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41.
We present new analytical three-dimensional solutions of the magnetohydrostatic equations, which are applicable to the co-rotating frame of reference outside a rigidly rotating cylindrical body, and have potential applications to planetary magnetospheres and stellar coronae. We consider the case with centrifugal force only, and use a transformation method in which the governing equation for the “pseudo-potential” (from which the magnetic field can be calculated) becomes the Laplace partial differential equation. The new solutions extend the set of previously found solutions to those of a “fractional multipole” nature, and offer wider possibilities for modelling than before. We consider some special cases, and present example solutions. 相似文献
42.
Asher Yahalom 《地球物理与天体物理流体动力学》2017,111(2):131-137
Cross helicity is not conserved in non-barotropic magnetohydro-dynamics (MHD) (as opposed to barotropic or incompressible MHD). Here we show that variational analysis suggests a new kind of local cross helicity which is conserved in the non-barotropic case. This local cross helicity can be integrated to a global non-barotropic cross helicity which was suggested in the work of Webb et al. (2014a,b). The non-barotropic cross helicity reduces to the standard cross helicity under barotropic assumptions. The new local cross helicity is conserved even for topologies for which the variational principle does not apply. 相似文献
43.
Matthew G. Baring 《Astrophysics and Space Science》2007,307(1-3):297-303
Particle acceleration at plasma shocks appears to be ubiquitous in the universe, spanning systems in the heliosphere, supernova
remnants, and relativistic jets in distant active galaxies and gamma-ray bursts. This review addresses some of the key issues
for shock acceleration theory that require resolution in order to propel our understanding of particle energization in astrophysical
environments. These include magnetic field amplification in shock ramps, the non-linear hydrodynamic interplay between thermal
ions and their extremely energetic counterparts possessing ultrarelativistic energies, and the ability to inject and accelerate
electrons in both non-relativistic and relativistic shocks. Recent observational developments that impact these issues are
summarized. While these topics are currently being probed by astrophysicists using numerical simulations, they are also ripe
for investigation in laboratory experiments, which potentially can provide valuable insights into the physics of cosmic shocks. 相似文献
44.
Abstract We introduce a general expansion approach to obtain a fully consistent closed set of magnetohydrodynamic equations in two independent variables, which is particularly useful to describe axially symmetric, time-dependent problems with weak variation of all quantities in the radial direction. This is done by considering the hierarchy of expanded magnetofluid equations in cylindrical coordinates and equating terms with equal powers in the radial coordinate r. From geometrical considerations it is shown that the radial expansions of the pertaining physical quantities are either even series or odd series in r; this introduces a significant reduction in the number of variables and equations. The closure of the system is provided by appropriate boundary conditions. Among other possible applications, the method is relevant for the analysis of structure and dynamics of magnetic field concentrations in stellar atmospheres. 相似文献
45.
R. Meinel 《地球物理与天体物理流体动力学》2013,107(1-3):79-84
Abstract It is shown that the equations of an α2-dynamo in a cylindrical disk with perfectly conducting surroundings allow an analytic representation of some special solutions. The related problem of free decay can be solved completely. 相似文献
46.
The magnetohydrodynamic turbulent cascade in the ecliptic solar wind: Study of Ulysses data 总被引：1，自引：0，他引：1
Raffaele Marino Luca Sorriso-Valvo Pierluigi Veltri Roberto Bruno 《Planetary and Space Science》2011,59(7):592-597
The occurrence of a nonlinear turbulent energy cascade in solar wind plasma has been recently established through the observation of an exact law from spacecraft measurements. The main results obtained in the fast, polar wind measured by Ulysses spacecraft are reviewed here. In particular, the turbulent cascade is seen as the mean to provide the energy necessary for the local heating in the non-adiabatic expansion of the solar wind. The importance of the density fluctuations in enhancing the turbulent energy transport is also evidenced. The ecliptic wind data measured by Ulysses are studied here in the same framework. This has been done by separating fast and slow streams, in order to avoid mixing of different physical conditions. The results further support the need for separate analysis of the two types of wind. 相似文献
47.
