Kinetic and Multiscale Simulations of Particle Acceleration in Astrophysical Shocks

该项目将研究粒子如何在天体物理源中加速到高速，重点是一种称为费米加速的特殊机制。 虽然人们相信，在恒星、超新星和其他强源周围流动的物质中，有相当一部分能量可以转化为非常快的粒子，这些粒子可以通过它们发出的辐射模式来识别，但其机制还没有得到很好的理解。 在这项研究之后，费米机制将得到更好的表征，并可能受到严格的约束。粒子在无碰撞冲击中的加速是大多数宇宙非热现象模型的核心。 对同步辐射的观测表明，脉冲星风星云、活动星系核的喷流、伽马射线爆发和超新星遗迹中的这种冲击，可以将相当大一部分能量流转化为具有幂律谱的相对论粒子。 然而，尽管它通常被援引为原因，但一阶费米机制的操作条件及其效率并没有从第一原理中得到理解。 本项目将通过对天体物理学无碰撞冲击进行大规模多维粒子模拟和混合模拟，构建冲击加速的自洽理论，并研究校准非线性扩散冲击加速理论所需的等离子体不稳定性、粒子散射和磁场产生的微观物理学。 具体问题包括：1）激波加速存在的判据及其效率; 2）加速粒子如何影响激波结构和演化; 3）实际激波中产生的质子和电子谱。 在这项工作中开发的工具将使冲击的多尺度建模，并将在非线性制度从第一原理推导出粒子谱。 这项研究将为无碰撞激波中的费米加速是高能宇宙射线和非热粒子的起源这一假设提供预测能力，并可能对磁化模型和天体物理流出物的组成施加严格的约束。由于费米加速是天体物理学中的一个基本过程，因此其结果将对观测者、实验者、以及研究高能天体物理学和宇宙学来源的理论家，甚至适用于太阳系中的冲击。 这些发现将指导新一代的实验室实验。 这项工作将涉及博士后、研究生和本科生，对他们进行数值建模和可视化培训，从而为他们在科学和技术领域的职业生涯做好准备，在这些领域，大规模计算发挥着越来越重要的作用。 将开发一个交互式等离子体物理教程，以便为专家和公众带来对这些棘手主题的直观理解。

This project will study how particles get accelerated to high speeds in astrophysical sources, focusing on a special mechanism known as Fermi acceleration. Although it is believed that a considerable fraction of the energy in material flowing around pulsars, supernovae, and other strong sources, can be converted into very fast particles, which are identified by the patterns of radiation they give off, the mechanism is not well understood. After this study, the Fermi mechanism will be much better characterized, and may be tightly constrained.Acceleration of particles in collisionless shocks is at the heart of most models of non-thermal phenomena in the Universe. Observations of synchrotron emission suggest that such shocks in pulsar wind nebulae, in jets from active galactic nuclei, in gamma-ray bursts, and in supernova remnants, can convert a significant fraction of the flow energy into relativistic particles with power-law spectra. However, although it is usually invoked as the cause, the conditions for operation of the first-order Fermi mechanism and its efficiency are not understood from first principles. This project will construct a self-consistent theory of shock acceleration by performing large-scale multidimensional particle-in-cell and hybrid simulations of astrophysical collisionless shocks, and study the microphysics of plasma instabilities, particle scattering, and magnetic field generation, necessary to calibrate nonlinear diffusive shock acceleration theory. Specific questions include: 1) the criteria for existence of shock acceleration, and its efficiency; 2) how accelerated particles influence shock structure and evolution; 3) the proton and electron spectra generated in realistic shocks. The tools developed during this work will enable multi-scale modeling of shocks and will derive particle spectra in the nonlinear regime from first principles. This research will provide predictive power to the hypothesis that Fermi acceleration in collisionless shocks is the origin of high-energy cosmic rays and non-thermal particles, and could place tight constraints on the models of magnetization and on the composition of astrophysical outflows.Because Fermi acceleration is a fundamental process in astrophysics, the results will be of value to observers, experimentalists, and theorists studying high-energy astrophysical and cosmological sources, and are even applicable to shocks in the solar system. The findings will guide a new generation of laboratory experiments. The work will involve postdocs, and graduate and undergraduate students, training them in numerical modeling and visualization, and thus preparing them for careers in science and technology fields, where large-scale computing increasingly plays an important role. An interactive plasma physics tutorial will be developed to bring intuitive understanding of these thorny subjects to specialists and the general public alike.