5,654 research outputs found
A dynamo driven by zonal jets at the upper surface: Applications to giant planets
We present a dynamo mechanism arising from the presence of barotropically
unstable zonal jet currents in a rotating spherical shell. The shear
instability of the zonal flow develops in the form of a global Rossby mode,
whose azimuthal wavenumber depends on the width of the zonal jets. We obtain
self-sustained magnetic fields at magnetic Reynolds numbers greater than 1000.
We show that the propagation of the Rossby waves is crucial for dynamo action.
The amplitude of the axisymmetric poloidal magnetic field depends on the
wavenumber of the Rossby mode, and hence on the width of the zonal jets. We
discuss the plausibility of this dynamo mechanism for generating the magnetic
field of the giant planets. Our results suggest a possible link between the
topology of the magnetic field and the profile of the zonal winds observed at
the surface of the giant planets. For narrow Jupiter-like jets, the poloidal
magnetic field is dominated by an axial dipole whereas for wide Neptune-like
jets, the axisymmetric poloidal field is weak.Comment: published in Icaru
Inertial waves in a differentially rotating spherical shell
We investigate the properties of small-amplitude inertial waves propagating
in a differentially rotating incompressible fluid contained in a spherical
shell. For cylindrical and shellular rotation profiles and in the inviscid
limit, inertial waves obey a second-order partial differential equation of
mixed type. Two kinds of inertial modes therefore exist, depending on whether
the hyperbolic domain where characteristics propagate covers the whole shell or
not. The occurrence of these two kinds of inertial modes is examined, and we
show that the range of frequencies at which inertial waves may propagate is
broader than with solid-body rotation. Using high-resolution calculations based
on a spectral method, we show that, as with solid-body rotation, singular modes
with thin shear layers following short-period attractors still exist with
differential rotation. They exist even in the case of a full sphere. In the
limit of vanishing viscosities, the width of the shear layers seems to weakly
depend on the global background shear, showing a scaling in E^{1/3} with the
Ekman number E, as in the solid-body rotation case. There also exist modes with
thin detached layers of width scaling with E^{1/2} as Ekman boundary layers.
The behavior of inertial waves with a corotation resonance within the shell is
also considered. For cylindrical rotation, waves get dramatically absorbed at
corotation. In contrast, for shellular rotation, waves may cross a critical
layer without visible absorption, and such modes can be unstable for small
enough Ekman numbers.Comment: 31 pages, 16 figures, accepted for publication in Journal of Fluid
Mechanic
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