Recognizing and interpreting the longest wavelength lithospheric magnetic signals obscured by...
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Recognizing and interpreting the longest wavelength lithospheric magnetic signals obscured by overlap with the core field 2004 Fall AGU: GP31A- 0821 Michael Purucker, Raytheon ITSS @ Geodynamics Branch GSFC, Greenbelt, MD USA 20771 Kathryn Whaler, School of Geosciences, University of Edinburgh, West Mains Rd, Edinburgh, EH9 3JW, UK [email protected] +1 301 614 6473 http://geodynamics.gsfc.nasa.gov/personal_pages/purucker/ purucker.html References, and suggested readings: References, and suggested readings: GMT, 2004, v..4, GMT, 2004, v..4, http://gmt.soest.hawaii.edu http://gmt.soest.hawaii.edu , P. Wessel and W. Smith. , P. Wessel and W. Smith. Jackson, A., Accounting for crustal magnetization in models of the core magnetic field, Geophys. J. Int., 103, 657-673, 1990. Langel, R. and Hinze, W., Langel, R. and Hinze, W., The magnetic field of the Earth’s lithosphere The magnetic field of the Earth’s lithosphere , Cambridge Univ. Press, Cambridge, 429 , Cambridge Univ. Press, Cambridge, 429 pp, 1998 pp, 1998 Nataf, H., and Ricard, Y., 3SMAC: an a priori tomographic model of the upper mantle based on Nataf, H., and Ricard, Y., 3SMAC: an a priori tomographic model of the upper mantle based on geophysical modeling, geophysical modeling, Phys. Earth Plan. Int Phys. Earth Plan. Int ., 95, 101-122, 1996. ., 95, 101-122, 1996. Maus, S., et al., Earth’s crustal magnetic field determined to spherical harmonic degre 90 from CHAMP Maus, S., et al., Earth’s crustal magnetic field determined to spherical harmonic degre 90 from CHAMP satellite measurements, Geophys. J. Int, submitted, 2005. Available electronically at satellite measurements, Geophys. J. Int, submitted, 2005. Available electronically at www.gfz- www.gfz- potsdam.de/pb2/pb23/SatMag/litmod3.html potsdam.de/pb2/pb23/SatMag/litmod3.html Mayhew, M. and Estes, R., Mayhew, M. and Estes, R., Equivalent source modeling of the core magnetic field using Magsat data , , J. J. Geomagnetism and Geoelectricity Geomagnetism and Geoelectricity , 35, 119-130, 1983 , 35, 119-130, 1983 Parker, R.L., Shure, L. and Hildebrand, J.A., The application of inverse theory to seamount magnetism, Rev. Geophys., 25, 17-40, 1987. Parker, R.L., Geophysical Inverse Theory, 386 pp., Princeton University Press, Princeton, 1994. Purucker, M.E.,T.J. Sabaka, and R.A. Langel, Conjugate gradient analysis: a new tool for studying satellite magnetic data sets, Geophys. Res. Lett., 23, 507-510, 1996. Purucker, M, Maus, S., and Luehr, H., From validation to prediction: Lithospheric field studies from Purucker, M, Maus, S., and Luehr, H., From validation to prediction: Lithospheric field studies from Magsat to Swarm, Magsat to Swarm, Earth Planets and Space Earth Planets and Space , available electronically at , available electronically at http://geodynamics.gsfc.nasa.gov/research/purucker/litho_eps_swarm_bibtex.pdf,submitted, 2004. http://geodynamics.gsfc.nasa.gov/research/purucker/litho_eps_swarm_bibtex.pdf,submitted, 2004. Reigber, C., Luhr, H., Schwintzer, P., and Wichert, J. (editors), Reigber, C., Luhr, H., Schwintzer, P., and Wichert, J. (editors), Earth Observation with CHAMP Earth Observation with CHAMP , Springer, , Springer, Heidelberg, 2004. Heidelberg, 2004. Sabaka, T.J., Olsen, N., and Purucker, M., Extending Comprehensive models of the Earth’s magnetic Sabaka, T.J., Olsen, N., and Purucker, M., Extending Comprehensive models of the Earth’s magnetic Abstract: We recognize and characterize two Abstract: We recognize and characterize two distinctive patterns evident in new maps of the distinctive patterns evident in new maps of the lithospheric magnetic field from the CHAMP lithospheric magnetic field from the CHAMP satellite, and new minimum amplitude satellite, and new minimum amplitude magnetization models that we deduce. The magnetization models that we deduce. The boundaries of these patterns define long- boundaries of these patterns define long- wavelength features in the lithospheric field not wavelength features in the lithospheric field not previously recognized because they were obscured previously recognized because they were obscured by overlap with the core field. These boundaries by overlap with the core field. These boundaries correspond to known crustal thickness variations. correspond to known crustal thickness variations. The major exceptions, the Sahara and most of The major exceptions, the Sahara and most of South America south of