756 research outputs found
Scientific management and implementation of the geophysical fluid flow cell for Spacelab missions
Scientific support for the spherical convection experiment to be flown on Spacelab 3 was developed. This experiment takes advantage of the zero gravity environment of the orbiting space laboratory to conduct fundamental fluid flow studies concerned with thermally driven motions inside a rotating spherical shell with radial gravity. Such a system is a laboratory analog of large scale atmospheric and solar circulations. The radial body force necessary to model gravity correctly is obtained by using dielectric polarization forces in a radially varying electric field to produce radial accelerations proportional to temperature. This experiment will answer fundamental questions concerned with establishing the preferred modes of large scale motion in planetary and stellar atmospheres
Theoretical and experimental studies in support of the geophysical fluid flow experiment
Computer programming was completed for digital acquisition of temperature and velocity data generated by the Geophysical Fluid Flow Cell (GFFC) during the upcoming Spacelab 3 mission. A set of scenarios was developed which covers basic electro-hydrodynamic instability, highly supercritical convection with isothermal boundaries, convection with imposed thermal forcing, and some stably stratified runs to look at large-scale thermohaline ocean circulations. The extent to which the GFFC experimental results apply to more complicated circumstances within the Sun or giant planets was assessed
Rossby and Magnetic Prandtl Number Scaling of Stellar Dynamos
Rotational scaling relationships are examined for the degree of equipartition
between magnetic and kinetic energies in stellar convection zones. These
scaling relationships are approached from two paradigms, with first a glance at
scaling relationship built upon an energy-balance argument and second a look at
a force-based scaling. The latter implies a transition between a
nearly-constant inertial scaling when in the asymptotic limit of minimal
diffusion and magnetostrophy, whereas the former implies a weaker scaling with
convective Rossby number. Both scaling relationships are then compared to a
suite of 3D convective dynamo simulations with a wide variety of domain
geometries, stratifications, and range of convective Rossby numbers.Comment: 15 pages, 6 figures, accepted in Ap
The dynamics of spiral arms in pure stellar disks
It has been believed that spirals in pure stellar disks, especially the ones
spontaneously formed, decay in several galactic rotations due to the increase
of stellar velocity dispersions. Therefore, some cooling mechanism, for example
dissipational effects of the interstellar medium, was assumed to be necessary
to keep the spiral arms. Here we show that stellar disks can maintain spiral
features for several tens of rotations without the help of cooling, using a
series of high-resolution three-dimensional -body simulations of pure
stellar disks. We found that if the number of particles is sufficiently large,
e.g., , multi-arm spirals developed in an isolated disk can
survive for more than 10 Gyrs. We confirmed that there is a self-regulating
mechanism that maintains the amplitude of the spiral arms. Spiral arms increase
Toomre's of the disk, and the heating rate correlates with the squared
amplitude of the spirals. Since the amplitude itself is limited by the value of
, this makes the dynamical heating less effective in the later phase of
evolution. A simple analytical argument suggests that the heating is caused by
gravitational scattering of stars by spiral arms, and that the self-regulating
mechanism in pure-stellar disks can effectively maintain spiral arms on a
cosmological timescale. In the case of a smaller number of particles, e.g.,
, spiral arms grow faster in the beginning of the simulation
(while is small) and they cause a rapid increase of . As a result, the
spiral arms become faint in several Gyrs.Comment: 18 pages, 19 figures, accepted for Ap
Solar Seismology from Space. a Conference at Snowmass, Colorado
The quality of the ground based observing environment suffers from several degrading factors: diurnal interruptions and thermal variations, atmospheric seeing and transparency fluctuations and adverse weather interruptions are among the chief difficulties. The limited fraction of the solar surface observable from only one vantage point is also a potential limitation to the quality of the data available without going to space. Primary conference goals were to discuss in depth the scientific return from current observations and analyses of solar oscillations, to discuss the instrumental and site requirements for realizing the full potential of the seismic analysis method, and to help bring new workers into the field by collecting and summarizing the key background theory. At the conclusion of the conference there was a clear consensus that ground based observation would not be able to provide data of the quality required to permit a substantial analysis of the solar convection zone dynamics or to permit a full deduction of the solar interior structure
Dynamically-Driven Star Formation In Models Of NGC 7252
We present new dynamical models of the merger remnant NGC 7252 which include
star formation simulated according to various phenomenological rules. By using
interactive software to match our model with the observed morphology and gas
velocity field, we obtain a consistent dynamical model for NGC 7252. In our
models, this proto-elliptical galaxy formed by the merger of two similar
gas-rich disk galaxies which fell together with an initial pericentric
separation of ~2 disk scale lengths approximately 620 Myr ago. Results from two
different star formation rules--- density-dependent and shock-induced--- show
significant differences in star formation during and after the first passage.
Shock-induced star formation yields a prompt and wide-spread starburst at the
time of first passage, while density-dependent star formation predicts a more
slowly rising and centrally concentrated starburst. A comparison of the
distributions and ages of observed clusters with results of our simulations
favors shock-induced mechanism of star formation in NGC 7252. We also present
simulated color images of our model of NGC 7252, constructed by incorporating
population synthesis with radiative transfer and dust attenuation. Overall the
predicted magnitudes and colors of the models are consistent with observations,
although the simulated tails are fainter and redder than observed. We suggest
that a lack of star formation in the tails, reflected by the redder colors, is
due to an incomplete description of star formation in our models rather than
insufficient gas in the tails.Comment: 11 pages, 9 figures, to be published in MNRA
Magnetic Wreaths and Cycles in Convective Dynamos
Solar-type stars exhibit a rich variety of magnetic activity. Seeking to
explore the convective origins of this activity, we have carried out a series
of global 3D magnetohydrodynamic (MHD) simulations with the anelastic spherical
harmonic (ASH) code. Here we report on the dynamo mechanisms achieved as the
effects of artificial diffusion are systematically decreased. The simulations
are carried out at a nominal rotation rate of three times the solar value
(3), but similar dynamics may also apply to the Sun. Our previous
simulations demonstrated that convective dynamos can build persistent toroidal
flux structures (magnetic wreaths) in the midst of a turbulent convection zone
and that high rotation rates promote the cyclic reversal of these wreaths. Here
we demonstrate that magnetic cycles can also be achieved by reducing the
diffusion, thus increasing the Reynolds and magnetic Reynolds numbers. In these
more turbulent models, diffusive processes no longer play a significant role in
the key dynamical balances that establish and maintain the differential
rotation and magnetic wreaths. Magnetic reversals are attributed to an
imbalance in the poloidal magnetic induction by convective motions that is
stabilized at higher diffusion levels. Additionally, the enhanced levels of
turbulence lead to greater intermittency in the toroidal magnetic wreaths,
promoting the generation of buoyant magnetic loops that rise from the deep
interior to the upper regions of our simulated domain. The implications of such
turbulence-induced magnetic buoyancy for solar and stellar flux emergence are
also discussed.Comment: 21 pages, 16 figures, accepted for publication in Ap
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