LCS-1: A high-resolution global model of the lithospheric magnetic field derived from CHAMP and Swarm satellite observations

Nils Olsen, Dhananjay Ravat, Chris Finlay, Livia Kathleen Kother

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Abstract

We derive a new model, named LCS-1, of Earth’s lithospheric field based on four years (Sept 2006 – Sept 2010) of magnetic observations taken by the CHAMP satellite at altitudes lower than 350 km, as well as almost three years (April 2014 to December 2016) of measurements taken by the two lower Swarm satellites Alpha and Charlie. The model is determined entirely from magnetic “gradient” data (approximated by finite differences): the North-South gradient is approximated by first differences of 15 second along-track data (for CHAMP and each of the two Swarm satellites), while the East-West gradient is approximated by the difference between observations taken by Swarm Alpha and Charlie. In total, we used 6.2 mio data points.The model is parametrized by 35,000 equivalent point sources located on an almost equal-area grid at a depth of 100 km below the surface (WGS84 ellipsoid). The amplitudes of these point sources are determined by minimizing the misfit to the magnetic satellite “gradient” data together with the global average of |Br| at the ellipsoid surface (i.e. applying a L1 model regularisation of Br). In a final step we transform the point-source representation to a spherical harmonic expansion.The model shows very good agreement with previous satellite-derived lithospheric field models at low degree (degree correlation above 0.8 for degrees n ≤ 133). Comparison with indepen-dent near-surface aeromagnetic data from Australia yields good agreement (coherency > 0.55) at horizontal wavelengths down to at least 250 km, corresponding to spherical harmonic degree n ≈ 160.The LCS-1 vertical component and field intensity anomaly maps at Earth’s surface show sim-ilar features to those exhibited by the WDMAM2 and EMM2015 lithospheric field models truncated at degree 185 in regions where they include near-surface data and provide unprece-dented detail where they do not. Example regions of improvement include the Bangui anomaly region in central Africa, the west African cratons, the East African Rift region, the Bay of Ben-gal, the southern 90° E ridge, the Cretaceous quiet zone south of the Walvis Ridge and the younger parts of the South Atlantic.
Original languageEnglish
JournalGeophysical Journal International
Volume211
Issue number3
Pages (from-to)1461–1477
ISSN0956-540X
DOIs
Publication statusPublished - 2017

Bibliographical note

This article has been accepted for publication in Geophysical Journal International ©: The authors 2017 Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

Keywords

  • Geomagnetism
  • Earth’s magnetic field
  • Satellite
  • CHAMP
  • Swarm
  • Spherical harmonics
  • Lithosphere

Cite this

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title = "LCS-1: A high-resolution global model of the lithospheric magnetic field derived from CHAMP and Swarm satellite observations",
abstract = "We derive a new model, named LCS-1, of Earth’s lithospheric field based on four years (Sept 2006 – Sept 2010) of magnetic observations taken by the CHAMP satellite at altitudes lower than 350 km, as well as almost three years (April 2014 to December 2016) of measurements taken by the two lower Swarm satellites Alpha and Charlie. The model is determined entirely from magnetic “gradient” data (approximated by finite differences): the North-South gradient is approximated by first differences of 15 second along-track data (for CHAMP and each of the two Swarm satellites), while the East-West gradient is approximated by the difference between observations taken by Swarm Alpha and Charlie. In total, we used 6.2 mio data points.The model is parametrized by 35,000 equivalent point sources located on an almost equal-area grid at a depth of 100 km below the surface (WGS84 ellipsoid). The amplitudes of these point sources are determined by minimizing the misfit to the magnetic satellite “gradient” data together with the global average of |Br| at the ellipsoid surface (i.e. applying a L1 model regularisation of Br). In a final step we transform the point-source representation to a spherical harmonic expansion.The model shows very good agreement with previous satellite-derived lithospheric field models at low degree (degree correlation above 0.8 for degrees n ≤ 133). Comparison with indepen-dent near-surface aeromagnetic data from Australia yields good agreement (coherency > 0.55) at horizontal wavelengths down to at least 250 km, corresponding to spherical harmonic degree n ≈ 160.The LCS-1 vertical component and field intensity anomaly maps at Earth’s surface show sim-ilar features to those exhibited by the WDMAM2 and EMM2015 lithospheric field models truncated at degree 185 in regions where they include near-surface data and provide unprece-dented detail where they do not. Example regions of improvement include the Bangui anomaly region in central Africa, the west African cratons, the East African Rift region, the Bay of Ben-gal, the southern 90° E ridge, the Cretaceous quiet zone south of the Walvis Ridge and the younger parts of the South Atlantic.",
keywords = "Geomagnetism, Earth’s magnetic field, Satellite, CHAMP, Swarm, Spherical harmonics, Lithosphere",
author = "Nils Olsen and Dhananjay Ravat and Chris Finlay and Kother, {Livia Kathleen}",
note = "This article has been accepted for publication in Geophysical Journal International {\circledC}: The authors 2017 Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.",
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LCS-1: A high-resolution global model of the lithospheric magnetic field derived from CHAMP and Swarm satellite observations. / Olsen, Nils; Ravat, Dhananjay ; Finlay, Chris; Kother, Livia Kathleen.

