Kink instabilities in jets from rotating magnetic fields

We have performed 2.5D and 3D simulations of conical jets driven by the rotation of an ordered, large-scale magnetic field in a stratified atmosphere. The simulations cover about three orders of magnitude in distance to capture the centrifugal acceleration as well as the evolution past the Alfven surface. We find that the jets develop kink instabilities, the characteristics of which depend on the velocity profile imposed at the base of the flow. The instabilities are especially pronounced with a rigid rotation profile, which induces a shearless magnetic field. The jet's expansion appears to be limiting the growth of Alfven mode instabilities.Comment: 10 pages, 13 figures, accepted for publication in A&

Cosine and Sine Operators Related with Orthogonal Polynomial Sets on the Intervall [-1,1]

The quantization of phase is still an open problem. In the approach of Susskind and Glogower so called cosine and sine operators play a fundamental role. Their eigenstates in the Fock representation are related with the Chebyshev polynomials of the second kind. Here we introduce more general cosine and sine operators whose eigenfunctions in the Fock basis are related in a similar way with arbitrary orthogonal polynomial sets on the intervall [-1,1]. To each polynomial set defined in terms of a weight function there corresponds a pair of cosine and sine operators. Depending on the symmetry of the weight function we distinguish generalized or extended operators. Their eigenstates are used to define cosine and sine representations and probability distributions. We consider also the inverse arccosine and arcsine operators and use their eigenstates to define cosine-phase and sine-phase distributions, respectively. Specific, numerical and graphical results are given for the classical orthogonal polynomials and for particular Fock and coherent states.Comment: 1 tex-file (24 pages), 11 figure

The Propagation of Magneto-Centrifugally Launched Jets: I

We present simulations of the propagation of magnetized jets. This work differs from previous studies in that the cross-sectional distributions of the jets's state variables are derived from analytical models for magneto-centrifugal launching. The source is a magnetized rotator whose properties are specfied as boundary conditions. The jets in these simulations are considerably more complex than the ``top-hat''constant density etc. profiles used in previous work. We find that density and magnetic field stratification (with radius) in the jet leads to new behavior including the separation of an inner jet core from a low density collar. We find this {\it jet within a jet} structure, along with the magnetic stresses, leads to propagation behaviors not observed in previous simulation studies. Our methodology allows us to compare MHD jets from different types of sources whose properties could ultimately be derived from the behavior of the propagating jets.Comment: 42 pages, accepted by the Ap

A Sample of Low Redshift BL Lacs. I. The Radio Data

We present a new sample of 30 nearby (z<0.2) BL Lacs, selected to study the nuclear as well as the large scale properties of low power radio sources. In this first paper, we show and discuss new radio data taken with the VLA (19 objects at 1.4 GHz, either in A or C configuration, or both) as well as with the VLBA (15 sources at 5 GHz). On the kiloparsec scale, all objects exhibit a compact core and a variety of radio morphologies (jets, halos, secondary compact components). On the parsec scale, we find weak cores and a few short, one-sided, jets. From the jet/counter-jet ratio, core dominance, and synchrotron self Compton model we estimate the intrinsic orientation and velocity of the jets. The resulting properties of BL Lacs are similar to those of a parent population composed of FR I radio galaxies.Comment: 46 pages, 12 figures, accepted for publication in Ap

3D Relativistic Magnetohydrodynamic Simulations of Magnetized Spine-Sheath Relativistic Jets

