1,134 research outputs found
Peaks and Troughs in Helioseismology: The Power Spectrum of Solar Oscillations
I present a matched-wave asymptotic analysis of the driving of solar
oscillations by a general localised source. The analysis provides a simple
mathematical description of the asymmetric peaks in the power spectrum in terms
of the relative locations of eigenmodes and troughs in the spectral response.
It is suggested that the difference in measured phase function between the
modes and the troughs in the spectrum will provide a key diagnostic of the
source of the oscillations. I also suggest a form for the asymmetric line
profiles to be used in the fitting of solar power spectra.
Finally I present a comparison between the numerical and asymptotic
descriptions of the oscillations. The numerical results bear out the
qualitative features suggested by the asymptotic analysis but suggest that
numerical calculations of the locations of the troughs will be necessary for a
quantitative comparison with the observations.Comment: 18 pages + 8 separate figures. To appear in Ap
Green's functions for far-side seismic images: a polar expansion approach
We have computed seismic images of magnetic activity on the far surface of
the Sun by using a seismic-holography technique. As in previous works, the
method is based on the comparison of waves going in and out of a particular
point in the Sun but we have computed here the Green's functions from a
spherical polar expansion of the adiabatic wave equations in the Cowling
approximation instead of using the ray-path approximation previously used in
the far-side holography. A comparison between the results obtained using the
ray theory and the spherical polar expansion is shown. We use the
gravito-acoustic wave equation in the local plane-parallel limit in both cases
and for the latter we take the asymptotic approximation for the radial
dependencies of the Green's function. As a result, improved images of the
far-side can be obtained from the polar-expansion approximation, especially
when combining the Green's functions corresponding to two and three skips. We
also show that the phase corrections in the Green's functions due to the
incorrect modeling of the uppermost layers of the Sun can be estimated from the
eigenfrequencies of the normal modes of oscillation.Comment: 8 pages, 5 figures, Astrophysical Journal, accepted (2010
Local models of stellar convection II: Rotation dependence of the mixing length relations
We study the mixing length concept in comparison to three-dimensional
numerical calculations of convection with rotation. In a limited range, the
velocity and temperature fluctuations are linearly proportional to the
superadiabaticity, as predicted by the mixing length concept and in accordance
with published results. The effects of rotation are investigated by varying the
Coriolis number, Co = 2 Omega tau, from zero to roughly ten, and by calculating
models at different latitudes. We find that \alpha decreases monotonically as a
function of the Coriolis number. This can be explained by the decreased spatial
scale of convection and the diminished efficiency of the convective energy
transport, the latter of which leads to a large increase of the
superadibaticity, \delta = \nabla - \nabla_ad as function of Co. Applying a
decreased mixing length parameter in a solar model yields very small
differences in comparison to the standard model within the convection zone. The
main difference is the reduction of the overshooting depth, and thus the depth
of the convection zone, when a non-local version of the mixing length concept
is used. Reduction of \alpha by a factor of roughly 2.5 is sufficient to
reconcile the difference between the model and helioseismic results. The
numerical results indicate reduction of \alpha by this order of magnitude.Comment: Final published version, 8 pages, 9 figure
Development of the opto-mechanical design for ICE-T
ICE-T (International Concordia Explorer Telescope) is a double 60 cm f/1.1
photometric robotic telescope, on a parallactic mount, which will operate at
Dome C, in the long Antarctic night, aiming to investigate exoplanets and
activity of the hosting stars. Antarctic Plateau site is well known to be one
of the best in the world for observations because of sky transparency in all
wavelengths and low scintillation noise. Due to the extremely harsh
environmental conditions (the lowest average temperature is -80C) the
criteria adopted for an optimal design are really challenging. Here we present
the strategies we have adopted so far to fulfill the mechanical and optical
requirements.Comment: 7 pages, 2 figures, contributed talk at 'An astronomical Observatory
at Concordia (Dome C, Antarctica) for the next decade', 11-15 May, Rome
(Italy
Indole-positive Vibrio vulnificus isolated from disease outbreaks on a Danish eel farm
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