the Equator, are regions South America south of the Equator, are regions where direct estimates of crustal thickness and where direct estimates of crustal thickness and heat flow are sparse. heat flow are sparse. Satellite-based magnetic field maps (MF-3) Satellite-based magnetic field maps (MF-3) In order to minimize time-variable fields associated with the In order to minimize time-variable fields associated with the interaction of the solar and terrestrial dynamos, we usually utilize interaction of the solar and terrestrial dynamos, we usually utilize spherical harmonic models built from data gathered during magnetically spherical harmonic models built from data gathered during magnetically quiet times, rather than the field data directly. Both CM4 (Sabaka et quiet times, rather than the field data directly. Both CM4 (Sabaka et al., 2004) and MF-3 (Maus et al., 2004) are models of this type. We al., 2004) and MF-3 (Maus et al., 2004) are models of this type. We prefer to use MF-3 for the purpose of this exercise because it goes to prefer to use MF-3 for the purpose of this exercise because it goes to higher spherical harmonic degree (90 vs 65). MF-3 is a lithospheric higher spherical harmonic degree (90 vs 65). MF-3 is a lithospheric field model only, and extends from degree 16 to 90. The CHAMP magnetic field model only, and extends from degree 16 to 90. The CHAMP magnetic field satellite input to MF-3 has had removed an internal field model field satellite input to MF-3 has had removed an internal field model to degree 15, an external field model of degree 2, and the predicted to degree 15, an external field model of degree 2, and the predicted signatures from eight main ocean tidal components. Additional external signatures from eight main ocean tidal components. Additional external fields are subsequently removed in a track-by-track scheme. Because of fields are subsequently removed in a track-by-track scheme. Because of its design philosophy, MF-3 can be considered a minimum estimate of the its design philosophy, MF-3 can be considered a minimum estimate of the lithospheric magnetic field, one in which there will be some lithospheric magnetic field, one in which there will be some suppression of along-track magnetic fields, which are N-S in equatorial suppression of along-track magnetic fields, which are N-S in equatorial and mid-latitudes. Regularization has been applied to degrees higher and mid-latitudes. Regularization has been applied to degrees higher than 60 to extract clusters of spherical harmonic coefficients that are than 60 to extract clusters of spherical harmonic coefficients that are well-resolved by the data. The highest noise levels remain in and well-resolved by the data. The highest noise levels remain in and Introduction Introduction Features of the lithospheric magnetic field with wavelengths in excess of Features of the lithospheric magnetic field with wavelengths in excess of 3000 km (spherical harmonic degree 13) are completely obscured by overlap 3000 km (spherical harmonic degree 13) are completely obscured by overlap with the core field. Between 2600 and 3000 km both core and lithospheric with the core field. Between 2600 and 3000 km both core and lithospheric signatures are present, hindering efforts at separation. Previous efforts signatures are present, hindering efforts at separation. Previous efforts (see for example Mayhew and Estes, 1983) at separation of the two fields (see for example Mayhew and Estes, 1983) at separation of the two fields have failed, and there is strong reason to believe that the two fields are have failed, and there is strong reason to believe that the two fields are not separable unless the core field is shut off, or changed signficantly. not separable unless the core field is shut off, or changed signficantly. However, new higher resolution models of the crustal field are becoming However, new higher resolution models of the crustal field are becoming available (Maus et al., 2005). In order to make some progress on available (Maus et al., 2005). In order to make some progress on qualitatively understanding the longest wavelengths, we borrow an old idea qualitatively understanding the longest wavelengths, we borrow an old idea from the exploration geophysics community, and visually characterize the from the exploration geophysics community, and visually characterize the field, and magnetization solutions deduced from that field. Because of field, and magnetization solutions deduced from that field. Because of the wider spectral content of the new