Galactic Spiral Arms and Dynamo Control Parameters     

Shukurov  A.  Sokoloff  D. 《Studia Geophysica et Geodaetica》1998,42(3):391-396

We discuss the effects of galactic spiral arms on the -coefficient, turbulent diffusivity and turbulent energy density of the interstellar turbulence. We argue that the -coefficient and the dynamo number are larger in the interarm regions, whereas the kinetic energy density of turbulence is larger in the arms; the turbulent magnetic diffusivity can be only weakly affected by the spiral pattern.  相似文献   

Computer modeling and simulation of coalbed methane resources   总被引：3，自引：0，他引：3  

G. B. C. Young 《International Journal of Coal Geology》1998,35(1-4)

Coal seam gas reservoirs are complex both geologically and in the mechanism of gas production. Understanding these naturally fractured reservoirs for two-phase (gas–water) flow conditions is often limited by a lack of data. This paper illustrates that reservoir simulation is a powerful tool which can be used to determine key data requirements, and how variability in reservoir properties and operating practices affect performance at the field level. The paper presents examples of how reservoir simulation can be used to assess the efficiency of well completions (fracturing or cavitation), identify candidate wells for remedial treatment, examine methane drainage in advance of mining, and assess the impact of errors in measured data on long-term gas production forecasts.  相似文献   

The magnetic field and rotational period of 53 Camelopardalis     

Hill  Bohlender  Landstreet  Wade  Manset  & Bastien 《Monthly notices of the Royal Astronomical Society》1998,297(1):236-238

The magnetic Ap star 53 Cam has been described as the best object for testing theories of elemental diffusion in the presence of a stellar magnetic field. It is thus vital to determine the rotational period of this very important star as accurately as possible. An improved period (8.026 81±0.000 04 d) is derived here by combining new measurements of the effective magnetic field with previously published values.  相似文献   

Modulation and symmetry changes in stellar dynamos     

E. Knobloch  S. M. Tobias  & N. O. Weiss 《Monthly notices of the Royal Astronomical Society》1998,297(4):1123-1138

Stellar dynamos are governed by non-linear partial differential equations (PDEs) which admit solutions with dipole, quadrupole or mixed symmetry (i.e. with different parities). These PDEs possess periodic solutions that describe magnetic cycles, and numerical studies reveal two different types of modulation. For modulations of Type 1 there are parity changes without significant changes of amplitude, while for Type 2 there are amplitude changes without significant changes in parity. In stars like the Sun, cyclic magnetic activity is interrupted by grand minima that correspond to Type 2 modulation. Although the Sun's magnetic field has maintained dipole symmetry for almost 300 yr, there was a significant parity change at the end of the Maunder Minimum. We infer that the solar field may have flipped from dipole to quadrupole polarity (and back) after deep minima in the past and may do so again in the future. Other stars, with different masses or rotation rates, may exhibit cyclic activity with dipole, quadrupole or mixed parity. The origins of such behaviour can be understood by relating the PDE results to solutions of appropriate low-order systems of ordinary differential equations (ODEs). Type 1 modulation is reproduced in a fourth-order system while Type 2 modulation occurs in a third-order system. Here we construct a new sixth-order system that describes both types of modulation and clarifies the interactions between symmetry-breaking and modulation of activity. Solutions of these non-linear ODEs reproduce the qualitative behaviour found for the PDEs, including flipping of polarity after a prolonged grand minimum. Thus we can be confident that these patterns of behaviour are robust, and will apply to stars that are similar to the Sun.  相似文献   

Superbubbles in magnetized interstellar media: blowout or confinement?     

Kohji Tomisaka 《Monthly notices of the Royal Astronomical Society》1998,298(3):797-810

The importance of the interstellar magnetic field is studied in relation to the evolution of superbubbles with a three-dimensional (3D) numerical magnetohydrodynamical (MHD) simulation. A superbubble is a large supernova remnant driven by sequential supernova explosions in an OB association. Its evolution is affected by the density stratification in the galactic disc. After the superbubble size reaches 2–3 times the density scaleheight, it expands preferentially in the z -direction, until finally it can punch out a hole in the gas disc (blowout). On the other hand, the magnetic field running parallel to the galactic disc has the effect of preventing it from expanding in the direction perpendicular to the field. The density stratification and the magnetic fields have completely opposite effects on the evolution of the superbubble. We present results of a 3D MHD simulation in which both effects are included. As a result, it is concluded that when the magnetic field has a much larger scaleheight than the density, even for a model in which the bubble would blow out from the disc if the magnetic field were absent, a magnetic field with a strength of 5 μG can confine the bubble in | z |≲300 pc for ≃ 20 Myr (confinement). In a model in which the field strength decreases in the halo as B  ∝ ρ^1/2, the superbubble eventually blows out like a model with B  = 0 even if the magnetic field in the mid-plane is as strong as B  = 5 μG.  相似文献   

