Magnetohydrodynamic jets from different magnetic field configurations

Using axisymmetric MHD simulations we investigate how the overall jet formation is affected by a variation in the disk magnetic flux profile and/or the existence of a central stellar magnetosphere. Our simulations evolve from an initial, hydrostatic equilibrium state in a force-free magnetic field configuration. We find a unique relation between the collimation degree and the disk wind magnetization power law exponent. The collimation degree decreases for steeper disk magnetic field profiles. Highly collimated outflows resulting from a flat profile tend to be unsteady. We further consider a magnetic field superposed of a stellar dipole and a disk field in parallel or anti-parallel alignment. Both stellar and disk wind may evolve in a pair of outflows, however, a reasonably strong disk wind component is essential for jet collimation. Strong flares may lead to a sudden change in mass flux by a factor two. We hypothesize that such flares may eventually trigger jet knots.Comment: 5 pages, 4 figures; proceedings from conference: Protostellar Jets in Context, held in Rhodes, July 7-12, 200

Radial and vertical angular momentum transport in protostellar discs

Angular momentum in protostellar discs can be transported either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field. We present a model of steady-state discs where these two mechanisms operate at the same radius and derive approximate criteria for their occurrence in an ambipolar diffusion dominated disc. We obtain "weak field'' solutions - which we associate with the MRI channel modes in a stratified disc - and transform them into accretion solutions with predominantly radial angular-momentum transport by implementing a turbulent-stress prescription based on published results of numerical simulations. We also analyze "intermediate field strength'' solutions in which both radial and vertical transport operate at the same radial location. Our results suggest, however, that this overlap is unlikely to occur in real discs.Comment: 5 pages, 2 figures, 1 table, aastex.cls. Accepted for publication in Astrophysics & Space Scienc

Magnetic Flux Transport by turbulent reconnection in astrophysical flows

The role of MHD turbulence in astrophysical environments is still highly debated. An important question that permeates this debate is the transport of magnetic flux. This is particularly important, for instance, in the context of star formation. When clouds collapse gravitationally to form stars, there must be some magnetic flux transport. otherwise the new born stars would have magnetic fields several orders of magnitude larger than the observed ones. Also, the magnetic flux that is dragged in the late stages of the formation of a star can remove all the rotational support from the accretion disk that grows around the protostar. The efficiency of the mechanism which is often invoked to allow the transport of magnetic fields in the different stages of star formation, namely, the ambipolar diffusion, has been lately put in check. We here discuss an alternative mechanism for magnetic flux transport which is based on turbulent fast magnetic reconnection. We review recent results obtained from 3D MHD numerical simulations that indicate that this mechanism is very efficient for decoupling and transport magnetic flux from the inner denser regions to the outskirts of collapsing clouds in the different stages of star formation. We also discuss this mechanism in the context of dynamo processes and speculate that it can play a role both in the solar dynamo and in accretion disk dynamo processes.Comment: 10 pages, 2 figures, review submitted to Physica Script

Outflows and inflows in astrophysical systems

We seek for self-similar solutions describing the time-dependent evolution of self-gravity systems with either spherical symmetry or axisymmetric disk geometry. By assuming self-similar variable $x\equiv r/at$ where $a$ is isothermal sound speed we find self-similar solutions extending from the initial instant $t=0$ to the final stage $t\to \infty$ using standard semi-analytical methods. Different types of solutions are constructed, which describe overall expansion or collapse, envelope expansion with core collapse (EECC), the formation of central rotationally supported quasi-equilibrium disk as well as shocks. Though infinitely many, these self-similarity solutions have similar asymptotic behaviors which may impose diagnosis on the velocity and density structures in astrophysical systems.Comment: ChJAA accepted; contribution to the proceedings of the 5th microquasar worksho

The Nucleon-Nucleon Interaction in a Chiral Constituent Quark Model

We study the short-range nucleon-nucleon interaction in a chiral constituent quark model by diagonalizing a Hamiltonian comprising a linear confinement and a Goldstone boson exchange interaction between quarks. The six-quark harmonic oscillator basis contains up to two excitation quanta. We show that the highly dominant configuration is $\mid s^4p^2[42]_O [51]_{FS}>$ due to its specific flavour-spin symmetry. Using the Born-Oppenheimer approximation we find a strong effective repulsion at zero separation between nucleons in both $^3S_1$ and $^1S_0$ channels. The symmetry structure of the highly dominant configuration implies the existence of a node in the S-wave relative motion wave function at short distances. The amplitude of the oscillation of the wave function at short range will be however strongly suppressed. We discuss the mechanism leading to the effective short-range repulsion within the chiral constituent quark model as compared to that related with the one-gluon exchange interaction.Comment: 31 pages, LaTe

The Atmospheric Chemistry Suite (ACS) of Three Spectrometers for the ExoMars 2016 Trace Gas Orbiter