solutions, we hope that larger the wider spectral content of the new solutions, we hope that larger patterns will become apparent, patterns that were not obvious when we were patterns will become apparent, patterns that were not obvious when we were examining very band-limited solutions. By way of analogy, we hope to be examining very band-limited solutions. By way of analogy, we hope to be able to differentiate ‘the forests from the fields’ by characterizing able to differentiate ‘the forests from the fields’ by characterizing features at smaller spatial scales (like the ‘trees and grasses’). This features at smaller spatial scales (like the ‘trees and grasses’). This analogy implies that our ‘imaging’ technique can’t see the ‘forests and analogy implies that our ‘imaging’ technique can’t see the ‘forests and fields’, just the ‘trees and grasses’, and that there are features at fields’, just the ‘trees and grasses’, and that there are features at small scales that give us clues into what is happening at the largest small scales that give us clues into what is happening at the largest scales. scales. A 3-component magnetization model from MF-3 A 3-component magnetization model from MF-3 In order to further characterize the magnetic field, we derive and show a three-component magnetization model from MF-3. Using all three components, we model magnetization as a linear combination of the Green's functions relating magnetization at any point in a 40 km thick magnetized crust to a satellite measurement of the magnetic field. This avoids subjective choices on the arrangement of equivalent source dipoles (Purucker et al., 2004), and produces a spatially continuous magnetization model. Details of the technique are presented below. The field predicted from the damped inversion is shown above, immediately to the right of the MF-3 model. To the right of that can be seen the three component magnetization solution, the calculated scalar magnetization, and the declination and inclination of the magnetization, plotted where those angles are well-determined. All are plotted at the Earth’s surface. Note that we are NOT assuming that the magnetization is in the direction of the core field. Conclusions Conclusions We recognize and characterize two distinctive patterns evident in We recognize and characterize two distinctive patterns evident in new maps of the lithospheric field deduced from CHAMP. The new maps of the lithospheric field deduced from CHAMP. The boundaries of these patterns define long-wavelength features in boundaries of these patterns define long-wavelength features in the lithospheric field not previously recognized because they the lithospheric field not previously recognized because they were obscured by overlap with the core field. These boundaries were obscured by overlap with the core field. These boundaries correspond in a general way to known magnetic crustal thickness correspond in a general way to known magnetic crustal thickness variations. The major exceptions, the Sahara and most of South variations. The major exceptions, the Sahara and most of South America south of the Equator, are regions where crustal thickness America south of the Equator, are regions where crustal thickness and heat flow are poorly known. and heat flow are poorly known. MF-3 and its recovery MF-3 and its recovery MF-3 model evaluated MF-3 model evaluated over North and over North and Central America (Maus Central America (Maus et al., 2004) et al., 2004) Characterizing the MF-3 model Characterizing the MF-3 model Over the North American region, there are two patterns Over the North American region, there are two patterns apparent in the vertical component map predicted at 300 km apparent in the vertical component map predicted at 300 km altitude at the left. The first pattern, which we will refer altitude at the left. The first pattern, which we will refer to as ‘C’, encompasses the North American land mass, the to as ‘C’, encompasses the North American land mass, the Caribbean and Gulf of Mexico, and northernmost South Caribbean and Gulf of Mexico, and northernmost South America. The peak-to-trough magnitude of anomalies in ‘C’ America. The peak-to-trough magnitude of anomalies in ‘C’ typically exceeds 50 nT, and the anomalies are either typically exceeds 50 nT, and the anomalies are either equidimensional or oriented in a direction subparallel to equidimensional or oriented in a direction subparallel to the nearest coastline or tectonic element. The second the nearest coastline or tectonic element. The second pattern, which we will refer