1,356 research outputs found
Three-dimensional stability of the solar tachocline
The three-dimensional, hydrodynamic stability of the solar tachocline is
investigated based on a rotation profile as a function of both latitude and
radius. By varying the amplitude of the latitudinal differential rotation, we
find linear stability limits at various Reynolds numbers by numerical
computations. We repeated the computations with different latitudinal and
radial dependences of the angular velocity. The stability limits are all higher
than those previously found from two-dimensional approximations and higher than
the shear expected in the Sun. It is concluded that any part of the tachocline
which is radiative is hydrodynamically stable against small perturbations.Comment: 6 pages, 8 figures, accepted by Astron. & Astrophy
On The Determination of MDI High-Degree Mode Frequencies
The characteristic of the solar acoustic spectrum is such that mode lifetimes
get shorter and spatial leaks get closer in frequency as the degree of a mode
increases for a given order. A direct consequence of this property is that
individual p-modes are only resolved at low and intermediate degrees, and that
at high degrees, individual modes blend into ridges. Once modes have blended
into ridges, the power distribution of the ridge defines the ridge central
frequency and it will mask the true underlying mode frequency. An accurate
model of the amplitude of the peaks that contribute to the ridge power
distribution is needed to recover the underlying mode frequency from fitting
the ridge.
We present the results of fitting high degree power ridges (up to l = 900)
computed from several two to three-month-long time-series of full-disk
observations taken with the Michelson Doppler Imager (MDI) on-board the Solar
and Heliospheric Observatory between 1996 and 1999.
We also present a detailed discussion of the modeling of the ridge power
distribution, and the contribution of the various observational and
instrumental effects on the spatial leakage, in the context of the MDI
instrument. We have constructed a physically motivated model (rather than some
ad hoc correction scheme) resulting in a methodology that can produce an
unbiased determination of high-degree modes, once the instrumental
characteristics are well understood.
Finally, we present changes in high degree mode parameters with epoch and
thus solar activity level and discuss their significance.Comment: 59 pages, 38 figures -- High-resolution version at
http://www-sgk.harvard.edu:1080/~sylvain/preprints/ -- Manuscript submitted
to Ap
A method for the estimation of p-mode parameters from averaged solar oscillation power spectra
A new fitting methodology is presented which is equally well suited for the
estimation of low-, medium-, and high-degree mode parameters from -averaged
solar oscillation power spectra of widely differing spectral resolution. This
method, which we call the "Windowed, MuLTiple-Peak, averaged spectrum", or
WMLTP Method, constructs a theoretical profile by convolving the weighted sum
of the profiles of the modes appearing in the fitting box with the power
spectrum of the window function of the observing run using weights from a
leakage matrix that takes into account both observational and physical effects,
such as the distortion of modes by solar latitudinal differential rotation. We
demonstrate that the WMLTP Method makes substantial improvements in the
inferences of the properties of the solar oscillations in comparison with a
previous method that employed a single profile to represent each spectral peak.
We also present an inversion for the internal solar structure which is based
upon 6,366 modes that we have computed using the WMLTP method on the 66-day
long 2010 SOHO/MDI Dynamics Run. To improve both the numerical stability and
reliability of the inversion we developed a new procedure for the
identification and correction of outliers in a frequency data set. We present
evidence for a pronounced departure of the sound speed in the outer half of the
solar convection zone and in the subsurface shear layer from the radial sound
speed profile contained in Model~S of Christensen-Dalsgaard and his
collaborators that existed in the rising phase of Solar Cycle~24 during
mid-2010
Energy balance of a laser ablation plume expanding in a background gas
The energy balance of a laser ablation plume in an
ambient gas for nanosecond pulses has been investigated on
the basis of the model of Predtechensky and Mayorov (PM),
which provides a relatively simple and clear description of
the essential hydrodynamics. This approach also leads to an
insightful description in dimensionless units of how the initial
kinetic energy of the plume is dissipated into kinetic and
thermal energy of the background gas. Eventually when the
plume has stopped, the initial kinetic energy of the plume is
converted into thermal energy of the plume and background
gas
The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Optimization of the Spectral Line Inversion Code
The Very Fast Inversion of the Stokes Vector (VFISV) is a Milne-Eddington
spectral line inversion code used to determine the magnetic and thermodynamic
parameters of the solar photosphere from observations of the Stokes vector in
the 6173 A Fe I line by the Helioseismic and Magnetic Imager (HMI) onboard the
Solar Dynamics Observatory (SDO). We report on the modifications made to the
original VFISV inversion code in order to optimize its operation within the HMI
data pipeline and provide the smoothest solution in active regions. The changes
either sped up the computation or reduced the frequency with which the
algorithm failed to converge to a satisfactory solution. Additionally, coding
bugs which were detected and fixed in the original VFISV release, are reported
here.Comment: Accepted for publication in Solar Physic
Analysis of Gamma Radiation from a Radon Source: Indications of a Solar Influence
This article presents an analysis of about 29,000 measurements of gamma
radiation associated with the decay of radon in a sealed container at the
Geological Survey of Israel (GSI) Laboratory in Jerusalem between 28 January
2007 and 10 May 2010. These measurements exhibit strong variations in time of
year and time of day, which may be due in part to environmental influences.
However, time-series analysis reveals a number of periodicities, including two
at approximately 11.2 year and 12.5 year. We have previously
found these oscillations in nuclear-decay data acquired at the Brookhaven
National Laboratory (BNL) and at the Physikalisch-Technische Bundesanstalt
(PTB), and we have suggested that these oscillations are attributable to some
form of solar radiation that has its origin in the deep solar interior. A
curious property of the GSI data is that the annual oscillation is much
stronger in daytime data than in nighttime data, but the opposite is true for
all other oscillations. This may be a systematic effect but, if it is not, this
property should help narrow the theoretical options for the mechanism
responsible for decay-rate variability.Comment: 9 pages, 21 figure
Differential rotation and meridional flow in the solar supergranulation layer: Measuring the eddy viscosity
We measure the eddy viscosity in the outermost layers of the solar convection
zone by comparing the rotation law computed with the Reynolds stress resulting
from f-plane simulations of the angular momentum transport in rotating
convection with the observed differential rotation pattern. The simulations
lead to a negative vertical and a positive horizontal angular momentum
transport. The consequence is a subrotation of the outermost layers, as it is
indeed indicated both by helioseismology and the observed rotation rates of
sunspots. In order to reproduce the observed gradient of the rotation rate a
value of about 1.5 x 10^{13} cm/s for the eddy viscosity is necessary.
Comparison with the magnetic eddy diffusivity derived from the sunspot decay
yields a surprisingly large magnetic Prandtl number of 150 for the
supergranulation layer. The negative gradient of the rotation rate also drives
a surface meridional flow towards the poles, in agreement with the results from
Doppler measurements. The successful reproduction of the abnormally positive
horizontal cross correlation (on the northern hemisphere) observed for bipolar
groups then provides an independent test for the resulting eddy viscosity.Comment: 6 pages, 8 figures, Astronomy and Astrophysics (subm.
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