Optimized boundary driven flows for dynamos in a sphere

We perform numerical optimization of the axisymmetric flows in a sphere to minimize the critical magnetic Reynolds number Rm_cr required for dynamo onset. The optimization is done for the class of laminar incompressible flows of von Karman type satisfying the steady-state Navier-Stokes equation. Such flows are determined by equatorially antisymmetric profiles of driving azimuthal (toroidal) velocity specified at the spherical boundary. The model is relevant to the Madison plasma dynamo experiment (MPDX), whose spherical boundary is capable of differential driving of plasma in the azimuthal direction. We show that the dynamo onset in this system depends strongly on details of the driving velocity profile and the fluid Reynolds number Re. It is found that the overall lowest Rm_cr~200 is achieved at Re~240 for the flow, which is hydrodynamically marginally stable. We also show that the optimized flows can sustain dynamos only in the range Rm_cr<Rm<Rm_cr2, where Rm_cr2 is the second critical magnetic Reynolds number, above which the dynamo is quenched. Samples of the optimized flows and the corresponding dynamo fields are presented

Predicting Understorey VegetationCover from Overstorey Attributes in Two Temperate MountainForests

Summary : It is important to develop a predictive understanding for the environmental controls on understorey vegetation, which harbor most of the plant biodiversity and are the source of food and cover for wildlife. Forest succession models (i. e. gap models) representing overstorey dynamics are not commonly linked to mathematical models of understorey dynamics. This is surprising, given that understorey vegetation clearly responds to changes in the overstorey that result in changing light availability. One difficulty may lie in the coarse representation of light regime captured by most gap models. Linkage of overstorey-understorey models might be facilitated if the diameter structure of simulated stands could be used to drive understorey change, as a proxy for light and other influences. The objective of this study was to determine whether understorey vegetation cover can be adequately predicted by variables derived from overstorey diameter structure alone, or if canopy cover and light availability are important, from additional predictors. Field sampling was conducted at a montane and a subalpine study area in the Swiss Alps. We used regression analysis to assess the relative importance of various overstorey predictors for understorey cover and composition. In the subalpine study area, the relative dominance of graminoids increased with increasing light availability, at the expense of forbs. In the montane study area, forb cover increased sharply with increasing light, while graminoid cover remained at low levels. As a result, the relative dominance of graminoid species declined with increasing light levels. This difference is attributed to the presence of Adenostyles alliariae, a tall, large-leaved forb. The effects of changes in the physical environment on plant community composition were thus mediated by interspecific interactions. This makes it difficult to predict overstorey-induced changes in understorey species composition at the level of functional groups. At both study sites, diameter structure variables were found to provide a reasonable approximation of total understorey cover, cover of the more common species, and species richness. Models of understorey community composition often improved (0-31% increased predictive ability) with inclusion of variables representing the light environment. In the context of gap model development, the great complexity associated with improved representation of light availability must be weighed against the relatively low gain in predictive power that is likely to result. We recommend that efforts to include forest understorey dynamics in gap models begin by considering empirical relationships between understorey patterns and overstorey diameter structur

Phase Structure of Compact Star in Modified Quark-Meson Coupling Model

The K$^-$ condensation and quark deconfinement phase transitions are investigated in the modified quark-meson coupling model. It is shown that K$^-$ condensation is suppressed because of the quark deconfinement when $B^{1/4}<$202.2MeV, where $B$ is the bag constant for unpaired quark matter. With the equation of state (EOS) solved self-consistently, we discuss the properties of compact stars. We find that the EOS of pure hadron matter with condensed K$^-$ phase should be ruled out by the redshift for star EXO0748-676, while EOS containing unpaired quark matter phase with $B^{1/4}$ being about 180MeV could be consistent with this observation and the best measured mass of star PSR 1913+16. We then probe into the change of the phase structures in possible compact stars with deconfinment phase as the central densities increase. But if the recent inferred massive star among Terzan 5 with M$>$1.68M$_{\odot}$ is confirmed, all the present EOSes with condensed phase and deconfined phase would be ruled out and therefore these exotic phases are unlikely to appear within neutron stars.Comment: 11 pages, 5 figure

A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses

Multipath propagation in the interstellar medium distorts radio pulses, an effect predominant for distant pulsars observed at low frequencies. Typically, broadened pulses are analyzed to determine the amount of propagation-induced pulse broadening, but with little interest in determining the undistorted pulse shapes. In this paper we develop and apply a method that recovers both the intrinsic pulse shape and the pulse broadening function that describes the scattering of an impulse. The method resembles the CLEAN algorithm used in synthesis imaging applications, although we search for the best pulse broadening function, and perform a true deconvolution to recover intrinsic pulse structre. As figures of merit to optimize the deconvolution, we use the positivity and symmetry of the deconvolved result along with the mean square residual and the number of points below a given threshold. Our method makes no prior assumptions about the intrinsic pulse shape and can be used for a range of scattering functions for the interstellar medium. It can therefore be applied to a wider variety of measured pulse shapes and degrees of scattering than the previous approaches. We apply the technique to both simulated data and data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical Journa

The Radial Extent and Warp of the Ionized Galactic Disk. I. A VLBA Survey of Extragalactic Sources Toward the Anticenter