to as ‘O’, encompasses the pattern, which we will refer to as ‘O’, encompasses the Eastern Pacific, the Cocos plate, and the western Atlantic Eastern Pacific, the Cocos plate, and the western Atlantic away from continental North America. The peak-to-trough away from continental North America. The peak-to-trough magnitude of anomalies in ‘O’ is typically less than 30 nT, magnitude of anomalies in ‘O’ is typically less than 30 nT, and the anomalies are commonly narrow and elongate in the and the anomalies are commonly narrow and elongate in the direction of the nearest spreading or subduction zone. The direction of the nearest spreading or subduction zone. The ‘C’ pattern can be discerned on the global maps above, when ‘C’ pattern can be discerned on the global maps above, when account is taken of the higher altitude. The ‘C’ pattern is account is taken of the higher altitude. The ‘C’ pattern is characteristic of much of the Asian landmass, a region characteristic of much of the Asian landmass, a region centered on but more extensive than Australia, and two broad centered on but more extensive than Australia, and two broad regions within the African landmass. The ‘O’ pattern is seen regions within the African landmass. The ‘O’ pattern is seen in the eastern Pacific, the North and South Atlantic, and in the eastern Pacific, the North and South Atlantic, and the Indian oceans. the Indian oceans. Characterizing the MF-3 based Characterizing the MF-3 based magnetization model magnetization model The ‘C’ and ‘O’ patterns are evident in the MF-3 based The ‘C’ and ‘O’ patterns are evident in the MF-3 based magnetization model. The magnitude of M shows these patterns magnetization model. The magnitude of M shows these patterns unambiguously. Regions with magnetizations greater than 0.1 A/m unambiguously. Regions with magnetizations greater than 0.1 A/m (red regions on above map) correspond to the ‘C’ pattern, and (red regions on above map) correspond to the ‘C’ pattern, and regions with magnetization less than 0.06 A/m (grey regions) regions with magnetization less than 0.06 A/m (grey regions) correspond to the ‘O’ pattern. Intermediate values of correspond to the ‘O’ pattern. Intermediate values of magnetization (between 0.06 and 0.1 A/m, pink on above map) magnetization (between 0.06 and 0.1 A/m, pink on above map) generally envelop regions displaying the ‘C’ pattern. In a generally envelop regions displaying the ‘C’ pattern. In a general way, the ‘C’ and ‘A’ patterns correspond to regions of general way, the ‘C’ and ‘A’ patterns correspond to regions of thick and thin magnetic crustal thickness, as defined by thick and thin magnetic crustal thickness, as defined by temperature and seismology in the 3SMAC model (Nataf and Ricard, temperature and seismology in the 3SMAC model (Nataf and Ricard, 1996) and shown immediately to the right of the scalar 1996) and shown immediately to the right of the scalar magnetization map. There are conspicuous exceptions to this magnetization map. There are conspicuous exceptions to this generalization. Most of the South American landmass south of the generalization. Most of the South American landmass south of the Equator is characterized by the ‘O’ pattern, yet crustal Equator is characterized by the ‘O’ pattern, yet crustal Acknowledgments Acknowledgments : : We thank Gauthier Hulot for providing some clarity to our early work on this subject, and Stefan Maus We thank Gauthier Hulot for providing some clarity to our early work on this subject, and Stefan Maus and the CHAMP team at GFZ for MF-3 and the CHAMP team at GFZ for MF-3 Details of technique Details of technique
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Transcript of Recognizing and interpreting the longest wavelength lithospheric magnetic signals obscured by...
Recognizing and interpreting the longest wavelength lithospheric magnetic signals obscured by overlap with the core field 2004 Fall AGU: GP31A-0821
Michael Purucker, Raytheon ITSS @ Geodynamics Branch
GSFC, Greenbelt, MD USA 20771
Kathryn Whaler, School of Geosciences,
University of Edinburgh, West Mains Rd, Edinburgh, EH9 3JW, UK
[email protected] +1 301 614 6473
http://geodynamics.gsfc.nasa.gov/personal_pages/purucker/purucker.html
References, and suggested readings:References, and suggested readings:GMT, 2004, v..4, GMT, 2004, v..4, http://gmt.soest.hawaii.eduhttp://gmt.soest.hawaii.edu, P. Wessel and W. Smith., P. Wessel and W. Smith.Jackson, A., Accounting for crustal magnetization in models of the core magnetic field, Geophys. J. Int., 103, 657-673, 1990.
