217 research outputs found
Dynamo Action in the Solar Convection Zone and Tachocline: Pumping and Organization of Toroidal Fields
We present the first results from three-dimensional spherical shell
simulations of magnetic dynamo action realized by turbulent convection
penetrating downward into a tachocline of rotational shear. This permits us to
assess several dynamical elements believed to be crucial to the operation of
the solar global dynamo, variously involving differential rotation resulting
from convection, magnetic pumping, and amplification of fields by stretching
within the tachocline. The simulations reveal that strong axisymmetric toroidal
magnetic fields (about 3000 G in strength) are realized within the lower stable
layer, unlike in the convection zone where fluctuating fields are predominant.
The toroidal fields in the stable layer possess a striking persistent
antisymmetric parity, with fields in the northern hemisphere largely of
opposite polarity to those in the southern hemisphere. The associated mean
poloidal magnetic fields there have a clear dipolar geometry, but we have not
yet observed any distinctive reversals or latitudinal propagation. The presence
of these deep magnetic fields appears to stabilize the sense of mean fields
produced by vigorous dynamo action in the bulk of the convection zone.Comment: 4 pages, 3 color figures (compressed), in press at ApJ
Velocity Amplitudes in Global Convection Simulations: The Role of the Prandtl Number and Near-Surface Driving
Several lines of evidence suggest that the velocity amplitude in global
simulations of solar convection, U, may be systematically over-estimated.
Motivated by these recent results, we explore the factors that determine U and
we consider how these might scale to solar parameter regimes. To this end, we
decrease the thermal diffusivity along two paths in parameter space.
If the kinematic viscosity is decreased proportionally with
(fixing the Prandtl number ), we find that U increases but
asymptotes toward a constant value, as found by Featherstone & Hindman (2016).
However, if is held fixed while decreasing (increasing ),
we find that U systematically decreases. We attribute this to an enhancement of
the thermal content of downflow plumes, which allows them to carry the solar
luminosity with slower flow speeds. We contrast this with the case of
Rayleigh-Benard convection which is not subject to this luminosity constraint.
This dramatic difference in behavior for the two paths in parameter space
(fixed or fixed ) persists whether the heat transport by unresolved,
near-surface convection is modeled as a thermal conduction or as a fixed flux.
The results suggest that if solar convection can operate in a high-
regime, then this might effectively limit the velocity amplitude. Small-scale
magnetism is a possible source of enhanced viscosity that may serve to achieve
this high- regime.Comment: 34 Pages, 8 Figures, submitted to a special issue of "Advances in
Space Research" on "Solar Dynamo Frontiers
A Spherical Plasma Dynamo Experiment
We propose a plasma experiment to be used to investigate fundamental
properties of astrophysical dynamos. The highly conducting, fast-flowing plasma
will allow experimenters to explore systems with magnetic Reynolds numbers an
order of magnitude larger than those accessible with liquid-metal experiments.
The plasma is confined using a ring-cusp strategy and subject to a toroidal
differentially rotating outer boundary condition. As proof of principle, we
present magnetohydrodynamic simulations of the proposed experiment. When a von
K\'arm\'an-type boundary condition is specified, and the magnetic Reynolds
number is large enough, dynamo action is observed. At different values of the
magnetic Prandtl and Reynolds numbers the simulations demonstrate either
laminar or turbulent dynamo action
Velocity Field Statistics in Star-Forming Regions. I. Centroid Velocity Observations
The probability density functions (pdfs) of molecular line centroid velocity
fluctuations and fluctuation differences at different spatial lags are
estimated for several nearby molecular clouds with active internal star
formation. The data consist of over 75,000 CO line profiles divided
among twelve spatially and/or kinematically distinct regions. Although three
regions (all in Mon R2) appear nearly Gaussian, the others show strong evidence
for non-Gaussian, often nearly exponential, centroid velocity pdfs, possibly
with power law contributions in the far tails. Evidence for nearly exponential
centroid pdfs in the neutral HI component of the ISM is also presented, based
on older optical and radio observations. These results are in contrast to pdfs
found in isotropic incompressible turbulence experiments and simulations.
Furthermore, no evidence is found for the scaling of difference pdf kurtosis
with Reynolds number which is seen in incompressible turbulence, and the
spatial distribution of high-amplitude velocity differences shows little
indication of the filamentary appearance predicted by decay simulations
dominated by vortical interactions. The variation with lag of the difference
pdf moments is presented as a constraint on future simulations.Comment: LaTeX, 23 pages, with 15 Figures included separately as gif image
files. Refereed/revised version accepted to the Astrophysical Journal. A
complete (but much larger) postscript version is available from
http://ktaadn.gsfc.nasa.gov/~miesc
Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?
