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| Titel |
Anelastic dynamo models of gas giants |
| VerfasserIn |
T. Gastine, L. Duarte, J. Wicht |
| 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 |
250063907
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| Zusammenfassung |
| Evolution models of the gas giants suggest that Jupiter and Saturn interiors
are separated into an outer non-conducting molecular envelope and an inner
metallic region, where the dynamo takes place.
Typical numerical models for planetary dynamos rely mainly on the Boussinesq
approximation, assuming that the background properties (temperature, density,
...) are constant with radius (Christensen & Aubert, 2006). While this
approximation is suitable for liquid iron cores of terrestrial planets, it
becomes more questionable in gas giants, where density increases by several
orders of magnitude (Guillot, 1999). The anelastic approximation thus provides
a more realistic framework to model the dynamics of Jupiter and Saturn as it
allows to incorporate effects of the density stratification, while filtering
out fast acoustic waves (Lantz & Fan, 1999). We also appropriately employ
stress-free rather than rigid flow boundary conditions, typical for
terrestrial dynamo models, which are known to promote a rich dynamical
behavior, including hemispherical dynamos, or bistability phenomena (e.g.
Simitev & Busse, 2009, Sasaki et al. 2011).
We present the results of a systematic parametric study on the effects of the
background density stratification. While the previous Boussinesq results
suggested that the dipolarity of the magnetic field may be a direct
consequence of the relative influence of inertial effects (through the local
Rossby number criterion developed by Christensen and Aubert (2006)), anelastic
dynamos tend to produce a broader range of field geometries, showing two
distinct dynamo branches: the first is characterised by dipole-dominated and
magnetostrophic dynamos and weak zonal flows, while the second shows
small-scale fields, is more geostrophic and zonal flows are significant. For
some cases, multiple solutions can be found depending on the starting
condition, reproducing a bistability (Simitev & Busse 2009). In conclusion,
the combination of density stratification effects and the use of stress-free
boundary conditions has a crucial influence on the geometry of the magnetic
field, indicating that these effects are important to appropriately model the
interior dynamics of gas giants. |
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