A two-layer αω-dynamo model with dynamic feedback on the ω-effect     

Colin B. Roald 《Monthly notices of the Royal Astronomical Society》1998,300(2):397-410

In an attempt to produce a simple representation of an interface dynamo, I examine a dynamo model composed of two one-dimensional (radially averaged) pseudo-spherical layers, one in the convection zone and possessing an α-effect, and the other in the tachocline and possessing an ω-effect. The two layers communicate by means of an analogue of Newton's law of cooling, and a dynamical back-reaction of the magnetic field on ω is provided. Extensive bifurcation diagrams are calculated for three separate values of η, the ratio of magnetic diffusivities of the two layers. I find recognizable similarities to, but also dramatic differences from, the comparable one-layer model examined by Roald &38; Thomas. In particular, the solar-like dynamo mode found previously is no longer stable in the two-layer version; in its place there is a sequence of periodic, quasi-periodic and chaotic modes probably created in a homoclinic bifurcation. These differences are important enough to provide support for the view that the solar dynamo cannot be meaningfully modelled in one dimension.  相似文献   

Galactic Dynamo and Spiral Arms - 3D MHD Simulations     

Elstner  D.  Lesch  H.  von Linden  Susane  Otmianowska-Mazur  Katarzyna  Urbanik  M. 《Studia Geophysica et Geodaetica》1998,42(3):373-381

In the present project we investigate the evolution of a three-dimensional (3D), large-scale galactic magnetic field under the influence of gas flows in spiral arms and in the presence of dynamo action. Our principal goal is to check how the dynamical evolution of gaseous spiral arms affects the global magnetic field structure and to what extent our models could explain the observed spiral patterns of polarization B-vectors in nearby galaxies. A two-step scheme is used: the N-body simulations of a two-component, self-gravitating disk provide the time-dependent velocity fields which are then used as the input to solve the mean-field dynamo equations. We found that the magnetic field is directly influenced by large-scale non-axisymmetric density wave flows yielding the magnetic field locally well-aligned with gaseous spiral arms in a manner similar to that discussed already by Otmianowska-Mazur et al. 1997. However, an additional field amplification, introduced by a non-zero -term in the dynamo equations, is required to cause a systematic increase of magnetic energy density against the diffusive losses. Our simulated magnetic fields are also used to construct the models of a high-frequency (Faraday rotation-free) polarized radio emission accounting for effects of projection and limited resolution, thus suitable for direct comparisons with observations.  相似文献   

Generation of Magnetic Fields During the Protogalactic Collapse     

Wiechen  H.  Birk  G.T.  Lesch  H. 《Studia Geophysica et Geodaetica》1998,42(3):397-403

Large-scale galactic magnetic fields are probably caused by some magnetic field amplification mechanism starting from a seed field. This seed field is still enigmatic. In this contribution it is shown that macroscopic sheared relative velocities of the charged and neutral components of a protogalactic partially ionized plasma generate magnetic fields during the protogalactic collapse. Plasma-neutral gas fluid simulations are performed in order to illustrate this magnetic field-self generation mechanism.  相似文献   

Eruption of an Active-Region Filament Driven by an Emerging Bipole     

Yun-Chun Jiang  Yuan-Deng Shen  Jing-Xiu Wang National Astronomical Observatories / Yunnan Observatory  Chinese Academy of Sciences  Kunming  National Astronomical Observatories  Chinese Academy of Sciences  Beijing 《中国天文和天体物理学报》2007,7(1):129-140

1 INTRODUCTION Filaments are cool, dense material suspended in the hot, tenuous corona. It is widely accepted that the global magnetic field surrounding the filaments plays a key role in their formation, structure and stability (Tandberg-Hanssen1995). Fil…  相似文献   

Planetary,solar and astrophysical relativistic electrons: Common energization mechanisms?     

Ilan Roth   《Planetary and Space Science》2007,55(15):2319-2323

Direct observations or deduced analysis indicate clearly that formation of intense fluxes of relativistic electrons is an important ingredient in the evolution of numerous active magnetized plasma systems. Examples of relativistic electron energization include the recovery phase of a planetary magnetic storm, post solar flare coronal activity and the afterglow of gamma ray bursts. It is suggested that there exists a universal mechanism, which may explain electron energization at the vastly different magnetized plasma environments. The favorite configuration consists of an inhomogeneous magnetic field anchored at a given magnetic structure and excitation of whistler waves due to external injection of low-energy non-isotropic electrons. The energization proceeds as a bootstrap process due to interaction with the propagating whistler waves along the inhomogeneous magnetic field.  相似文献