Magnetic bubbles and magnetic towers – I. General properties and simple analytical models
Magnetic bubbles and magnetic towers – I. General properties and simple analytical models
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磁气泡和磁塔 – I. 一般性质和简单分析模型
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发表时间：
2012
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通讯作者：
T. Amari
中科院分区：
文献类型：
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作者：
J. Aly;T. Amari
我们考虑包含磁化等离子体的有界区域D完全受无磁场外部介质限制的静磁平衡--磁泡平衡(MBEqs)--或者同时受这种介质和密集等离子体区域中的线束约束--磁塔平衡(MTEqs)。我们首先建立它们的一些一般性质。特别地，我们导出了一系列有用的积分方程，它们分别与磁场和D内外的热压有关。我们用它们证明了具有纯极向场的轴对称MBEq的不存在，并讨论了Braithwaite从初始非平衡态松弛形成MBEq的一些最新结果。接下来，我们提出了两族精确的轴对称MBEqs，它们分别具有球形和环形形状。第一族是从Prendergast的自引力磁化体模型中提取出来的，第二族是利用Palumbo的等动力平衡理论构造的，对于这个平衡，磁压和热压在任何磁通面上都是恒定的。因此，具有多种结构的MTEqs以一种简单的方式得到：我们从任意的MBEq开始，只考虑它在给定平面上切割气泡D的部分。对于任一族的MBEqs和MTEqs，我们都以封闭的形式计算大多数有趣的物理量(如能量、磁螺旋度和扭度)。我们的结果可望用于建立几个天体的简单模型(如星系团内介质中的X射线空穴、星盘吸积致密天体的喷流、星冕中的喷发事件及其喷发)。
We consider magnetostatic equilibria in which a bounded region D containing a magnetized plasma is either fully confined by a field-free external medium – magnetic bubble equilibria (MBEqs) – or is confined by both such a medium and line-tying in a dense plasma region – magnetic tower equilibria (MTEqs). We first establish some of their general properties. In particular, we derive a series of useful integral equalities relating the magnetic field and the thermal pressures inside and outside D, respectively. We use them to prove the non-existence of an axisymmetric MBEq with a purely poloidal field, and to discuss some recent results of Braithwaite on MBEq formation by relaxation from an initial non-equilibrium state. We next present two families of exact analytical axisymmetric MBEqs with, respectively, spherical and toroidal shapes. The first family is extracted from Prendergast’s model of a self-gravitating magnetized body, while the second one is constructed by using Palumbo’s theory of isodynamic equilibria, for which both magnetic and thermal pressures take constant values on any flux surface. MTEqs with a large variety of structures are thus obtained in a simple way: we start from an arbitrary MBEq and just consider the part of it above a given plane cutting the bubble D. For MBEqs and MTEqs in either family, we compute in closed form most of the interesting physical quantities (such as energy, magnetic helicity and twist). Our results are expected to be useful for building up simple models of several astrophysical objects (such as X-ray cavities in the intracluster medium, jets emitted by disc accreting compact objects, eruptive events in stellar coronae and their ejecta).