Langel, R. and Hinze, W., Langel, R. and Hinze, W., The magnetic field of the Earth’s lithosphereThe magnetic field of the Earth’s lithosphere , Cambridge Univ. Press, Cambridge, 429 pp, 1998, Cambridge Univ. Press, Cambridge, 429 pp, 1998
Nataf, H., and Ricard, Y., 3SMAC: an a priori tomographic model of the upper mantle based on geophysical modeling, Nataf, H., and Ricard, Y., 3SMAC: an a priori tomographic model of the upper mantle based on geophysical modeling, Phys. Earth Plan. IntPhys. Earth Plan. Int., ., 95, 101-122, 1996.95, 101-122, 1996.
Maus, S., et al., Earth’s crustal magnetic field determined to spherical harmonic degre 90 from CHAMP satellite measurements, Geophys. J. Maus, S., et al., Earth’s crustal magnetic field determined to spherical harmonic degre 90 from CHAMP satellite measurements, Geophys. J. Int, submitted, 2005. Available electronically at Int, submitted, 2005. Available electronically at www.gfz-potsdam.de/pb2/pb23/SatMag/litmod3.htmlwww.gfz-potsdam.de/pb2/pb23/SatMag/litmod3.html
Mayhew, M. and Estes, R., Mayhew, M. and Estes, R., Equivalent source modeling of the core magnetic field using Magsat data, , J. Geomagnetism and GeoelectricityJ. Geomagnetism and Geoelectricity, 35, , 35, 119-130, 1983119-130, 1983Parker, R.L., Shure, L. and Hildebrand, J.A., The application of inverse theory to seamount magnetism, Rev. Geophys., 25, 17-40, 1987.Parker, R.L., Geophysical Inverse Theory, 386 pp., Princeton University Press, Princeton, 1994.Purucker, M.E.,T.J. Sabaka, and R.A. Langel, Conjugate gradient analysis: a new tool for studying satellite magnetic data sets, Geophys. Res. Lett., 23, 507-510, 1996.
Purucker, M, Maus, S., and Luehr, H., From validation to prediction: Lithospheric field studies from Magsat to Swarm, Purucker, M, Maus, S., and Luehr, H., From validation to prediction: Lithospheric field studies from Magsat to Swarm, Earth Planets and Earth Planets and SpaceSpace, available electronically at http://geodynamics.gsfc.nasa.gov/research/purucker/litho_eps_swarm_bibtex.pdf,submitted, 2004. , available electronically at http://geodynamics.gsfc.nasa.gov/research/purucker/litho_eps_swarm_bibtex.pdf,submitted, 2004.
Reigber, C., Luhr, H., Schwintzer, P., and Wichert, J. (editors), Reigber, C., Luhr, H., Schwintzer, P., and Wichert, J. (editors), Earth Observation with CHAMPEarth Observation with CHAMP, Springer, Heidelberg, 2004., Springer, Heidelberg, 2004.
Sabaka, T.J., Olsen, N., and Purucker, M., Extending Comprehensive models of the Earth’s magnetic field with Orsted and CHAMP data, Sabaka, T.J., Olsen, N., and Purucker, M., Extending Comprehensive models of the Earth’s magnetic field with Orsted and CHAMP data, Geophys. J. IntGeophys. J. Int., 159, 521-547, 2004.., 159, 521-547, 2004.Shure, L., Parker, R.L. and Backus G.E., Harmonic splines for geomagnetic modelling, Phys. Earth Planet. Int., 28, 215-229, 1982.
Stauning, P., et al. (editors), Stauning, P., et al. (editors), Proceedings of the 4th Orsted International Science Team ConferenceProceedings of the 4th Orsted International Science Team Conference , Danish Met. Inst., Copenhagen, 2003, 300 p., Danish Met. Inst., Copenhagen, 2003, 300 p.
