10,280 research outputs found
Magnetic energy cascade in spherical geometry: I. The stellar convective dynamo case
We present a method to characterize the spectral transfers of magnetic energy
between scales in simulations of stellar convective dynamos. The full triadic
transfer functions are computed thanks to analytical coupling relations of
spherical harmonics based on the Clebsch-Gordan coefficients. The method is
applied to mean field dynamo models as benchmark tests. From the
physical standpoint, the decomposition of the dynamo field into primary and
secondary dynamo families proves very instructive in the case.
The same method is then applied to a fully turbulent dynamo in a solar
convection zone, modeled with the 3D MHD ASH code. The initial growth of the
magnetic energy spectrum is shown to be non-local. It mainly reproduces the
kinetic energy spectrum of convection at intermediate scales. During the
saturation phase, two kinds of direct magnetic energy cascades are observed in
regions encompassing the smallest scales involved in the simulation. The first
cascade is obtained through the shearing of magnetic field by the large scale
differential rotation that effectively cascades magnetic energy. The second is
a generalized cascade that involves a range of local magnetic and velocity
scales. Non-local transfers appear to be significant, such that the net
transfers cannot be reduced to the dynamics of a small set of modes. The
saturation of the large scale axisymmetric dipole and quadrupole are detailed.
In particular, the dipole is saturated by a non-local interaction involving the
most energetic scale of the magnetic energy spectrum, which points out the
importance of the magnetic Prandtl number for large-scale dynamos.Comment: 21 pages, 14 figures, 1 table, accepted for publication in the
Astrophysical Journa
A simple model of quantum trajectories
Quantum trajectory theory, developed largely in the quantum optics community
to describe open quantum systems subjected to continuous monitoring, has
applications in many areas of quantum physics. In this paper I present a simple
model, using two-level quantum systems (q-bits), to illustrate the essential
physics of quantum trajectories and how different monitoring schemes correspond
to different ``unravelings'' of a mixed state master equation. I also comment
briefly on the relationship of the theory to the Consistent Histories formalism
and to spontaneous collapse models.Comment: 42 pages RevTeX including four figures in encapsulated postscript.
Submitted to special issue of American Journal of Physic
Influence of the Tachocline on Solar Evolution
Recently helioseismic observations have revealed the presence of a shear
layer at the base of the convective zone related to the transition from
differential rotation in the convection zone to almost uniform rotation in the
radiative interior, the tachocline. At present, this layer extends only over a
few percent of the solar radius and no definitive explanations have been given
for this thiness. Following Spiegel and Zahn (1992, Astron. Astrophys.), who
invoke anisotropic turbulence to stop the spread of the tachocline deeper in
the radiative zone as the Sun evolves, we give some justifications for their
hypothesis by taking into account recent results on rotating shear instability
(Richard and Zahn 1999, Astron. Astrophys.). We study the impact of the
macroscopic motions present in this layer on the Sun's structure and evolution
by introducing a macroscopic diffusivity in updated solar models. We find
that a time dependent treatment of the tachocline significantly improves the
agreement between computed and observed surface chemical species, such as the
Li and modify the internal structure of the Sun (Brun, Turck-Chi\`eze and
Zahn, 1999, in Astrophys. J.).Comment: to appear in Annals of the New York Academy of Sciences, vol 898.
Postscript file, 9 pages and 5 figures New Email Address for A. S. Brun:
[email protected]
Magnetic games between a planet and its host star: the key role of topology
Magnetic interactions between a star and a close-in planet are postulated to
be a source of enhanced emissions and to play a role in the secular evolution
of the orbital system. Close-in planets generally orbit in the sub-alfv\'enic
region of the stellar wind, which leads to efficient transfers of energy and
angular momentum between the star and the planet. We model the magnetic
interactions occurring in close-in star-planet systems with three-dimensional,
global, compressible magneto-hydrodynamic numerical simulations of a planet
orbiting in a self-consistent stellar wind. We focus on the cases of magnetized
planets and explore three representative magnetic configurations. The Poynting
flux originating from the magnetic interactions is an energy source for
enhanced emissions in star-planet systems. Our results suggest a simple
geometrical explanation for ubiquitous on/off enhanced emissions associated
with close-in planets, and confirm that the Poynting fluxes can reach powers of
the order of W. Close-in planets are also showed to migrate due to
magnetic torques for sufficiently strong stellar wind magnetic fields. The
topology of the interaction significantly modifies the shape of the magnetic
obstacle that leads to magnetic torques. As a consequence, the torques can vary
by at least an order of magnitude as the magnetic topology of the interaction
varies.Comment: 15 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Global Dynamics of Subsurface Solar Active Regions
We present three-dimensional numerical simulations of a magnetic loop
evolving in either a convectively stable or unstable rotating shell. The
magnetic loop is introduced in the shell in such a way that it is buoyant only
in a certain portion in longitude, thus creating an \Omega-loop. Due to the
action of magnetic buoyancy, the loop rises and develops asymmetries between
its leading and following legs, creating emerging bipolar regions whose
characteristics are similar to the ones of observed spots at the solar surface.
In particular, we self-consistently reproduce the creation of tongues around
the spot polarities, which can be strongly affected by convection. We moreover
emphasize the presence of ring-shaped magnetic structures around our simulated
emerging regions, which we call "magnetic necklace" and which were seen in a
number of observations without being reported as of today. We show that those
necklaces are markers of vorticity generation at the periphery and below the
rising magnetic loop. We also find that the asymmetry between the two legs of
the loop is crucially dependent on the initial magnetic field strength. The
tilt angle of the emerging regions is also studied in the stable and unstable
cases and seems to be affected both by the convective motions and the presence
of a differential rotation in the convective cases.Comment: 23 pages (ApJ 2-column format), 19 figures, accepted for publication
in Ap
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