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| Titel |
The influence of magnetic fields in planetary dynamo models |
| VerfasserIn |
Krista Soderlund, Eric King, Jonathan Aurnou |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072069
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| Zusammenfassung |
| Magnetic fields are common throughout the solar system with properties as diverse as the
planets themselves. Since these fields likely result from convectively driven dynamo action,
the coupling between magnetic fields, fluid flow, and heat transfer must be understood
in order to determine what controls the strength, morphology, and evolution of
planetary magnetic fields. Towards this end, we have carried out a suite of dynamo and
non-magnetic convection simulations to investigate the effect of the presence of
magnetic fields on convection, the effect of varying convective vigor, and the effect
of varying the rotation rate. This survey considers models with Prandtl number
Pr = 1; magnetic Prandtl numbers up to Pm = 5; Ekman numbers in the range
10-3 -¥ E -¥ 10-5; and Rayleigh numbers from near onset to more than 1000 times critical.
We measure the strengths and structures of magnetic fields and fluid motions, as
well as heat transfer efficiency and in situ force balances. These analyses illustrate
that dynamo action does not necessitate a fundamental change to the overall flow
field, although the impact of magnetic fields is found to increase for lower Ekman
numbers. By directly calculating the forces in each of our simulations, we show that the
traditionally defined Elsasser number, Îi, overestimates the role of the Lorentz force in
dynamos. The Coriolis force remains greater than the Lorentz force even in cases
with Îi -ă 100, explaining the persistence of columnar flows in Îi > 1 dynamo
simulations, a quasigeostrophic phenomena. We argue that a dynamic Elsasser
number, Îd, better represents the Lorentz to Coriolis force ratio. By applying the
Îd parametrization to planetary settings, we predict that the convective dynamics
(excluding zonal flows) in planetary interiors are only weakly influenced by their
large-scale magnetic fields. Our survey also provides new insight into the breakdown of
dipolar magnetic field generation since we observe a sharp transition from dipolar
to multipolar dynamos in models with moderate to high Ekman numbers. Force
calculations show that this transition occurs when the inertial and viscous forces
become comparable. These results suggest that viscous effects are important for
dipolar field generation in many present day dynamo simulations and imply that
dynamo simulations with moderate Ekman numbers may not correctly capture
the physics of planetary dynamos where viscosity is expected to be negligible. |
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