Whaler, K. and Langel, R., Minimal crustal magnetization from satellite data, Whaler, K. and Langel, R., Minimal crustal magnetization from satellite data, Phys. Earth Planet. IntPhys. Earth Planet. Int., 98, 303-319, 1996.., 98, 303-319, 1996.
Abstract: We recognize and characterize two distinctive Abstract: We recognize and characterize two distinctive patterns evident in new maps of the lithospheric magnetic patterns evident in new maps of the lithospheric magnetic field from the CHAMP satellite, and new minimum amplitude field from the CHAMP satellite, and new minimum amplitude magnetization models that we deduce. The boundaries of magnetization models that we deduce. The boundaries of these patterns define long-wavelength features in the these patterns define long-wavelength features in the lithospheric field not previously recognized because they lithospheric field not previously recognized because they were obscured by overlap with the core field. These were obscured by overlap with the core field. These boundaries correspond to known crustal thickness variations. boundaries correspond to known crustal thickness variations. The major exceptions, the Sahara and most of South America The major exceptions, the Sahara and most of South America south of the Equator, are regions where direct estimates of south of the Equator, are regions where direct estimates of crustal thickness and heat flow are sparse. crustal thickness and heat flow are sparse.
Satellite-based magnetic field maps (MF-3)Satellite-based magnetic field maps (MF-3)In order to minimize time-variable fields associated with the interaction of the solar and terrestrial In order to minimize time-variable fields associated with the interaction of the solar and terrestrial
dynamos, we usually utilize spherical harmonic models built from data gathered during magnetically dynamos, we usually utilize spherical harmonic models built from data gathered during magnetically quiet times, rather than the field data directly. Both CM4 (Sabaka et al., 2004) and MF-3 (Maus et quiet times, rather than the field data directly. Both CM4 (Sabaka et al., 2004) and MF-3 (Maus et al., 2004) are models of this type. We prefer to use MF-3 for the purpose of this exercise because it al., 2004) are models of this type. We prefer to use MF-3 for the purpose of this exercise because it goes to higher spherical harmonic degree (90 vs 65). MF-3 is a lithospheric field model only, and goes to higher spherical harmonic degree (90 vs 65). MF-3 is a lithospheric field model only, and
extends from degree 16 to 90. The CHAMP magnetic field satellite input to MF-3 has had removed extends from degree 16 to 90. The CHAMP magnetic field satellite input to MF-3 has had removed an internal field model to degree 15, an external field model of degree 2, and the predicted signatures an internal field model to degree 15, an external field model of degree 2, and the predicted signatures
from eight main ocean tidal components. Additional external fields are subsequently removed in a from eight main ocean tidal components. Additional external fields are subsequently removed in a track-by-track scheme. Because of its design philosophy, MF-3 can be considered a minimum track-by-track scheme. Because of its design philosophy, MF-3 can be considered a minimum
estimate of the lithospheric magnetic field, one in which there will be some suppression of along-estimate of the lithospheric magnetic field, one in which there will be some suppression of along-track magnetic fields, which are N-S in equatorial and mid-latitudes. Regularization has been track magnetic fields, which are N-S in equatorial and mid-latitudes. Regularization has been
applied to degrees higher than 60 to extract clusters of spherical harmonic coefficients that are well-applied to degrees higher than 60 to extract clusters of spherical harmonic coefficients that are well-resolved by the data. The highest noise levels remain in and around the auroral zones, and we will resolved by the data. The highest noise levels remain in and around the auroral zones, and we will
defer characterization of the fields in those areas because of the very band-limited nature of the defer characterization of the fields in those areas because of the very band-limited nature of the lithospheric signal in those areas.lithospheric signal in those areas.