We report multifrequency Very Long Baseline Array observations of twelve active galactic nuclei seen toward the Galactic anticenter. All of the sources are at |b| < 10 degrees and seven have |b| < 0.5 degrees. Our VLBA observations can detect an enhancement in the angular broadening of these sources due to an extended H II disk, if the orientation of the H II disk in the outer Galaxy is similar to that of the H I disk. Such an extended H II disk is suggested by the C IV absorption in a quasar's spectrum, the appearance of H I disks of nearby spiral galaxies, and models of Ly-alpha cloud absorbers and the Galactic fountain. We detect eleven of the twelve sources at one or more frequencies; nine of the sources are compact and suitable for an angular broadening analysis. A preliminary analysis of the observed angular diameters suggests that the H II disk does not display considerable warping or flaring and does not extend to large Galactocentric distances (R >~ 100 kpc). A companion paper (Lazio & Cordes 1997) combines these observations with those in the literature and presents a more comprehensive analysis.Comment: 19 pages, LaTeX2e with AASTeX macro aaspp4, accepted for publication in ApJS, Vol. 115, 1998 April; Figures 1, 3, and 4 included, for figures of individual sources see http://astrosun.tn.cornell.edu/students/lazio/Anticenter/anticenterI.htm

Improving the establishment submodel of a forest patch model to assess the long-term protective effect of mountain forests

Simulation models such as forest patch models can be used to forecast the development of forest structural attributes over time. However, predictions of such models with respect to the impact of forest dynamics on the long-term protective effect of mountain forests may be of limited accuracy where tree regeneration is simulated with little detail. For this reason, we improved the establishment submodel of the ForClim forest patch model by implementing a more detailed representation of tree regeneration. Our refined submodel included canopy shading and ungulate browsing, two important constraints to sapling growth in mountain forests. To compare the old and the new establishment submodel of ForClim, we simulated the successional dynamics of the Stotzigwald protection forest in the Swiss Alps over a 60-year period. This forest provides protection for an important traffic route, but currently contains an alarmingly low density of tree regeneration. The comparison yielded a significantly longer regeneration period for the new model version, bringing the simulations into closer agreement with the known slow stand dynamics of mountain forests. In addition, the new model version was applied to forecast the future ability of the Stotzigwald forest to buffer the valley below from rockfall disturbance. Two scenarios were simulated: (1) canopy shading but no browsing impact, and (2) canopy shading and high browsing impact. The simulated stand structures were then compared to stand structure targets for rockfall protection, in order to assess their long-term protective effects. Under both scenarios, the initial sparse level of tree regeneration affected the long-term protective effect of the forest, which considerably declined during the first 40years. In the complete absence of browsing, the density of small trees increased slightly after 60years, raising hope for an eventual recovery of the protective effect. In the scenario that included browsing, however, the density of small trees remained at very low levels. With our improved establishment submodel, we provide an enhanced tool for studying the impacts of structural dynamics on the long-term protective effect of mountain forests. For certain purposes, it is important that predictive models of forest dynamics adequately represent critical processes for tree regeneration, such as sapling responses to low light levels and high browsing pressur

On Pulsar Distance Measurements and their Uncertainties

Accurate distances to pulsars can be used for a variety of studies of the Galaxy and its electron content. However, most distance measures to pulsars have been derived from the absorption (or lack thereof) of pulsar emission by Galactic HI gas, which typically implies that only upper or lower limits on the pulsar distance are available. We present a critical analysis of all measured HI distance limits to pulsars and other neutron stars, and translate these limits into actual distance estimates through a likelihood analysis that simultaneously corrects for statistical biases. We also apply this analysis to parallax measurements of pulsars in order to obtain accurate distance estimates and find that the parallax and HI distance measurements are biased in different ways, because of differences in the sampled populations. Parallax measurements typically underestimate a pulsar's distance because of the limited distance to which this technique works and the consequential strong effect of the Galactic pulsar distribution (i.e. the original Lutz-Kelker bias), in HI distance limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading to overestimated distances because the bright pulsars on which this technique is applicable are more likely to be nearby given their brightness.Comment: 32 pages, 1 figure, 2 tables; Accepted for publication in the Astrophysical Journa

Neutron star properties in a chiral SU(3) model

We investigate various properties of neutron star matter within an effective chiral $SU(3)_L \times SU(3)_R$ model. The predictions of this model are compared with a Walecka-type model. It is demonstrated that the importance of hyperon degrees are strongly depending on the interaction used, even if the equation of state near saturation density is nearly the same in both models. While the Walecka-type model predicts a strange star core with strangeness fraction $f_S \approx 4/3$, the chiral model allows only for $f_S \approx 1/3$ and predicts that $\Sigma^0$, $\Sigma^+$ and $\Xi^0$ will not exist in star, in contrast to the Walecka-type model.Comment: 13 pages, Revtex, 5 figs include

Toward a Theory of Legislative Decision

Recent developments in formal political analysis have spawned two seemingly related theories of democratic political processes. The more familiar of the two is the theory of electoral competition based on Downs' (1957) heuristics and greatly elaborated by Davis, Hinich and Ordeshook (1970), Kramer (1975), McKelvey (1976), and others. Somewhat less familiar (perhaps because the intellectual movement is less well integrated) is the theory of legislative decision which has grown from roots in game theory and the theory of social choice. Black (1958), Riker (1962), Plott (1967), Wilson (1969), Schwartz (1970), Kadane (1972), and several others have nurtured the rudimentary models which compose this theory