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| Titel |
The magnetic field and secular variation of Jupiter |
| VerfasserIn |
V. A. Ridley, R. Holme |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063536
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| Zusammenfassung |
| Planetary dynamos, resulting from fluid flow in electrically conductive parts of their interior,
are thought to be highly time dependent. Currently, our understanding of temporal variation
of these fields is limited because we only have observations for one example, the Earth. To
overcome this, data acquired by 6 NASA space missions between 1973-2003 are
used to investigate possible time variation (secular variation) of Jupiter’s magnetic
field.
Previous attempts to model jovimagnetic secular variation have been inconclusive or
ineffective for a number of reasons, including limited data usage, inadequate consideration of
the external current disk field and the modelling approach taken. We attempt to resolve these
issues by using all data available within 12 Jovian radii, establishing and removing the
current disk field for each individual orbit and taking a regularised minimum norm approach
to modelling the internal planetary field. This approach allows construction of numerically
stable models with constrained small-scale (high spherical harmonic degree) structure that
directly fit the observations. Two models of Jupiter’s magnetic field are presented:
the first time-averaged over the whole dataset, whilst the second allows for linear
time variation of the field. Comparison of these allow inferences to be made about
jovimagnetic secular variation with our favoured model indicating a ~0.042%yr-1
decrease in the dipole magnetic moment over the investigated time period; this
value is comparable with Earth (~0.06%yr-1). Simple models of jovian “core
flow”, while highly speculative, also show patterns not dissimilar from those of the
Earth.
These models are calculated with reference to the Jovian System III 1965.0 reference
frame, itself defined by the magnetic field. Thus, some of the secular variation could result
from inaccuracies in this determination; however, such an effect cannot explain all the
observed secular variation. The constraint on changes in planetary rotation rate allow a bound
to be placed on angular momentum transfer between the atmosphere and deep interior,
as seen for variations in Earth’s observed length-of-day. Using changes in zonal
atmospheric wind structure, if the winds were to extend to depths only 2% into the planet,
observed atmospheric changes between 1979-1996 would translate to a ~10° rotation
of the planetary interior via angular momentum exchange, inconsistent with the
magnetic observations. We thus provide strong observational evidence against models
linking surface winds to deep Jovian convection, particular deep convection on
cylinders. |
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