The Atmospheric Chemistry Suite (ACS) package is an element of the Russian contribution to the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. ACS consists of three separate infrared spectrometers, sharing common mechanical, electrical, and thermal interfaces. This ensemble of spectrometers has been designed and developed in response to the Trace Gas Orbiter mission objectives that specifically address the requirement of high sensitivity instruments to enable the unambiguous detection of trace gases of potential geophysical or biological interest. For this reason, ACS embarks a set of instruments achieving simultaneously very high accuracy (ppt level), very high resolving power (>10,000) and large spectral coverage (0.7 to 17 μm—the visible to thermal infrared range). The near-infrared (NIR) channel is a versatile spectrometer covering the 0.7–1.6 μm spectral range with a resolving power of ∼20,000. NIR employs the combination of an echelle grating with an AOTF (Acousto-Optical Tunable Filter) as diffraction order selector. This channel will be mainly operated in solar occultation and nadir, and can also perform limb observations. The scientific goals of NIR are the measurements of water vapor, aerosols, and dayside or night side airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the 2.2–4.4 μm range. MIR achieves a resolving power of >50,000. It has been designed to accomplish the most sensitive measurements ever of the trace gases present in the Martian atmosphere. The thermal-infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer encompassing the spectral range of 1.7–17 μm with apodized resolution varying from 0.2 to 1.3 cm−1. TIRVIM is primarily dedicated to profiling temperature from the surface up to ∼60 km and to monitor aerosol abundance in nadir. TIRVIM also has a limb and solar occultation capability. The technical concept of the instrument, its accommodation on the spacecraft, the optical designs as well as some of the calibrations, and the expected performances for its three channels are described

Moscow-type NN-potentials and three-nucleon bound states

A detailed description of Moscow-type (M-type) potential models for the NN interaction is given. The microscopic foundation of these models, which appear as a consequence of the composite quark structure of nucleons, is discussed. M-type models are shown to arise naturally in a coupled channel approach when compound or bag-like six-quark states, strongly coupled to the NN channel, are eliminated from the complete multiquark wave function. The role of the deep-lying bound states that appear in these models is elucidated. By introducing additional conditions of orthogonality to these compound six-quark states, a continuous series of almost on-shell equivalent nonlocal interaction models, characterized by a strong reduction or full absence of a local repulsive core (M-type models), is generated. The predictions of these interaction models for 3N systems are analyzed in detail. It is shown that M-type models give, under certain conditions, a stronger binding of the 3N system than the original phase-equivalent model with nodeless wave functions. An analysis of the 3N system with the new versions of the Moscow NN potential describing also the higher even partial waves is presented. Large deviations from conventional NN force models are found for the momentum distribution in the high momentum region. In particular, the Coulomb displacement energy for nuclei ^3He - ^3H displays a promising agreement with experiment when the ^3H binding energy is extrapolated to the experimental value.Comment: 23 pages Latex, 9 figures, to appear in Phys.Rev.

Models of Venus neutral upper atmosphere: Structure and composition

Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed. Tables are provided for density, temperature, composition (CO2, O, CO, He, N, N2, and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day- and nightsides.Estimates are made of other species, including O2 and D. Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, O, C, and He) as well as the hot components (H, N, C, O) on the day- and nightsides.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25840/1/0000403.pd

Enhanced transport in the polar mesosphere of Jupiter: Evidence from Cassini UVIS helium 584 Å airglow

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95607/1/jgre2060.pd

Self-Similar Solutions for Viscous and Resistive ADAF

In this paper, the self-similar solution of resistive advection dominated accretion flows (ADAF) in the presence of a pure azimuthal magnetic field is investigated. The mechanism of energy dissipation is assumed to be the viscosity and the magnetic diffusivity due to turbulence in the accretion flow. It is assumed that the magnetic diffusivity and the kinematic viscosity are not constant and vary by position and $\alpha$-prescription is used for them. In order to solve the integrated equations that govern the behavior of the accretion flow, a self-similar method is used. The solutions show that the structure of accretion flow depends on the magnetic field and the magnetic diffusivity. As, the radial infall velocity and the temperature of the flow increase, and the rotational velocity decreases. Also, the rotational velocity for all selected values of magnetic diffusivity and magnetic field is sub-Keplerian. The solutions show that there is a certain amount of magnetic field that the rotational velocity of the flow becomes zero. This amount of the magnetic field depends on the gas properties of the disc, such as adiabatic index and viscosity, magnetic diffusivity, and advection parameters. The solutions show the mass accretion rate increases by adding the magnetic diffusivity and in high magnetic pressure case, the ratio of the mass accretion rate to the Bondi accretion rate decreases as magnetic field increases. Also, the study of Lundquist and magnetic Reynolds numbers based on resistivity indicates that the linear growth of magnetorotational instability (MRI) of the flow decreases by resistivity. This property is qualitatively consistent with resistive magnetohydrodynamics (MHD) simulations.Comment: 18 pages, 3 figures, accepted by JA&