3,573 research outputs found
Global shallow water magnetohydrodynamic waves in the solar tachocline
We derive analytical solutions and dispersion relations of global magnetic
Poincar\'e (magneto-gravity) and magnetic Rossby waves in the approximation of
shallow water magnetohydrodynamics. The solutions are obtained in a rotating
spherical coordinate system for strongly and weakly stable stratification
separately in the presence of toroidal magnetic field. In both cases magnetic
Rossby waves split into fast and slow magnetic Rossby modes. In the case of
strongly stable stratification (valid in the radiative part of the tachocline)
all waves are slightly affected by the layer thickness and the toroidal
magnetic field, while in the case of weakly stable stratification (valid in the
upper overshoot layer of the tachocline) magnetic Poincar\'e and fast magnetic
Rossby waves are found to be concentrated near the solar equator, leading to
equatorially trapped waves. However, slow magnetic Rossby waves tend to
concentrate near the poles, leading to polar trapped waves. The frequencies of
all waves are smaller in the upper weakly stable stratification region than in
the lower strongly stable stratification one
Data base management system analysis and performance testing with respect to NASA requirements
Several candidate Data Base Management Systems (DBM's) that could support the NASA End-to-End Data System's Integrated Data Base Management System (IDBMS) Project, later rescoped and renamed the Packet Management System (PMS) were evaluated. The candidate DBMS systems which had to run on the Digital Equipment Corporation VAX 11/780 computer system were ORACLE, SEED and RIM. Oracle and RIM are both based on the relational data base model while SEED employs a CODASYL network approach. A single data base application which managed stratospheric temperature profiles was studied. The primary reasons for using this application were an insufficient volume of available PMS-like data, a mandate to use actual rather than simulated data, and the abundance of available temperature profile data
Probing Solar Convection
In the solar convection zone acoustic waves are scattered by turbulent sound
speed fluctuations. In this paper the scattering of waves by convective cells
is treated using Rytov's technique. Particular care is taken to include
diffraction effects which are important especially for high-degree modes that
are confined to the surface layers of the Sun. The scattering leads to damping
of the waves and causes a phase shift. Damping manifests itself in the width of
the spectral peak of p-mode eigenfrequencies. The contribution of scattering to
the line widths is estimated and the sensitivity of the results on the assumed
spectrum of the turbulence is studied. Finally the theoretical predictions are
compared with recently measured line widths of high-degree modes.Comment: 26 pages, 7 figures, accepted by MNRA
Results of data base management system parameterized performance testing related to GSFC scientific applications
The results of a variety of tests designed to demonstrate and evaluate the performance of several commercially available data base management system (DBMS) products compatible with the Digital Equipment Corporation VAX 11/780 computer system are summarized. The tests were performed on the INGRES, ORACLE, and SEED DBMS products employing applications that were similar to scientific applications under development by NASA. The objectives of this testing included determining the strength and weaknesses of the candidate systems, performance trade-offs of various design alternatives and the impact of some installation and environmental (computer related) influences
Numerical simulations of the kappa-mechanism with convection
A strong coupling between convection and pulsations is known to play a major
role in the disappearance of unstable modes close to the red edge of the
classical Cepheid instability strip. As mean-field models of time-dependent
convection rely on weakly-constrained parameters, we tackle this problem by the
means of 2-D Direct Numerical Simulations (DNS) of kappa-mechanism with
convection.
Using a linear stability analysis, we first determine the physical conditions
favourable to the kappa-mechanism to occur inside a purely-radiative layer.
Both the instability strips and the nonlinear saturation of unstable modes are
then confirmed by the corresponding DNS. We next present the new simulations
with convection, where a convective zone and the driving region overlap. The
coupling between the convective motions and acoustic modes is then addressed by
using projections onto an acoustic subspace.Comment: 5 pages, 6 figures, accepted for publication in Astrophysics and
Space Science, HELAS workshop (Rome june 2009
Adiabatic elimination in quantum stochastic models
We consider a physical system with a coupling to bosonic reservoirs via a
quantum stochastic differential equation. We study the limit of this model as
the coupling strength tends to infinity. We show that in this limit the
solution to the quantum stochastic differential equation converges strongly to
the solution of a limit quantum stochastic differential equation. In the
limiting dynamics the excited states are removed and the ground states couple
directly to the reservoirs.Comment: 17 pages, no figures, corrected mistake
Detecting solar g-modes with ASTROD
We present an up-to-date estimate for the prospect of using the
Astrodynamical Space Test of Relativity using Optical Devices (ASTROD) for an
unambiguous detection of solar g modes (f < 400 micro Hertz) through their
gravitational signature. There are currently two major efforts to detect
low-frequency gravitational effects, ASTROD and the Laser Interferometer Space
Antenna (LISA). Using the most recent g mode surface amplitude estimates, both
observational and theoretical, it is unclear whether LISA will be capable of
successfully detecting these modes. The ASTROD project may be better suited for
detection as its sensitivity curve is shifted towards lower frequencies with
the best sensitivity occurring in the range 100-300 micro Hertz.Comment: HELAS II international conference "Helioseismology, asteroseismology
and MHD connections", 20-24 August 2007, Goettingen, German
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A Network of SCOP Hidden Markov Models and Its Analysis
<p>Abstract</p> <p>Background</p> <p>The Structural Classification of Proteins (SCOP) database uses a large number of hidden Markov models (HMMs) to represent families and superfamilies composed of proteins that presumably share the same evolutionary origin. However, how the HMMs are related to one another has not been examined before.</p> <p>Results</p> <p>In this work, taking into account the processes used to build the HMMs, we propose a working hypothesis to examine the relationships between HMMs and the families and superfamilies that they represent. Specifically, we perform an all-against-all HMM comparison using the HHsearch program (similar to BLAST) and construct a network where the nodes are HMMs and the edges connect similar HMMs. We hypothesize that the HMMs in a connected component belong to the same family or superfamily more often than expected under a random network connection model. Results show a pattern consistent with this working hypothesis. Moreover, the HMM network possesses features distinctly different from the previously documented biological networks, exemplified by the exceptionally high clustering coefficient and the large number of connected components.</p> <p>Conclusions</p> <p>The current finding may provide guidance in devising computational methods to reduce the degree of overlaps between the HMMs representing the same superfamilies, which may in turn enable more efficient large-scale sequence searches against the database of HMMs.</p
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