Stochastic Parker Spirals in the Solar Wind
Stochastic Parker Spirals in the Solar Wind
复制标题
太阳风中的随机帕克螺旋
DOI：
10.3847/1538-4357/abd39a
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发表时间：
2021
期刊：
影响因子：
--
通讯作者：
G. Li
中科院分区：
文献类型：
--
作者：
N. Bian;G. Li
本文提出了太阳风和太阳源表面湍流引起的磁力线角色散的解析模型。模型中的日球磁力线是由一对正则共轭变量--日面余纬余弦μ和经度余弦ω--的哈密顿量导出的。在扩散近似中，帕克螺线由一组随机微分方程来模拟，θ和θ是r的函数。这些随机的帕克螺旋线是在半径增加的球体上的标准随机行走的实现，叠加在由于太阳旋转引起的角漂移上。用球谐函数的形式给出了描述场线密度角扩散的Fokker-Planck方程的绿色解。从观测者追踪到太阳的磁力线是布朗桥的实现。我们的模型结合了随机的足迹运动的影响，在源表面，这是与零频率分量的太阳风湍流。假设足点运动是扩散的，其贡献的角扩散率的随机帕克螺旋，然后给出的角扩散率的足点除以太阳风的速度，并由一个独特的参数，这是久保数控制。
An analytic model for the angular dispersion of magnetic field lines resulting from the turbulence in the solar wind and at the solar source surface is presented. The heliospheric magnetic field lines in our model are derived from a Hamiltonian with the pair of canonically conjugated variables the cosine of the heliographic colatitude μ and the longitude ϕ. In the diffusion approximation, the Parker spirals are modeled by a set of stochastic differential equations for θ and ϕ as functions of r. These stochastic Parker spirals are realizations of a standard random walk on a sphere of increasing radius, superimposed on an angular drift due to solar rotation. The Green function solution of the Fokker–Planck equation describing the angular diffusion of the field line density is obtained in terms of spherical harmonics. Magnetic field lines traced from an observer back to the Sun are realizations of a Brownian bridge. Our model incorporates the effect of the random footpoint motions at the source surface, which is associated with the zero-frequency component of the solar wind turbulence. Assuming that the footpoint motion is diffusive, its contribution to the angular diffusivity of the stochastic Parker spirals is then given by the angular diffusivity of the footpoints divided by the solar wind speed and is controlled by a unique parameter, which is the Kubo number.