Time-dependent interaction of granules with magnetic flux tubes

The time-dependent interaction of the granulation velocity field with a magnetic flux tube is investigated here. It is seen that when a magnetic field line is displaced normal to itself so as to simulate thebuffeting action of granules, a flow of gas is initiated along the field. By choosing a lateral velocity field which is consistent with observations of granules, it is found that the resulting gas motion is a downward flow with a velocity compatible with the observed downflow in isolated photospheric flux tubes. It is therefore proposed that the observed photospheric downflow is a manifestation of the interaction of granules with flux tubes.     

Numerical simulation of slow shocks in the solar wind

The evolutionary state of slow forward shock waves is examined with the use of two MHD numerical codes. Our study is intended to be exploratory rather than a detailed parametric one. The first code is one-dimensional (with three components of velocity and magnetic field) which is used to follow a slow shock that propagates into a positive gradient of density versus distance. It is found that the slow shock evolves into an extraneous (intermediate) shock wave. The second code has a spherical, one-dimensional, planar geometry (with two velocity and magnetic field components) which is used to follow a spiral interplanetary magnetic field. It is found that a slow shock type perturbation can generate a forward slow shock; a fast forward shock is generated in the front of the slow shock; a contact discontinuity is formed behind the slow shock, and a compound nonlinear MHD wave is formed behind the contact discontinuity with a fast reverse shock formed further behind. Thus, we demonstrate that the evolution of a slow shock into (solely) a fast shock, as suggested by Whang (1987), is much more complicated.     

W3(OH)致密HⅡ区磁场研究

本文以W3(OH)为实例,建立了与OH脉泽成协的HII区气体-尘埃壳层的磁场模型,由此可见,致密HII区所在分子云核心中的磁场强度,与分子云核心的分子数密度之间存在指数α=1/2的幂律关系;而在HII区气体-尘埃壳层中,磁场强度与分子数密度之间存在α=1的正比关系,根据讨论可知,与OH脉泽成协的HII区,其气体-尘埃壳层的分子数密度为10~6cm~(-3)量级,磁场强度为几个mG;一旦分子数密度达10~7cm~(-3)量级,OH脉泽便将熄灭。     

Light propagation in a universe with spatial inhomogeneities

A light propagation in a universe which is homogeneous only in average systematically differs from a light propagation in a strictly homogeneous Universe. We demonstrate a link between this effect and the general theory of transport phenomenon in random media. The effective spatial curvature of a universe which is homogeneous only in average is introduced. This curvature is governing a light propagation in such universe. We show that the effective spatial curvature is lower than the average curvature. It implies that a universe with critical mean density looks like a space with negative curvature.     

On the existence of the continuous spectrum of ideal MHD in a 2D magnetostatic equilibrium

The continuous spectrum of a 2D magnetostatic equilibrium with y-invariance is derived. It is shown that the continuous spectrum is given by an eigenvalue problem on each magnetic surface and is related to the different behaviour of the equilibrium quantities in different magnetic surfaces. The special case of a uniform poloidal magnetic field in a 1D equilibrium that is stratified with height, has been considered in detail and it is found that there is no continuous spectrum. It is shown that this result relies completely on the artificial property that the behaviour of the equilibrium quantities along a magnetic field line is independent of the field line considered. As a consequence the non-existence of a continuous spectrum in a 1D equilibrium with a uniform magnetic field cannot be used to argue that the continuous spectrum has no physical relevance.Research Assistant of the Belgian National Fund for Scientific Research.     

Monte Carlo Modeling of the Thermal Conductivity of Porous Cometary Ice

The basic structure of comet nuclei is an aggregation of grains, with a size distribution that extends over several orders of magnitude and a similar distribution of pores. Although attempts have been made to assess the effect of porosity on the thermal conductivity, the effect of pore size distribution has been ignored. Modeling a porous structure with a wide size distribution would require a very fine 3-D grid, so as to accommodate the smallest and largest voids. In order to circumvent this difficulty, we adopt a hierarchical procedure. Thus we assume a random and fractal porous structure and use a 3-D Monte Carlo model. The basic configuration is a cube made of unit cells of two types, (ice) filled and void, randomly distributed. Their relative number corresponds to a prescribed porosity. We solve the heat transport equation for this cube until a steady state is obtained, and from this solution the effective thermal conductivity is derived. The calculations are repeated for a range of porosities and temperatures, since the ice conductivity is temperature dependent. The basic cube serves as a unit filled cell in a larger cube, and in this way the hierarchical structure of the medium is built up.We find that the thermal conductivity is lowered by several orders of magnitude at high porosities. The correction factor, obtained as a fit to the results of our calculations, is expressed as a smooth function of the porosity, which tends to zero as the porosity approaches the percolation threshold of the solid. If only the porosity of the medium is known, this correction is not uniquely determined, but rather a range of values is possible. Only if the size distribution of the pores is known does the correction become uniquely determined.     

