The Relation between Escape and Scattering Times of Energetic Particles in a Turbulent Magnetized Plasma: Application to Solar Flares
The Relation between Escape and Scattering Times of Energetic Particles in a Turbulent Magnetized Plasma: Application to Solar Flares
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DOI：
10.3847/2041-8213/aaedb3
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发表时间：
2018-08
期刊：
影响因子：
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通讯作者：
F. Effenberger;V. Petrosian
中科院分区：
文献类型：
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作者：
F. Effenberger;V. Petrosian
从许多空间和天体物理源的加速区获得粒子逃逸时间的知识对于分析这些粒子产生的发射特征以及确定起作用的加速和传输机制至关重要。这封信解决了这个普遍的问题，特别是在太阳耀斑中，除了湍流的散射外，从加速区向其边界会聚的磁场也影响粒子逃逸。我们在Malyshkin和Kulsrud工作的基础上，用粒子输运的数值模型检验了逃逸时间和散射时间之间的(近似)解析关系，以及场收敛速度，该关系对注入粒子的强扩散极限和弱扩散极限以及各向同性俯仰角分布都适用。为此，在假定不同的初始俯仰角分布的情况下，用随机微分方程组格式求解了粒子的Fokker-Planck动力学输运模型。这种方法能够进一步洞察运输过程的相空间动态，否则将无法访问。我们发现，在各向同性情况下，数值结果与解析方程总体上符合得很好；但是，在非同位素情况下，特别是在沿磁场线传播的情况下，弱扩散区有明显的差异。这些结果对于解释太阳耀斑和其他类似的空间和天体物理加速站点中的高能粒子的观测结果以及确定福克-普朗克型动力学方程中常用的加速度-输运系数具有重要意义。
A knowledge of the particle escape time from the acceleration regions of many space and astrophysical sources is of critical importance in the analysis of emission signatures produced by these particles and in the determination of the acceleration and transport mechanisms at work. This Letter addresses this general problem, in particular in solar flares, where in addition to scattering by turbulence, the magnetic field convergence from the acceleration region toward its boundaries also influences the particle escape. We test an (approximate) analytic relation between escape and scattering times, and the field convergence rate, based on the work of Malyshkin & Kulsrud, valid for both strong and weak diffusion limits and isotropic pitch-angle distributions of the injected particles, with a numerical model of particle transport. To this end, a kinetic Fokker–Planck transport model of particles is solved with a stochastic differential equation scheme, assuming different initial pitch-angle distributions. This approach enables further insights into the phase-space dynamics of the transport process, which would otherwise not be accessible. We find that in general the numerical results agree well with the analytic equation for the isotropic case; however, there are significant differences in the weak diffusion regime for non-isotopic cases, especially for distributions beamed along the magnetic field lines. The results are important in the interpretation of observations of energetic particles in solar flares and other similar space and astrophysical acceleration sites, and for the determination of acceleration-transport coefficients, commonly used in Fokker–Planck–type kinetic equations.