 |
| Titel |
How to make an Earth-like geodynamo model |
| VerfasserIn |
Ulrich Christensen, Julien Aubert, Gauthier Hulot |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040205
|
|
|
|
|
|
| Zusammenfassung |
| For many published dynamo models an Earth-like magnetic field has been claimed.
However, it has also been noted that as the Ekman number (viscosity) is lowered to
less unrealistic values, the magnetic field tends to become less Earth-like. Here
we define quantitative criteria for the degree of similarity between the field of a
dynamo model and the geomagnetic field, concentrating on the agreement in field
morphology. Main criterion is (1) the ratio between the axial dipole component and
non axial dipole parts of the field. In addition, we include (2) the ratio between
equatorially symmetric and antisymmetric components and (3) between zonal and
non-zonal components in the non-dipole field and (4) a measure for the degree of
concentration of magnetic flux at the core surface into small patches. We test a large
number of dynamo models that cover the accessible parameter space concerning
their compliance with these criteria. We tentatively order models according to to
magnetic Reynolds number Rm and magnetic Ekman number Em (ratio between
rotation period and magnetic diffusion time). Requirements for an Earth-like field
morphology are that Em -¤ 10-4 and that Rm falls into a limited range that depends
on Em and ranges between 140 and 450 at Em=10-4 and increases to 1000-5000
when extrapolated to the Earth value Em=5 Ã 10-9. Earth-like models seem to
exist in a contiguous swath of parameter space that links the present high-Ekman
number models with the geodynamo. Hence the models are probably generically
similar to the true geodynamo. The thermal boundary condition (fixed temperature or
fixed flux) and the distribution of sources and sinks of buoyancy has a secondary
influence on the field morphology. The computationally least expensive Earth-like
models can be constructed at Em - 3x10-4 for compositionally driven convection. |
|
|
|
|
|
|