Numerical simulations of weakly magnetized and strongly magnetized relativistic jets embedded in a weakly magnetized and strongly magnetized stationary or weakly relativistic (v = c/2) sheath have been performed. A magnetic field parallel to the flow is used in these simulations performed by the new GRMHD numerical code RAISHIN used in its RMHD configuration. In the numerical simulations the Lorentz factor $\gamma = 2.5$ jet is precessed to break the initial equilibrium configuration. In the simulations sound speeds are $\lesssim c/\sqrt 3$ in the weakly magnetized simulations and $\lesssim 0.3c$ in the strongly magnetized simulations. The Alfven wave speed is $\lesssim 0.07c$ in the weakly magnetized simulations and $\lesssim 0.56c$ in the strongly magnetized simulations. The results of the numerical simulations are compared to theoretical predictions from a normal mode analysis of the linearized relativistic magnetohydrodynamic (RMHD) equations capable of describing a uniform axially magnetized cylindrical relativistic jet embedded in a uniform axially magnetized relativistically moving sheath. The theoretical dispersion relation allows investigation of effects associated with maximum possible sound speeds, Alfven wave speeds near light speed and relativistic sheath speeds. The prediction of increased stability of the weakly magnetized system resulting from c/2 sheath speeds and the stabilization of the strongly magnetized system resulting from c/2 sheath speeds is verified by the numerical simulation results.Comment: 31 pages, 8 figures, accepted for publicatin in ApJ. A paper with high resolution figures available at http://gammaray.nsstc.nasa.gov/~mizuno/research_new.htm

Sheared Flow As A Stabilizing Mechanism In Astrophysical Jets

It has been hypothesized that the sustained narrowness observed in the asymptotic cylindrical region of bipolar outflows from Young Stellar Objects (YSO) indicates that these jets are magnetically collimated. The j cross B force observed in z-pinch plasmas is a possible explanation for these observations. However, z-pinch plasmas are subject to current driven instabilities (CDI). The interest in using z-pinches for controlled nuclear fusion has lead to an extensive theory of the stability of magnetically confined plasmas. Analytical, numerical, and experimental evidence from this field suggest that sheared flow in magnetized plasmas can reduce the growth rates of the sausage and kink instabilities. Here we propose the hypothesis that sheared helical flow can exert a similar stabilizing influence on CDI in YSO jets.Comment: 13 pages, 2 figure

Three-Dimensional Simulations of Inflows Irradiated by a Precessing Accretion Disk in Active Galactic Nuclei: Formation of Outflows

We present three-dimensional (3-D) hydrodynamical simulations of gas flows in the vicinity of an active galactic nucleus (AGN) powered by a precessing accretion disk. We consider the effects of the radiation force from such a disk on its environment on a relatively large scale (up to ~10 pc. We implicitly include the precessing disk by forcing the disk radiation field to precess around a symmetry axis with a given period ($P$) and a tilt angle ($\Theta$). We study time evolution of the flows irradiated by the disk, and investigate basic dependencies of the flow morphology, mass flux, angular momentum on different combinations of $\Theta$ and $P$. We find the gas flow settles into a configuration with two components, (1) an equatorial inflow and (2) a bipolar inflow/outflow with the outflow leaving the system along the poles (the directions of disk normals). However, the flow does not always reach a steady state. We find that the maximum outflow velocity and the kinetic outflow power at the outer boundary can be reduced significantly with increasing $\Theta$. We also find that of the mass inflow rate across the inner boundary does not change significantly with increasing $\Theta$. (Abbreviated)Comment: Accepted for publication in ApJ. 15 pages, 7 figures. A version with full resolution figures can be downloaded from http://www.physics.unlv.edu/~rk/preprint/precess.pd

Magnetic Field Effects on the Structure and Evolution of Overdense Radiatively Cooling Jets

We investigate the effect of magnetic fields on the propagation dynamics and morphology of overdense, radiatively cooling, supermagnetosonic jets, with the help of fully three-dimensional SPMHD simulations. Evaluated for a set of parameters which are mainly suitable for protostellar jets (with density ratios between the jet and the ambient medium 3-10, and ambient Mach number ~ 24), these simulations are also compared with baseline non-magnetic and adiabatic calculations. We find that, after amplification by compression and re-orientation in nonparallel shocks at the working surface, the magnetic field that is carried backward with the shocked gas into the cocoon improves the jet collimation relative to the purely hydrodynamic (HD) systems. Low-amplitude, approximately equally spaced internal shocks (which are absent in the HD systems) are produced by MHD K-H reflection pinch modes. The longitudinal field geometry also excites non-axisymmetric helical modes which cause some beam wiggling. The strength and amount of these modes are, however, reduced (by ~ twice) in the presence of radiative cooling relative to the adiabatic cases. Besides, a large density ratio between the jet and the ambient medium also reduces, in general, the number of the internal shocks. As a consequence, the weakness of the induced internal shocks makes it doubtful that the magnetic pinches could produce by themselves the bright knots observed in the overdense, radiatively cooling protostellar jets.Comment: To appear in ApJ; 36 pages + 16 (gif) figures. PostScript files of figures are available at http://www.iagusp.usp.br/preprints/preprint.htm

