 |
| Titel |
A dynamo driven by zonal jets at the upper surface in the giant planets |
| VerfasserIn |
Céline Guervilly, Philippe Cardin, Nathanael Schaeffer |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053253
|
|
|
|
|
|
| Zusammenfassung |
| A dynamo driven by zonal jets at the upper surface in the giant planets
Céline Guervilly, Philippe Cardin, Nathanael Scaheffer
We present a new possible dynamo mechanism for generating the magnetic fields of the
giant planets. The mechanism relies on the presence of barotropically unstable differential
rotation. We assume that zonal jet currents within the outer molecular hydrogen layer exert a
drag at the top of the deep electrically conducting region. Because of the planet’s rapid
rotation, this boundary forcing drives nearly geostrophic axisymmetric motions in the
conducting region.
For a given forcing, measured by the critical Rossby number Roc, a shear instability of
the zonal flow develops in the form of a global Rossby mode. The wavenumber of the mode
depends on the width of the zonal jets. For Ro -¥ Roc, we obtain self-sustained magnetic
fields at magnetic Reynolds numbers greater than 103. The propagation of the Rossby
wave and its non-axisymmetric structure are both crucial for dynamo action. The
amplitude of self-sustained axisymmetric poloidal magnetic field plausibly depends
on the wavenumber of the shear instability, and hence on the width of the zonal
jets.
For narrow jets, the poloidal magnetic field is dominated by an axial dipole whereas in the
case of wide jets, the axisymmetric poloidal field is weak. Hence, this new dynamo
mechanism explains the differences between the magnetic fields of Saturn and Jupiter on one
hand and Uranus and Neptune on the other hand by the observed features of their
atmosphere. |
|
|
|
|
|
|