Can the turbulent galactic dynamo generate large-scale magnetic fields?

Large-scale magnetic fields in galaxies are thought to be generated by a turbulent dynamo. However, the same turbulence also leads to a small-scale dynamo which generates magnetic noise at a more rapid rate. The efficiency of the large-scale dynamo depends on how this noise saturates. We examine this issue, taking into account ambipolar drift, which obtains in a galaxy with significant neutral gas. We argue as follows.
(i) The small-scale dynamo generated field does not fill the volume, but is concentrated into intermittent rope-like structures. The flux ropes are curved on the turbulent eddy scales. Their thickness is set by the diffusive scale determined by the effective ambipolar diffusion.
(ii) For a largely neutral galactic gas, the small-scale dynamo saturates, as a result of inefficient random stretching, when the peak field in a flux rope has grown to a few times the equipartition value.
(iii) The average energy density in the saturated small-scale field is subequipartition, since it does not fill the volume.
(iv) Such fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale into wave-like motion. The diffusive effects needed for the large-scale dynamo operation are then preserved until the large-scale field itself grows to near equipartition levels.     

Viscosity alpha in rotating spherical shear flows with an external magnetic field

We study the turbulent behaviour induced by the magnetic shear instability for a magnetized, incompressible fluid in a spherical shell. A differential rotation that is decreasing outwards but hydrodynamically stable according to the Rayleigh criterion is prescribed, and an external, uniform magnetic field is imposed parallel to the rotation axis. Our main concern in this paper is the fully global treatment of this magnetohydrodynamical system, so we focus particular attention on the influence of the boundary conditions. Non-linear, steady solutions are presented for stress-free as well as for rigid boundary conditions for one specific model with a fixed strength of the external magnetic field and a fixed differential rotation rate. We calculate the eddy viscosity ν_T and the viscosity alpha α_SS resulting from the total stress tensor. These turbulence parameters turn out to differ drastically depending on the boundary conditions for the flow. An investigation of the radial structure of the viscosity alpha (whilst varying the differential rotation law) shows that the enhanced generation of turbulence takes place mainly in the boundary layers of the shell.     

Flux expulsion by inhomogeneous turbulence

Flux expulsion is an important consequence of the interaction of magnetic fields with fluid convection and has been well studied for particular cases of steady, single-cell flows. Here we examine a related phenomenon in inhomogeneous turbulence using direct numerical simulations. To understand our numerical results, we analyse average properties of our model, and obtain mean transport coefficients which can be used to describe the approach of the system to its final state. For the kinematic problem these transport coefficients give an excellent prediction of the expulsion process; however, the enhanced transport is suppressed by dynamical back-reaction of the Lorentz force. Finally, we discuss the astrophysical implications for magnetic fields in stellar convection zones. Segregation of magnetic fields from turbulent motion not only allows strong toroidal fields to accumulate in regions of convective overshoot but also permits significant poloidal fields to be maintained by dynamo action in stars like the Sun.     

浙江省的地质灾害现状及防治对策

浙江省发育有崩塌、滑坡、泥石流、地面塌陷、水土流失、地面沉降等10余种地质灾害,人类工程经济活动是地质灾害发生的主要因素,减轻地质灾害危害的对策应以防为主。     

极射赤平投影中的数学方法

本文提供了严格依据赤平投影原理并利用空间解析几何知识推导出的公式计算方法，可以写出已知产状的线或面的赤平投影方程，也能计算两线所共的面，两面的交线，线或面之间的夹角以及它们的平分线或平分面，可以进行真倾斜和视倾斜的换算，倾斜和侧伏的换算；模拟线面绕轴的旋转以及恢复节理的原始产状；最后阐述用计算机进行节理统计和绘制极点等密图的原理。所用的向量表示方法直观表达了相关构造要素之间的空间关系，可用于计算机辅助教学，也能用于生产实践，精度可达0.01°。     

固定点源测深法定量解释研究

本文在简要介绍由B.A.柯马罗夫近年提出的“用固定点源测深资料作地电断面图”的定量解释方法(相对强度法、积分参数法和微分参数法)基础上,重点讨论了球体、板状体及组合板状体上用计算机对其理论计算、数值模拟和物理模拟视极化率测深曲线进行定量解释(确定埋藏体中心深度、产状、上界面位置及轮廓)的效果。并以一个野外试验结果说明了方法的有效性。     

河南及邻区中等地震前地震活动性异常

采用“八五”地震预报攻关研究成果－－地震算法复杂性Ｃ（ｎ）值及地震强度因子Ｍｆ值时间扫描，对河南及邻区１２次中等以上地震进行单项和综合分析研究。结果表明，对应率达８０％，显示出这些方法具有一定的预报意义。     

冀东油田钻井岩芯的磁学研究

论述了利用古地磁、磁组构综合研究进行钻井岩芯定向和确定沉积组构（沉积层面在现代地理坐标系下的）方向的方法．通过对冀东油田100块钻井岩芯实测结果的系统分析，指出该方法在岩芯磁组构方向确定和沉积剩磁成分能够分离出来的情况下，可用于地层产状未知和井孔近似直立情况下的钻井岩芯定向，并能给出沉积组构方向．对岩芯样品中的磁性矿物成分、成因进行了研究，分析了岩芯磁性特征与烃类富集的关系，指出磁参数分布特征，能为油气勘探开发、指示油气富集部位提供参考．     

1996年11月9日长江口外6.1级地震影响场与震中区地震地质概述

1996年11月9日长江口外6.1级地震震中位于北纬31°43’,东经123°04’震级(Ms)6.1级,震源深度7km,陆上最大地震烈度为4度。上海虽普遍有感,但无明显损失。在解译长江口外地球物理场异常的基础上,发现这次地震发生在东西向崇明—启东断裂与北东向镇海断裂的交汇部位,是北东向断裂右旋滑移的结果。     

粤东复杂地形上空的大气湍流强度及扩散参数

在具有复杂下垫面的粤东梅州市采用追踪平衡球的双经纬仪观测和美制ＧＩＬＬ三轴风速仪观测，分别测定了该地区拉格朗日系统和欧拉系统的湍流脉动量。然后用不同参照系的泰勒公式，分别计算出大气的水平和垂直的湍流强度及扩散参数，并与ＰＧ法之计算值和ＢＮＬ实验结果比较，发现距地面１００ｍ以上高空在各类稳定度层结条件下大气扩散参数均比ＰＧ值和ＢＮＬ值偏大。