Towards sustainable agriculture: fossil-free ammonia

Citation: Pfromm, P. H. (2017). Towards sustainable agriculture: Fossil-free ammonia. Journal of Renewable and Sustainable Energy, 9(3), 034702. https://doi.org/10.1063/1.4985090About 40% of our food would not exist without synthetic ammonia (NH3) for fertilization. Yet, NH3 production is energy intensive. About 2% of the world's commercial energy is consumed as fossil fuels for NH3 synthesis based on the century-old Haber-Bosch (H.-B.) process. The state of the art and the opportunities for reducing the fossil energy footprint of industrial H.-B. NH3 synthesis are discussed. It is shown that even a hypothetical utterly revolutionary H.-B. catalyst could not significantly reduce the energy demand of H.-B. NH3 as this is governed by hydrogen production. Renewable energy-enabled, fossil-free NH3 synthesis is then evaluated based on the exceptional and continuing cost decline of renewable electricity. H.-B. syngas (H2, N2) is assumed to be produced by electrolysis and cryogenic air separation, and then supplied to an existing H.-B. synthesis loop. Fossil-free NH3 could be produced for energy costs of about $232 per tonne NH3 without claiming any economic benefits for the avoidance of about 1.5 tonnes of CO2 released per tonne NH3 compared to the most efficient H.-B. implementations. Research into alternatives to the H.-B. process might be best targeted at emerging markets with currently little NH3 synthesis capacity but significant future population growth such as Africa. Reduced capital intensity, good scale-down economics, tolerance for process upsets and contamination, and intermittent operability are some desirable characteristics of NH3 synthesis in less developed markets, and for stranded resources. Processes that are fundamentally different from H.-B. may come to the fore under these specific boundary conditions

Three-Dimensional Simulations of Jets from Keplerian Disks: Self--Regulatory Stability

We present the extension of previous two-dimensional simulations of the time-dependent evolution of non-relativistic outflows from the surface of Keplerian accretion disks, to three dimensions. The accretion disk itself is taken to provide a set of fixed boundary conditions for the problem. The 3-D results are consistent with the theory of steady, axisymmetric, centrifugally driven disk winds up to the Alfv\'en surface of the outflow. Beyond the Alfv\'en surface however, the jet in 3-D becomes unstable to non-axisymmetric, Kelvin-Helmholtz instabilities. We show that jets maintain their long-term stability through a self-limiting process wherein the average Alfv\'enic Mach number within the jet is maintained to order unity. This is accomplished in at least two ways. First, poloidal magnetic field is concentrated along the central axis of the jet forming a ``backbone'' in which the Alfv\'en speed is sufficiently high to reduce the average jet Alfv\'enic Mach number to unity. Second, the onset of higher order Kelvin-Helmholtz ``flute'' modes (m \ge 2) reduce the efficiency with which the jet material is accelerated, and transfer kinetic energy of the outflow into the stretched, poloidal field lines of the distorted jet. This too has the effect of increasing the Alfv\'en speed, and thus reducing the Alfv\'enic Mach number. The jet is able to survive the onset of the more destructive m=1 mode in this way. Our simulations also show that jets can acquire corkscrew, or wobbling types of geometries in this relatively stable end-state, depending on the nature of the perturbations upon them. Finally, we suggest that jets go into alternating periods of low and high activity as the disappearance of unstable modes in the sub-Alfv\'enic regime enables another cycle of acceleration to super-Alfv\'enic speeds.Comment: 57 pages, 22 figures, submitted to Ap