Recognizing and interpreting the longest wavelength lithospheric magnetic signals obscured by overla

1
Recognizing and interpreting the longest
wavelength lithospheric magnetic signals obscured
by overlap with the core field
2004 Fall AGU GP31A-0821
MF-3 and its recovery
Michael Purucker, Raytheon ITSS _at_ Geodynamics
Branch GSFC, Greenbelt, MD USA 20771 Kathryn
Whaler, School of Geosciences, University of
Edinburgh, West Mains Rd, Edinburgh, EH9 3JW,
UK purucker_at_geomag.gsfc.nasa.gov 1 301 614
6473 http//geodynamics.gsfc.nasa.gov/personal_pag
es/purucker/purucker.html





Abstract We recognize and characterize two
distinctive patterns evident in new maps of the
lithospheric magnetic field from the CHAMP
satellite, and new minimum amplitude
magnetization models that we deduce. The
boundaries of these patterns define
long-wavelength features in the lithospheric
field not previously recognized because they were
obscured by overlap with the core field. These
boundaries correspond to known crustal thickness
variations. The major exceptions, the Sahara and
most of South America south of the Equator, are
regions where direct estimates of crustal
thickness and heat flow are sparse.


Characterizing the MF-3 model 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
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 anomalies in C
typically exceeds 50 nT, and the anomalies are
either 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 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
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 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 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 Indian oceans.

MF-3 model evaluated over North and Central
America (Maus et al., 2004)

Introduction 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 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 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). 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
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 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). This analogy
implies that our imaging technique cant 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 largest scales.



Conclusions 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-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 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 poorly known.
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 Earths surface. Note that we are NOT
assuming that the magnetization is in the
direction of the core field.




Acknowledgments 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
Satellite-based magnetic field maps (MF-3) 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 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 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 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 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-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-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 lithospheric signal in
those areas.
References, and suggested readings GMT, 2004,
v..4, http//gmt.soest.hawaii.edu, P. Wessel and
W. Smith. Jackson, A., Accounting for crustal
magnetization in models of the core magnetic
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1990. Langel, R. and Hinze, W., The magnetic
field of the Earths lithosphere, 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, Phys. Earth Plan. Int., 95, 101-122,
1996. Maus, S., et al., Earths crustal magnetic
field determined to spherical harmonic degre 90
from CHAMP satellite measurements, Geophys. J.
Int, submitted, 2005. Available electronically at
www.gfz-potsdam.de/pb2/pb23/SatMag/litmod3.html Ma
yhew, M. and Estes, R., Equivalent source
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data, J. Geomagnetism and Geoelectricity, 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
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M, Maus, S., and Luehr, H., From validation to
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Details of technique
Characterizing the MF-3 based magnetization
model 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 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
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 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) and shown immediately to
the right of the scalar magnetization map. There
are 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
typical of continental crust. The other major
exception is the Sahara desert, again
characterized by the O pattern but with typical
continental crustal thicknesses.