许多行星 (如木卫三 ,水星 ,地球 ,木星和土星 )和恒星 (如太阳 )具有内部磁场。对这些磁场的存在和变化的解释对行星科学家和天体物理学家是一个巨大的挑战。本文试图总结行星和恒星的导电流体内部磁流体力学研究的新近发展和困难。一般由热对流驱动的流动通过磁流体力学过程产生并维持在行星和恒星中的磁场。在行星中磁流体力学过程强烈地受到转动 ,磁场和球几何位型的综合影响。其动力学的关键方面涉及科里奥利力和洛伦兹力间的相互作用。在太阳中其流线 ,即处于对流层的薄的剪切流层在太阳的磁流体力学过程中扮演了一个基本的角色 ,并由之产生了 1 1年的太阳黑子周期。本文也给出了一个新的非线性三维太阳发电机模型。 相似文献
48.
Magnetic reconnection at a three-dimensional null point is a natural extension of the familiar two-dimensional X-point reconnection. A model is set up here for reconnection at a null point with current directed parallel to the fan plane, by solving the kinematic, steady, resistive magnetohydrodynamic equations in its vicinity. The magnetic field is assumed to be steady, and a localised diffusion region surrounding the null point is also assumed, outside which the plasma is ideal. Particular attention is focussed on the way that the magnetic flux changes its connections as a result of the reconnection. The resultant plasma flow is found to cross the spine and fan of the null, and thus transfer magnetic flux between topologically distinct regions. Solutions are also found in which the flow crosses either the spine or fan only. 相似文献
49.
V.V. Pipin 《地球物理与天体物理流体动力学》2013,107(1):25-43
We explore the f -effect and the small-scale current helicity, , for the case of weakly compressible magnetically driven turbulence that is subjected to the differential rotation. No restriction is applied to the amplitude of angular velocity, i.e., the derivations presented are valid for an arbitrary Coriolis number, z * = 2 z cor , though the differential rotation itself is assumed to be weak. The expressions obtained are used to explore the possible distributions of f -effect and h c in convection zones (CZ) of the solar-type stars. Generally, our theory gives f { { > 0 in the northern hemisphere of the Sun and the opposite case in the southern hemisphere. In most cases the h c has the opposite sign to f { { . However, we show that in the depth of CZ where the influence of rotation upon turbulence (associated with z *) and the radial shear of angular velocity are strong, the distribution of f { { might be drastically different from a classical cos è -dependence, where è is colatitude. It is shown that f { { has a negative sign at the bottom and below of CZ at mid latitudes. There, the distribution of h c is also different from cos è , but it does not change its sign with the depth. Further, we briefly consider these quantities in the disk geometry. The application of the developed theory to dynamos in the accretion disk is more restrictive because they usually have a strong differential rotation, | ‘ log z / ‘ log r | > 1. 相似文献
50.
Martin Schrinner Karl-Heinz Rädler Matthias Rheinhardt Ulrich R. Christensen 《地球物理与天体物理流体动力学》2013,107(2):81-116
Mean-field theory describes magnetohydrodynamic processes leading to large-scale magnetic fields in various cosmic objects. In this study magnetoconvection and dynamo processes in a rotating spherical shell are considered. Mean fields are defined by azimuthal averaging. In the framework of mean-field theory, the coefficients which determine the traditional representation of the mean electromotive force, including derivatives of the mean magnetic field up to the first order, are crucial for analyzing and simulating dynamo action. Two methods are developed to extract mean-field coefficients from direct numerical simulations of the mentioned processes. While the first method does not use intrinsic approximations, the second one is based on the second-order correlation approximation. There is satisfying agreement of the results of both methods for sufficiently slow fluid motions. Both methods are applied to simulations of rotating magnetoconvection and a quasi-stationary geodynamo. The mean-field induction effects described by these coefficients, e.g., the α-effect, are highly anisotropic in both examples. An α2-mechanism is suggested along with a strong γ-effect operating outside the inner core tangent cylinder. The turbulent diffusivity exceeds the molecular one by at least one order of magnitude in the geodynamo example. With the aim to compare mean-field simulations with corresponding direct numerical simulations, a two-dimensional mean-field model involving all previously determined mean-field coefficients was constructed. Various tests with different sets of mean-field coefficients reveal their action and significance. In the magnetoconvection and geodynamo examples considered here, the match between direct numerical simulations and mean-field simulations is only satisfying if a large number of mean-field coefficients are involved. In the magnetoconvection example, the azimuthally averaged magnetic field resulting from the numerical simulation is in good agreement with its counterpart in the mean-field model. However, this match is not completely satisfactory in the geodynamo case anymore. Here the traditional representation of the mean electromotive force ignoring higher than first-order spatial derivatives of the mean magnetic field is no longer a good approximation. 相似文献