Unsteady flow of a relativistic radiating gas in a gravitational field

The unsteady flow of a relativistic radiating neutrino gas is studied in a gravitational field. The curved body is assumed to be a vertical flat plate on which is imposed a time-dependent perturbation on a basic flow. For small perturbations, the ill-posed problem is reduced to a well-posed one and analytical solutions are developed.     

Doppler effect and nature of light

The problem of the frequency change of light hitting a moving detector from the point of view of an observer who is at rest in the reference frame of an emitter is discussed. It is pointed out that wave and corpuscular characters of light yield different results: a formula analogical to the famous Doppler one is obtained if a wave nature of light is assumed, however, no shift in the frequency is obtained if a corpuscular nature of light is only considered.     

On the turbulent decay of strong magnetic fields and the development of sunspot areas

This paper deals with the conception that two-dimensional turbulence is present in a sunspot where the magnetic field is strong. This conception is based upon the incapacity of even a strong magnetic field to influence an arbitrary two-dimensional turbulence, if the magnetic field is parallel to a constant direction and the motion occurs in planes orthogonal to it. It is, moreover, shown that such a two-dimensional turbulence provides for a turbulent decay of the magnetic field. The decay rate possesses nearly the same dependence on the scales as for three-dimensional turbulence. Finally, the turbulent decay is studied by investigating a simple model and comparing the results with those deduced by Bumba from the observed decay of sunspot groups areas. By means of our conception even a good quantitative agreement is stated.     

The stability of line tied force-free cylindrical arcades: Is an active region filament a requirement for a two-ribbon flare?

The MHD stability of force-free, cylindrical arcades is investigated, including the stabilising effect of photospheric line tying. It is found that a wide variety of fields are stable. This suggests that either a departure from a force free equilibrium or suppression of line tying is necessary if a two-ribbon flare is to be triggered. It is postulated that in both circumstances, the existence of an active region filament is an essential preflare requirement for a two-ribbon flare.     

The structure of polar caps and the equatorial belt of pulsars

The self-consistent balanced pulsar magnetosphere of a magnetic neutron star with aligned magnetic and rotational axes is considered. It is shown that the magnetosphere consists of electron polar caps separated by empty space from a positron equatorial belt. The shape of the cold polar caps at a large distance from the star is calculated. It is shown that the cap shape at a large distance is independent of the magnetospheric structure near the neutron star. The shape of the equatorial belt is calculated. It is shown that a part of the equatorial belt rotates differentially, and its angular velocity is larger than that of the star (superrotation). It is shown that under certain conditions the space charge density of the belt can be very large. In principle, the formation of a surface charge placed in vacuum on a magnetic surface is possible. Magnetospheric vibrations are considered. A connection is established between drifting subpulses and the equatorial belt superrotation and also between drifting subpulses and cap vibrations. The characteristic frequency of vibrations and the angular velocity of superrotation are estimated.     

Photon in a cavity – a Gedankenexperiment

The inertial and gravitational mass of electromagnetic radiation (i.e., a photon distribution) in a cavity with reflecting walls has been treated by many authors for over a century. After many contending discussions, a consensus has emerged that the mass of such a photon distribution is equal to its total energy divided by the square of the speed of light. Nevertheless, questions remain unsettled on the interaction of the photons with the walls of the box. In order to understand some of the details of this interaction, a simple case of a single photon with an energy $E_{\nu }=h\nu$ bouncing up and down in a static cavity with perfectly reflecting walls in a constant gravitational field $\mathit{g}$, constant in space and time, is studied and its contribution to the weight of the box is determined as a temporal average.     

Compressible,conductive, steady MHD flow in a gravitational field

The magnetohydrodynamics of a compressible fluid with finite electrical conductivity in a gravitational field is treated analytically. For the case of one ignorable coordinate in cylindrical and Cartesian coordinates the problem is reduced to a scalar partial differential equation. The ideal gas equation of state is considered. For simplicity, the new equation is derived for a two-component motion. The application of this result to mass flow in sunspots is discussed.     

Accounting for the viscosity of the fluid core in the problem of the physical libration of the moon

A two-component theoretical model of the physical libration of the Moon in longitude is constructed with account taken of the viscosity of the core. In the new version, a hydrodynamic problem of motion of a fluid filling a solid rotating shell is solved. It is found that surfaces of equal angular velocity are spherical, and a velocity field of the fluid core of the Moon is described by elementary functions. A distribution of the internal pressure in the core is found. An angular momentum exchange between the fluid core and solid mantle is described by a third-order differential equation with a right-hand side. The roots of a characteristic equation are studied and the stability of rotation is proved. A libration angle as a function of time is found using the derived solution of the differential equation. Limiting cases of infinitely large and infinitely small viscosity are considered and an effect of lag of a libration phase from a phase of action of an external moment of forces is ascertained. This makes it possible to estimate the viscosity and sizes of the lunar fluid core from data of observations.     