In: Geophysical Journal International, Vol. 211, No. 3, 2017, p. 1461–1477.

Research output: Contribution to journalJournal articleResearchpeer-review

TY - JOUR

T1 - LCS-1: A high-resolution global model of the lithospheric magnetic field derived from CHAMP and Swarm satellite observations

AU - Olsen, Nils

AU - Ravat, Dhananjay

AU - Finlay, Chris

AU - Kother, Livia Kathleen

N1 - This article has been accepted for publication in Geophysical Journal International ©: The authors 2017 Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

PY - 2017

Y1 - 2017

N2 - We derive a new model, named LCS-1, of Earth’s lithospheric field based on four years (Sept 2006 – Sept 2010) of magnetic observations taken by the CHAMP satellite at altitudes lower than 350 km, as well as almost three years (April 2014 to December 2016) of measurements taken by the two lower Swarm satellites Alpha and Charlie. The model is determined entirely from magnetic “gradient” data (approximated by finite differences): the North-South gradient is approximated by first differences of 15 second along-track data (for CHAMP and each of the two Swarm satellites), while the East-West gradient is approximated by the difference between observations taken by Swarm Alpha and Charlie. In total, we used 6.2 mio data points.The model is parametrized by 35,000 equivalent point sources located on an almost equal-area grid at a depth of 100 km below the surface (WGS84 ellipsoid). The amplitudes of these point sources are determined by minimizing the misfit to the magnetic satellite “gradient” data together with the global average of |Br| at the ellipsoid surface (i.e. applying a L1 model regularisation of Br). In a final step we transform the point-source representation to a spherical harmonic expansion.The model shows very good agreement with previous satellite-derived lithospheric field models at low degree (degree correlation above 0.8 for degrees n ≤ 133). Comparison with indepen-dent near-surface aeromagnetic data from Australia yields good agreement (coherency > 0.55) at horizontal wavelengths down to at least 250 km, corresponding to spherical harmonic degree n ≈ 160.The LCS-1 vertical component and field intensity anomaly maps at Earth’s surface show sim-ilar features to those exhibited by the WDMAM2 and EMM2015 lithospheric field models truncated at degree 185 in regions where they include near-surface data and provide unprece-dented detail where they do not. Example regions of improvement include the Bangui anomaly region in central Africa, the west African cratons, the East African Rift region, the Bay of Ben-gal, the southern 90° E ridge, the Cretaceous quiet zone south of the Walvis Ridge and the younger parts of the South Atlantic.

AB - We derive a new model, named LCS-1, of Earth’s lithospheric field based on four years (Sept 2006 – Sept 2010) of magnetic observations taken by the CHAMP satellite at altitudes lower than 350 km, as well as almost three years (April 2014 to December 2016) of measurements taken by the two lower Swarm satellites Alpha and Charlie. The model is determined entirely from magnetic “gradient” data (approximated by finite differences): the North-South gradient is approximated by first differences of 15 second along-track data (for CHAMP and each of the two Swarm satellites), while the East-West gradient is approximated by the difference between observations taken by Swarm Alpha and Charlie. In total, we used 6.2 mio data points.The model is parametrized by 35,000 equivalent point sources located on an almost equal-area grid at a depth of 100 km below the surface (WGS84 ellipsoid). The amplitudes of these point sources are determined by minimizing the misfit to the magnetic satellite “gradient” data together with the global average of |Br| at the ellipsoid surface (i.e. applying a L1 model regularisation of Br). In a final step we transform the point-source representation to a spherical harmonic expansion.The model shows very good agreement with previous satellite-derived lithospheric field models at low degree (degree correlation above 0.8 for degrees n ≤ 133). Comparison with indepen-dent near-surface aeromagnetic data from Australia yields good agreement (coherency > 0.55) at horizontal wavelengths down to at least 250 km, corresponding to spherical harmonic degree n ≈ 160.The LCS-1 vertical component and field intensity anomaly maps at Earth’s surface show sim-ilar features to those exhibited by the WDMAM2 and EMM2015 lithospheric field models truncated at degree 185 in regions where they include near-surface data and provide unprece-dented detail where they do not. Example regions of improvement include the Bangui anomaly region in central Africa, the west African cratons, the East African Rift region, the Bay of Ben-gal, the southern 90° E ridge, the Cretaceous quiet zone south of the Walvis Ridge and the younger parts of the South Atlantic.

KW - Geomagnetism

KW - Earth’s magnetic field

KW - Satellite

KW - CHAMP

KW - Swarm

KW - Spherical harmonics

KW - Lithosphere

U2 - 10.1093/gji/ggx381

DO - 10.1093/gji/ggx381

M3 - Journal article

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SP - 1461

EP - 1477

JO - Geophysical Journal International

JF - Geophysical Journal International

SN - 0956-540X

IS - 3

ER -