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| Titel |
On magnetic estimation of the Earth's core angular momentum variation |
| VerfasserIn |
Seiki Asari, Ingo Wardinski |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250088925
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| Publikation (Nr.) |
 EGU/EGU2014-3105.pdf |
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| Zusammenfassung |
| Inversely modelling the core surface flow from a geomagnetic field model C3FM2 for 1957.0
to 2008.0, we systematically investigate temporal evolution of the Earth’s core angular
momentum (CAM). Various models are built by varying two parameters involved in the
inversion. The first parameter, p(= -1,0,1 and 3), controls the regularization implemented
in the inversion, where weights for each spherical harmonic degree l are such that a
power-law - l-p is imposed on resulting flow power spectrum. The second parameter
concerns tangential geostrophy (TG) constraint that is imposed in a weak form, and regulates
the fraction of ageostrophic flow content admitted. All the models meet tangential
magnetostrophy constraint tightly, and their misfits to the C3FM2 secular variation are fixed
at a constant level.
CAM variations derived from these flow models are then examined in the time domain,
with particular reference to the two notable features of the observed length-of-day
(LOD) variation: the secular trend and 6-year variation. The moderately negative
trend of the observed LOD is consistent only with the CAM variations for flow
models of two contrasting types, each obtained with either p = -1 under a strong TG
constraint, and p ≥ 0 under a weak TG constraint. For the strong TG case the power
spectrum peaks at spherical harmonic degrees at 10 to 15, while the spectra are
dominated by powers at the lowest degrees for the weak TG cases. These models
may be interpreted as corresponding to the two distinct dynamo regimes identified
in previous numerical simulations, i.e. the strong/weak TG case corresponding
to geostrophic/magnetostrophic state associated predominantly with weak/strong
magnetic field and intermediate/large scale flows. Temporal derivatives of the CAM
variations reveal rapid behaviours, with their extrema well synchronized for any models
estimated with the wide range of parameters. The rapid CAM phase thus seems
robustly determined for a given magnetic model and inversion scheme. The extrema
sometimes match those of the observed 6-year LOD variation. However, a more detailed
analysis is needed to quantify the CAM variability and its relation to the rapid LOD
variations. |
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