unusual CMB composition defined by an LLSVP (large low shear wave velocity province) beneath South Africa has been suggested as the reason for the SAA and the LLSVP's longevity could play a role in recurrent weak field or persistent triggering of transitional events in this region (Bloxham & Gubbins, 1987;Gubbins, 1987;Tarduno et al, 2015;Terra-Nova et al, 2019).…”

Section: Field Morphologymentioning

confidence: 99%

One Hundred Thousand Years of Geomagnetic Field Evolution

Panovska

Korte

Constable

2019

Reviews of Geophysics

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Paleomagnetic records from sediments, archeological artifacts, and lava flows provide the foundation for studying geomagnetic field changes over 0-100 ka. Late Quaternary time-varying spherical harmonic models for 0-100 ka produce a global view used to evaluate new data records, study the paleomagnetic secular variation on centennial to multimillennial timescales, and investigate extreme regional or global events such as the Laschamp geomagnetic excursion. Recent modeling results (GGF100k and LSMOD.2) are compared to previous studies based on regional or global stacks and averages of relative geomagnetic paleointensity variations. Time-averaged field structure is similar on Holocene, 100 ky, and million-year timescales. Paleosecular variation activity varies greatly over 0-100 ka, with large changes in field strength and significant morphological changes that are especially evident when field strength is low. GGF100k exhibits a factor of 4 variation in geomagnetic axial dipole moment, and higher-resolution models suggest that much larger changes are likely during global excursions. There is some suggestion of recurrent field states resembling the present-day South Atlantic Anomaly, but these are not linked to initiation or evolution of excursions. Several properties used to characterize numerical dynamo simulations as "Earth-like" are evaluated and, in future, improved models may yet reveal systematic changes linked to the onset of geomagnetic excursions. Modeling results are useful in applications ranging from ground truth and data assimilation in geodynamo simulations to providing geochronological constraints and modeling the influence of geomagnetic variations on cosmogenic isotope production rates. Plain Language SummaryEarth's magnetic field is generated by dynamo activity in the planet's liquid outer core. It provides the foundation for modern navigation, protects us from space weather, and through its variations over time provides a mechanism for studying the deep interior of our planet. On timescales beyond a few centuries there are no direct observations, and paleomagnetic records from sediments, archeological artifacts, and lava flows are used for studying the geomagnetic field. They provide a global view of the field used to evaluate new data records, study the field changes (known as paleosecular variation) on centennial to multimillennial timescales, and investigate extreme regional or global events. The average field structure is similar on 10 ky, 100 ky, and million-year timescales. The magnetic dipole moment varies by at least a factor of 4 over 0-100 ka, with higher-resolution models suggesting much larger changes during global excursions. Recent results provide a global view, allowing analysis of geomagnetic excursions, including the Laschamp excursion. Results are useful in applications ranging from ground truth and data assimilation in numerical geodynamo simulations, to providing age constraints for climate records, archeological artifacts, and sediments and for studying impact of Earth's mag...

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Preferred locations of weak surface field in numerical dynamos with heterogeneous core–mantle boundary heat flux: consequences for the South Atlantic Anomaly

Cited by 28 publications

References 80 publications

Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

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

One Hundred Thousand Years of Geomagnetic Field Evolution

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