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 $\kappa$ along two paths in parameter space. If the kinematic viscosity $\nu$ is decreased proportionally with $\kappa$ (fixing the Prandtl number $P_r = \nu/\kappa$), we find that U increases but asymptotes toward a constant value, as found by Featherstone & Hindman (2016). However, if $\nu$ is held fixed while decreasing $\kappa$ (increasing $P_r$), 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 $P_r$ or fixed $\nu$) 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-$P_r$ 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-$P_r$ 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 $^{13}$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$^3$, fully ionized, magnetic-field free plasma in a spherical geometry. Plasma parameters of $T_{e}\approx5$ to $20$ eV and $n_{e}\approx10^{11}$ to $5\times10^{12}$ cm$^{-3}$ 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&