It is known that there must be some weak form of transport (called cool
bottom processing, or CBP) acting in low mass RGB and AGB stars, adding nuclei,
newly produced near the hydrogen-burning shell, to the convective envelope. We
assume that this extra-mixing originates in a stellar dynamo operated by the
differential rotation below the envelope, maintaining toroidal magnetic fields
near the hydrogen-burning shell. We use a phenomenological approach to the
buoyancy of magnetic flux tubes, assuming that they induce matter circulation
as needed by CBP models. This establishes requirements on the fields necessary
to transport material from zones where some nuclear burning takes place,
through the radiative layer, and into the convective envelope. Magnetic field
strengths are determined by the transport rates needed by CBP for the model
stellar structure of a star of initially 1.5 solar mass, in both the AGB and
RGB phases. The field required for the AGB star in the processing zone is B_0 ~
5x10^6 G; at the base of the convective envelope this yields an intensity B_E <
10^4 G (approximately). For the RGB case, B_0 ~ 5x10^4 to 4x10^5 G, and the
corresponding B_E are ~ 450 to 3500 G. These results are consistent with
existing observations on AGB stars. They also hint at the basis for high field
sources in some planetary nebulae and the very large fields found in some white
dwarfs. It is concluded that transport by magnetic buoyancy should be
considered as a possible mechanism for extra mixing through the radiative zone,
as is required by both stellar observations and the extensive isotopic data on
circumstellar condensates found in meteorites.Comment: 26 pages, 4 figures, accepted by Astrophysical Journa
Vitamin D5 in Arabidopsis thaliana
Abstract Vitamin D3 is a secosterol hormone critical for bone growth and calcium homeostasis, produced in vertebrate skin by photolytic conversion of the cholesterol biosynthetic intermediate provitamin D3. Insufficient levels of vitamin D3 especially in the case of low solar UV-B irradiation is often compensated by an intake of a dietary source of vitamin D3 of animal origin. Small amounts of vitamin D3 were described in a few plant species and considered as a peculiar feature of their phytochemical diversity. In this report we show the presence of vitamin D5 in the model plant Arabidopsis thaliana. This plant secosterol is a UV-B mediated derivative of provitamin D5, the precursor of sitosterol. The present work will allow a further survey of vitamin D distribution in plant species
The Wisconsin Plasma Astrophysics Laboratory
The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a flexible user
facility designed to study a range of astrophysically relevant plasma processes
as well as novel geometries that mimic astrophysical systems. A multi-cusp
magnetic bucket constructed from strong samarium cobalt permanent magnets now
confines a 10 m, fully ionized, magnetic-field free plasma in a spherical
geometry. Plasma parameters of to eV and
to cm provide an ideal testbed
for a range of astrophysical experiments including self-exciting dynamos,
collisionless magnetic reconnection, jet stability, stellar winds, and more.
This article describes the capabilities of WiPAL along with several
experiments, in both operating and planning stages, that illustrate the range
of possibilities for future users.Comment: 21 pages, 12 figures, 2 table
2MASS wide field extinction maps: II. The Ophiuchus and the Lupus cloud complexe
We present an extinction map of a ~1,700 deg sq region that encloses the
Ophiuchus, the Lupus, and the Pipe dark complexes using 42 million stars from
the Two Micron All Sky Survey (2MASS) point source catalog. The use of a robust
and optimal near-infrared method to map dust column density (Nicer, described
in Lombardi & Alves 2001) allow us to detect extinction as low as A_K = 0.05
mag with a 2-sigma significance, and still to have a resolution of 3 arcmin on
our map. We also present a novel, statistically sound method to characterize
the small-scale inhomogeneities in molecular clouds. Finally, we investigate
the cloud structure function, and show that significant deviations from the
results predicted by turbulent models are observed.Comment: 16 pages, A&A in pres
Turbulent Driving Scales in Molecular Clouds
Supersonic turbulence in molecular clouds is a dominant agent that strongly
affects the clouds' evolution and star formation activity. Turbulence may be
initiated and maintained by a number of processes, acting at a wide range of
physical scales. By examining the dynamical state of molecular clouds, it is
possible to assess the primary candidates for how the turbulent energy is
injected. The aim of this paper is to constrain the scales at which turbulence
is driven in the molecular interstellar medium, by comparing simulated
molecular spectral line observations of numerical magnetohydrodynamic (MHD)
models and molecular spectral line observations of real molecular clouds. We
use principal component analysis, applied to both models and observational
data, to extract a quantitative measure of the driving scale of turbulence. We
find that only models driven at large scales (comparable to, or exceeding, the
size of the cloud) are consistent with observations. This result applies also
to clouds with little or no internal star formation activity. Astrophysical
processes acting on large scales, including supernova-driven turbulence,
magnetorotational instability, or spiral shock forcing, are viable candidates
for the generation and maintenance of molecular cloud turbulence. Small scale
driving by sources internal to molecular clouds, such as outflows, can be
important on small scales, but cannot replicate the observed large-scale
velocity fluctuations in the molecular interstellar medium.Comment: 8 pages, 7 figures, accepted for publication in A&
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