The anisotropic diffusion equation is of crucial importance in understanding cosmic ray (CR) diffusion across the Galaxy and its interplay with the Galactic magnetic field. This diffusion term contributes to the highly stiff nature of the CR transport equation. In order to conduct numerical simulations of time-dependent cosmic ray transport, implicit integrators have been traditionally favoured over the CFL-bound explicit integrators in order to be able to take large step sizes. We propose exponential methods that directly compute the exponential of the matrix to solve the linear anisotropic diffusion equation. These methods allow us to take even larger step sizes; in certain cases, we are able to choose a step size as large as the simulation time, i.e., only one time step. This can substantially speed-up the simulations whilst generating highly accurate solutions (l2 error $\leq 10^{-10}$). Additionally, we test an approach based on extracting a constant coefficient from the anisotropic diffusion equation where the constant coefficient term is solved implicitly or exponentially and the remainder is treated using some explicit method. We find that this approach, for linear problems, is unable to improve on the exponential-based methods that directly evaluate the matrix exponential.
翻译:在理解银河系统的宇宙射线扩散及其与银河磁场的相互作用方面,天体扩散方程式至关重要。这个扩散术语有助于CR传输方程式的高度僵硬性。为了对时间依赖宇宙射线传输进行数字模拟,隐含的集成器传统上优于CFL的显性集成器,以便能够采取大步尺寸。我们建议了直接计算矩阵指数的指数的方法,以解决线性反向扩散方程式的线性扩散方程式。这些方法允许我们采取更大的步骤尺寸;在某些情况下,我们可以选择一个与模拟时间一样大的步数,也就是说,只有一步。这可以大大加速模拟,同时产生非常精确的解决方案(l2误差 $\leq 10 ⁇ - 10美元)。此外,我们测试了一种方法,即从异体扩散方方方程式中提取一个恒定的系数,即恒定的系数术语是隐含或指数的,其余的则使用某种明确的方法处理。我们发现,对于线性矩阵问题来说,这一方法无法直接改进指数式矩阵方法。