The role of the Yoshizawa effect in the Archontis dynamo

The generation of mean magnetic fields is studied for a simple non-helical flow where a net cross-helicity of either sign can emerge. This flow, which is also known as the Archontis flow, is a generalization of the Arnold–Beltrami–Childress flow, but with the cosine terms omitted. The presence of cross-helicity leads to a mean-field dynamo effect that is known as the Yoshizawa effect. Direct numerical simulations of such flows demonstrate the presence of magnetic fields on scales larger than the scale of the flow. Contrary to earlier expectations, the Yoshizawa effect is found to be proportional to the mean magnetic field and can therefore lead to its exponential instead of just linear amplification for magnetic Reynolds numbers that exceed a certain critical value. Unlike α effect dynamos, it is found that the Yoshizawa effect is not notably constrained by the presence of a conservation law. It is argued that this is due to the presence of a forcing term in the momentum equation, which leads to a non-zero correlation with the magnetic field. Finally, the application to energy convergence in solar wind turbulence is discussed.     

Numerical simulation of type III solar radio bursts on meter- and hectometer-waves

Numerical simulation of type III bursts is made by the use of fully numerical scheme showing a general rule for obtaining a numerically stable difference scheme. Although the electron distribution function is one-dimensional in velocity space, the plasma waves is cylindrically symmetric two-dimensional in K-space.It is confirmed that the previous simulation made by the use of semi-analytical method assuming the plateau distribution of electron distribution is qualitatively correct, but the number density of electron beam to have a typical type III burst was overestimated by a factor of about 3.It is demonstrated that a tentative neglection of a term for the induced scattering of plasma waves into nonresonant K-range gives no remarkable effect on the energy loss of the electron beam, though the scattering is strong. The reason is that the scattering reduces the saturation level of plasma waves resulting in a reduction of the energy loss, while a part of the energy of electron beam is indirectly lost by the scattering.     

Quantum and thermal phenomena in the matter-gravity system

The matter-gravity system is examined in a path integral approach for the case of conformal matter coupled to a Friedman-Robertson-Walker space time. In particular the case of gravitational potentials of interest in cosmology for which the universe tunnels from a small radius is examined. It is observed that in the presence of such gravitational horizons the universe evolves in a complex time and it is shown how a classical time and temperature emerge. Correspondingly, one will have compensating quantum and thermal fluctuations for the matter and gravity system and it is noted that the unstable mode of gravity corresponding to the universe tunneling into existence will be compensated by an analogous mode for matter corresponding to its creation. This last point is examined in a simple De Sitter model with conformal matter and a relation is found between the cosmological constant, the number of matter fields and the self coupling of matter responsable for its instability.     

A new regularization of the restricted three-body problem and an application

A new regularizing transformation for the three-dimensional restricted three-body problem is constructed. It is explicitly derived and is equivalent to a simple rational map. Geometrically it is equivalent to a rotation of the 3-sphere. Unlike the KS map it is dimension preserving and is valid inn dimensions. This regularizing map is applied to the restricted problem in order to prove the existence of a family of periodic orbits which continue from a family of collision orbits.     

点在球面多边形内外的判定方法及应用

精确判断卫星进出南大西洋辐射异常区(SAA)对于保护卫星上重要仪器、延长卫星寿命和制定观测计划至关重要,这就是所谓点定位问题,简单说来就是判定球面上一点P是否位于指定的球面区域R内的问题。传统的研究方法是把球面简化为平面,研究点与平面多边形的关系。但SAA区域为球面上的区域,用平面简化得出的点定位误差较大。提出判断点在球面区域内外的算法,处理了可能出现的奇异情况,实现了点在球面区域内的准确定位。     

应用于射电天文的低噪声温度测量方法

噪声温度是接收机和低噪声放大器最重要的性能指标,是了解设备性能好坏的关键因素。随着电子技术的快速发展,接收机和低噪声放大器的噪声温度变得越来越低,准确而快速地测量接收机和低噪声放大器的噪声温度变得非常困难。介绍了6种在射电天文中经常使用的测量低噪声温度的方法,这些测量方法具有准确可靠、简单易行的优点。叙述了测量原理并给出了一些测量方法的测量结果,对影响测量噪声温度精度并且易被忽视的因素也做了详细讨论。