IntroductionIntroductionFeatures of the lithospheric magnetic field with wavelengths in excess of 3000 km (spherical harmonic Features of the lithospheric magnetic field with wavelengths in excess of 3000 km (spherical harmonic
degree 13) are completely obscured by overlap with the core field. Between 2600 and 3000 km both core degree 13) are completely obscured by overlap with the core field. Between 2600 and 3000 km both core and lithospheric signatures are present, hindering efforts at separation. Previous efforts (see for and lithospheric signatures are present, hindering efforts at separation. Previous efforts (see for
example Mayhew and Estes, 1983) at separation of the two fields have failed, and there is strong reason example Mayhew and Estes, 1983) at separation of the two fields have failed, and there is strong reason to believe that the two fields are not separable unless the core field is shut off, or changed signficantly. to believe that the two fields are not separable unless the core field is shut off, or changed signficantly. However, new higher resolution models of the crustal field are becoming available (Maus et al., 2005). However, new higher resolution models of the crustal field are becoming available (Maus et al., 2005). In order to make some progress on qualitatively understanding the longest wavelengths, we borrow an In order to make some progress on qualitatively understanding the longest wavelengths, we borrow an
old idea from the exploration geophysics community, and visually characterize the field, and old idea from the exploration geophysics community, and visually characterize the field, and magnetization solutions deduced from that field. Because of the wider spectral content of the new magnetization solutions deduced from that field. Because of the wider spectral content of the new
solutions, we hope that larger patterns will become apparent, patterns that were not obvious when we solutions, we hope that larger patterns will become apparent, patterns that were not obvious when we were examining very band-limited solutions. By way of analogy, we hope to be able to differentiate ‘the were examining very band-limited solutions. By way of analogy, we hope to be able to differentiate ‘the forests from the fields’ by characterizing features at smaller spatial scales (like the ‘trees and grasses’). forests from the fields’ by characterizing features at smaller spatial scales (like the ‘trees and grasses’). This analogy implies that our ‘imaging’ technique can’t see the ‘forests and fields’, just the ‘trees and This analogy implies that our ‘imaging’ technique can’t see the ‘forests and fields’, just the ‘trees and
grasses’, and that there are features at small scales that give us clues into what is happening at the grasses’, and that there are features at small scales that give us clues into what is happening at the largest scales.largest scales.
A 3-component magnetization model from MF-3A 3-component magnetization model from MF-3In order to further characterize the magnetic field, we derive and show a three-component magnetization model from MF-3. Using all three components, we model magnetization as a linear combination of the Green's functions relating magnetization at any point in a 40 km thick magnetized crust to a satellite measurement of the magnetic field. This avoids subjective choices on the arrangement of equivalent source dipoles (Purucker et al., 2004), and produces a spatially continuous magnetization model. Details of the technique are presented below. The field predicted from the damped inversion is shown above, immediately to the right of the MF-3 model. To the right of that can be seen the three component magnetization solution, the calculated scalar magnetization, and the declination and inclination of the magnetization, plotted where those angles are well-determined. All are plotted at the Earth’s surface. Note that we are NOT assuming that the magnetization is in the direction of the core field.
ConclusionsConclusionsWe recognize and characterize two distinctive patterns evident in new maps of the We recognize and characterize two distinctive patterns evident in new maps of the
lithospheric field deduced from CHAMP. The boundaries of these patterns define long-lithospheric field deduced from CHAMP. The boundaries of these patterns define long-wavelength features in the lithospheric field not previously recognized because they were wavelength features in the lithospheric field not previously recognized because they were obscured by overlap with the core field. These boundaries correspond in a general way to obscured by overlap with the core field. These boundaries correspond in a general way to
known magnetic crustal thickness variations. The major exceptions, the Sahara and most of known magnetic crustal thickness variations. The major exceptions, the Sahara and most of South America south of the Equator, are regions where crustal thickness and heat flow are South America south of the Equator, are regions where crustal thickness and heat flow are
poorly known.poorly known.
