Magnetic field decay in isolated neutron stars
Magnetic field decay in isolated neutron stars
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DOI：
10.1086/171646
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发表时间：
1992-08
期刊：
影响因子：
--
通讯作者：
P. Goldreich;A. Reisenegger
中科院分区：
文献类型：
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作者：
P. Goldreich;A. Reisenegger
我们研究三种机制，促进磁通量的损失从一个孤立的中子星星。欧姆衰减产生磁场相对于带电粒子的扩散。它以与电导率成反比且与磁场强度无关的速率进行。欧姆衰变发生在中子星星的流体核和固体壳中，但它太慢，无法直接影响恒星尺度的磁场。双极扩散涉及磁场和带电粒子相对于中子的漂移。如果质子形成正常流体，则漂移速度与磁场强度的二次幂成比例。双极扩散的变体包括磁通管的浮力上升和超流中子涡旋的拖曳。双极扩散在流体核的外部部分中操作，其中带电粒子组成是均匀的，质子和电子是唯一的物质。与双极扩散相关联的带电粒子通量分解成螺线管和无旋分量。这两个分量都受到摩擦阻力的作用。无旋分量扰动中子、质子和电子之间的化学平衡，从而产生有效抑制它的压力梯度。螺线管分量能够在短时间内将磁通量从外核输送到地壳。穿过内核的磁通量，在那里带电粒子的组成是不均匀的，将被永久地捕获，除非粒子相互作用可以迅速平滑偏离化学平衡。磁场经历与电场的霍尔分量相关的霍尔漂移。漂移速度与磁场强度成正比。霍尔漂移发生在整个中子星星上。与耗散的欧姆衰减和双极扩散不同，霍尔漂移保存磁能。因此，它本身不能对磁场衰减负责。然而，它可以提高欧姆耗散率。在固体地壳中，只有电子是移动的，并且霍尔角的正切很大。在那里，磁场的演变类似于不可压缩流体中的大雷诺数下的涡量。这使我们推测，磁场在小尺度下经历了一个由欧姆耗散终止的湍流级联。磁场的小尺度分量也通过霍尔漂移波从欧姆耗散慢的内地壳传输到欧姆耗散快的外地壳。磁通量通过地壳的扩散需要~5 x 10 ^8/B_（12）yr，其中B_（12）是以10 ^（12）G为单位测量的地壳磁场强度。
We investigate three mechanisms that promote the loss of magnetic flux from an isolated neutron star. Ohmic decay produces a diffusion of the magnetic field with respect to the charged particles. It proceeds at a rate that is inversely proportional to the electric conductivity and independent of the magnetic field strength. Ohmic decay occurs in both the fluid core and solid crust of a neutron star, but it is too slow to directly affect magnetic fields of stellar scale. Ambipolar diffusion involves a drift of the magnetic field and charged particles relative to the neutrons. The drift speed is proportional to the second power of the magnetic field strength if the protons form a normal fluid. Variants of ambipolar diffusion include both the buoyant rise and the dragging by superfluid neutron vortices of magnetic flux tubes. Ambipolar diffusion operates in the outer part of the fluid core where the charged particle composition is homogeneous, protons and electrons being the only species. The charged particle flux associated with ambipolar diffusion decomposes into a solenoidal and an irrotational component. Both components are opposed by frictional drag. The irrotational component perturbs the chemical equilibrium between neutrons, protons, and electrons, thus generating pressure gradients that effectively choke it. The solenoidal component is capable of transporting magnetic flux from the outer core to the crust on a short time scale. Magnetic flux that threads the inner core, where the charged particle composition is inhomogeneous, would be permanently trapped unless particle interactions could rapidly smooth departures from chemical equilibrium. Magnetic fields undergo a Hall drift related to the Hall component of the electric field. The drift speed is proportional to the magnetic field strength. Hall drift occurs throughout a neutron star. Unlike ohmic decay and ambipolar diffusion which are dissipative, Hall drift conserves magnetic energy. Thus, it cannot by itself be responsible for magnetic field decay. However, it can enhance the rate of ohmic dissipation. In the solid crust, only the electrons are mobile and the tangent of the Hall angle is large. There, the evolution of the magnetic field resembles that of vorticity in an incompressible fluid at large Reynolds number. This leads us to speculate that the magnetic field undergoes a turbulent cascade terminated by ohmic dissipation at small scales. The small-scale components of the magnetic field are also transported by Hall drift waves from the inner crust where ohmic dissipation is slow to the outer crust where it is rapid. The diffusion of magnetic flux through the crust takes ~ 5 x 10^8/B_(12) yr, where B_(12) is the crustal magnetic field strength measured in units of 10^(12) G.