12,373 research outputs found
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]
Novel String Banana Template Method of Track Reconstruction for high Multiplicity Events with Significant Multiple Scattering
Novel String Banana Template Method (SBTM) for track reconstruction in high
multiplicity events in non-uniform magnetic field spectrometer with emphasis on
the lowest momenta tracks with significant Multiple Scattering (MS) is
described. Two steps model of track with additional parameter/s which takes
into account MS for this particular track is introduced. SBTM is time efficient
and demonstrates better resolutions than another method equivalent to the Least
Squares method (LSM).Comment: 3 pages, 3 figures, DPF2004 Proceeding, International Journal of
Modern Physics
Nonextensive statistics in stellar plasma and solar neutrinos
Nonextensive and quantum uncertainty effects (related to the quasiparticles
composing the stellar core) have strong influence on the nuclear rates and, of
course, affect solar neutrino fluxes. Both effects do coexist and are due to
the frequent collisions among the ions. The weakly nonextensive nature of the
solar core is confirmed. The range of predictions for the neutrino fluxes is
enlarged and the solar neutrino problem becomes less dramatic.Comment: 4 pages. Proc. of TAUP99, Sept. 6-10 1999, Paris. To appear in Nucl.
Phys. B, Proc. Supp
Antiproton and Positron Signal Enhancement in Dark Matter Mini-Spikes Scenarios
The annihilation of dark matter (DM) in the Galaxy could produce specific
imprints on the spectra of antimatter species in Galactic cosmic rays, which
could be detected by upcoming experiments such as PAMELA and AMS02. Recent
studies show that the presence of substructures can enhance the annihilation
signal by a "boost factor" that not only depends on energy, but that is
intrinsically a statistical property of the distribution of DM substructures
inside the Milky Way. We investigate a scenario in which substructures consist
of "mini-spikes" around intermediate-mass black holes. Focusing on
primary positrons and antiprotons, we find large boost factors, up to a few
thousand, that exhibit a large variance at high energy in the case of positrons
and at low energy in the case of antiprotons. As a consequence, an estimate of
the DM particle mass based on the observed cut-off in the positron spectrum
could lead to a substantial underestimate of its actual value.Comment: 13 pages, 9 figures, minor changes, version accepted for publication
in PR
Exploring the vs relation with flux transport dynamo models of solar-like stars
Aims: To understand stellar magnetism and to test the validity of the
Babcock-Leighton flux transport mean field dynamo models with stellar activity
observations Methods: 2-D mean field dynamo models at various rotation rates
are computed with the STELEM code to study the sensitivity of the activity
cycle period and butterfly diagram to parameter changes and are compared to
observational data. The novelty is that these 2-D mean field dynamo models
incorporate scaling laws deduced from 3-D hydrodynamical simulations for the
influence of rotation rate on the amplitude and profile of the meridional
circulation. These models make also use of observational scaling laws for the
variation of differential rotation with rotation rate. Results: We find that
Babcock-Leighton flux transport dynamo models are able to reproduce the change
in topology of the magnetic field (i.e. toward being more toroidal with
increasing rotation rate) but seem to have difficulty reproducing the cycle
period vs activity period correlation observed in solar-like stars if a
monolithic single cell meridional flow is assumed. It may however be possible
to recover the vs relation with more complex meridional
flows, if the profile changes in a particular assumed manner with rotation
rate. Conclusions: The Babcock-Leighton flux transport dynamo model based on
single cell meridional circulation does not reproduce the vs
relation unless the amplitude of the meridional circulation is
assumed to increase with rotation rate which seems to be in contradiction with
recent results obtained with 3-D global simulations.Comment: 12 pages, 8 figures, accepted for publication by A&A 1: AIM,
CEA/DSM-CNRS-Univ. Paris 7, IRFU/SAp, France, 2: D.A.M.T.P., Centre for
Mathematical Sciences, Univ. of Cambridge, UK, 3: JILA and Department of
Astrophysical and Planetary Sciences, Univ. of Colorado, US
Flux-tube geometry and solar wind speed during an activity cycle
The solar wind speed at 1 AU shows variations in latitude and in time which
reflect the evolution of the global background magnetic field during the
activity cycle. It is commonly accepted that the terminal wind speed in a
magnetic flux-tube is anti-correlated with its expansion ratio, which motivated
the definition of widely-used semi-empirical scaling laws relating one to the
other. In practice, such scaling laws require ad-hoc corrections. A predictive
law based solely on physical principles is still missing. We test whether the
flux-tube expansion is the controlling factor of the wind speed at all phases
of the cycle and at all latitudes using a very large sample of wind-carrying
open magnetic flux-tubes. We furthermore search for additional physical
parameters based on the geometry of the coronal magnetic field which have an
influence on the terminal wind flow speed. We use MHD simulations of the corona
and wind coupled to a dynamo model to provide a large statistical ensemble of
open flux-tubes which we analyse conjointly in order to identify relations of
dependence between the wind speed and geometrical parameters of the flux-tubes
which are valid globally (for all latitudes and moments of the cycle). Our
study confirms that the terminal speed of the solar wind depends very strongly
on the geometry of the open magnetic flux-tubes through which it flows. The
total flux-tube expansion is more clearly anti-correlated with the wind speed
for fast rather than for slow wind flows, and effectively controls the
locations of these flows during solar minima. Overall, the actual asymptotic
wind speeds attained are also strongly dependent on field-line inclination and
magnetic field amplitude at the foot-points. We suggest ways of including these
parameters on future predictive scaling-laws for the solar wind speed.Comment: Accepted for publicaton on Astronomy & Astrophysic
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