MF-3 and its recoveryMF-3 and its recovery
MF-3 model evaluated over MF-3 model evaluated over North and Central America North and Central America
(Maus et al., 2004)(Maus et al., 2004)
Characterizing the MF-3 modelCharacterizing the MF-3 modelOver the North American region, there are two patterns apparent in the vertical Over the North American region, there are two patterns apparent in the vertical
component map predicted at 300 km altitude at the left. The first pattern, which we component map predicted at 300 km altitude at the left. The first pattern, which we will refer to as ‘C’, encompasses the North American land mass, the Caribbean and will refer to as ‘C’, encompasses the North American land mass, the Caribbean and Gulf of Mexico, and northernmost South America. The peak-to-trough magnitude of Gulf of Mexico, and northernmost South America. The peak-to-trough magnitude of
anomalies in ‘C’ typically exceeds 50 nT, and the anomalies are either anomalies in ‘C’ typically exceeds 50 nT, and the anomalies are either equidimensional or oriented in a direction subparallel to the nearest coastline or equidimensional or oriented in a direction subparallel to the nearest coastline or
tectonic element. The second pattern, which we will refer to as ‘O’, encompasses the tectonic element. The second pattern, which we will refer to as ‘O’, encompasses the Eastern Pacific, the Cocos plate, and the western Atlantic away from continental Eastern Pacific, the Cocos plate, and the western Atlantic away from continental
North America. The peak-to-trough magnitude of anomalies in ‘O’ is typically less North America. The peak-to-trough magnitude of anomalies in ‘O’ is typically less than 30 nT, and the anomalies are commonly narrow and elongate in the direction of than 30 nT, and the anomalies are commonly narrow and elongate in the direction of
the nearest spreading or subduction zone. The ‘C’ pattern can be discerned on the the nearest spreading or subduction zone. The ‘C’ pattern can be discerned on the global maps above, when account is taken of the higher altitude. The ‘C’ pattern is global maps above, when account is taken of the higher altitude. The ‘C’ pattern is
characteristic of much of the Asian landmass, a region centered on but more characteristic of much of the Asian landmass, a region centered on but more extensive than Australia, and two broad regions within the African landmass. The extensive than Australia, and two broad regions within the African landmass. The
‘O’ pattern is seen in the eastern Pacific, the North and South Atlantic, and the ‘O’ pattern is seen in the eastern Pacific, the North and South Atlantic, and the Indian oceans.Indian oceans.
Characterizing the MF-3 based magnetization modelCharacterizing the MF-3 based magnetization modelThe ‘C’ and ‘O’ patterns are evident in the MF-3 based magnetization model. The The ‘C’ and ‘O’ patterns are evident in the MF-3 based magnetization model. The magnitude of M shows these patterns unambiguously. Regions with magnetizations magnitude of M shows these patterns unambiguously. Regions with magnetizations greater than 0.1 A/m (red regions on above map) correspond to the ‘C’ pattern, and greater than 0.1 A/m (red regions on above map) correspond to the ‘C’ pattern, and regions with magnetization less than 0.06 A/m (grey regions) correspond to the ‘O’ regions with magnetization less than 0.06 A/m (grey regions) correspond to the ‘O’
pattern. Intermediate values of magnetization (between 0.06 and 0.1 A/m, pink on above pattern. Intermediate values of magnetization (between 0.06 and 0.1 A/m, pink on above map) generally envelop regions displaying the ‘C’ pattern. In a general way, the ‘C’ and map) generally envelop regions displaying the ‘C’ pattern. In a general way, the ‘C’ and
‘A’ patterns correspond to regions of thick and thin magnetic crustal thickness, as ‘A’ patterns correspond to regions of thick and thin magnetic crustal thickness, as defined by temperature and seismology in the 3SMAC model (Nataf and Ricard, 1996) defined by temperature and seismology in the 3SMAC model (Nataf and Ricard, 1996)
and shown immediately to the right of the scalar magnetization map. There are and shown immediately to the right of the scalar magnetization map. There are conspicuous exceptions to this generalization. Most of the South American landmass conspicuous exceptions to this generalization. Most of the South American landmass south of the Equator is characterized by the ‘O’ pattern, yet crustal thicknesses are south of the Equator is characterized by the ‘O’ pattern, yet crustal thicknesses are typical of continental crust. The other major exception is the Sahara desert, again typical of continental crust. The other major exception is the Sahara desert, again characterized by the ‘O’ pattern but with typical continental crustal thicknesses.characterized by the ‘O’ pattern but with typical continental crustal thicknesses.
AcknowledgmentsAcknowledgments::We thank Gauthier Hulot for providing some clarity to our early work on this subject, and Stefan Maus and the CHAMP team at GFZ for MF-3We thank Gauthier Hulot for providing some clarity to our early work on this subject, and Stefan Maus and the CHAMP team at GFZ for MF-3
Details of techniqueDetails of technique
