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Frozen-Flux Modelling for Epochs 1915 and 1980The frozen-flux hypothesis for the Earth's liquid core assumes that convective terms dominate diffusive terms in the induction equation governing the behaviour of the magnetic field at the surface of the core. While highly plausible on the basis of estimates of physical parameters, the hypothesis has been questioned. To study this hypothesis, we improve the method which tests the consistency of magnetic observations with the hypothesis by constructing simple, flux-conserving core-field models fitting the data at pairs of epochs. We introduce a new approach that fixes the patch configurations at each of the two epochs before inversion, so that each configuration is consistent with its respective data set but possesses the same patch topology. We expand upon the inversion algorithm, using quadratic programming to maintain the proper flux sign within patches; the modelling calculations are also extended to include data types that depend non-linearly on the model. Every test of a hypothesis depends on the characterization of the observational uncertainties; we undertake a thorough review of this question. For main-field models, the primary source of uncertainty comes from the crustal field. We base our analysis on statistical models of the crustal magnetization, adjusted to bring it into better conformity with our data set. The noise model permits us to take into account the correlations between the measurements and requires that a different weighting be given to horizontal and vertical components. It also indicates that the observations should be fit more closely than has been the practice heretofore. We apply the revised method to Magsat data from 1980 and survey and observatory data from 1915.5, two data sets believed to be particularly difficult to reconcile with the frozen-flux hypothesis. We compute a pair of simple, flux-conserving models that fit the averaged data from each epoch. We therefore conclude that present knowledge of the geomagnetic fields of 1980 and 1915.5 is consistent with the frozen-flux hypothesis.
Document ID
20000013933
Acquisition Source
Headquarters
Document Type
Reprint (Version printed in journal)
Authors
OBrien, Michael S.
(Scripps Institution of Oceanography La Jolla, CA United States)
Constable, Catherine G.
(Scripps Institution of Oceanography La Jolla, CA United States)
Parker, Robert L.
(Scripps Institution of Oceanography La Jolla, CA United States)
Date Acquired
August 19, 2013
Publication Date
January 1, 1997
Publication Information
Publication: Geophysics Journal International
Publisher: Royal Astronomical Society
Volume: 128
Subject Category
Geophysics
Funding Number(s)
CONTRACT_GRANT: NAGW-3932
CONTRACT_GRANT: NAGW-2966
CONTRACT_GRANT: NAG5-2968
Distribution Limits
Public
Copyright
Other

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