Theory of Kinetic Alfven Waves on Auroral Field Lines

这个项目将研究地球磁层中沿极光场线的动力学Alfven波和波包的理论。这种波在将能量从外层磁层输送到较低海拔的过程中发挥了重要作用，在那里，波加速了导致极光的沉淀电子和离子。阿尔芬波在产生充斥在外层磁层的流出等离子体方面也起到了重要作用。虽然磁流体力学(MHD)理论足以描述各种空间等离子体现象，但主导极光场线下部物理的等离子体过程本质上是动力学的，因此MHD理论不能很好地描述这一过程。此外，极光加速区是一个强烈动态和不均匀的区域，这意味着标准的局域等离子体动力学理论对这些现象的描述能力也是有限的。特别是，极光加速区与碰撞电离层耦合，实际上可以由于带电粒子的沉淀而改变电离层的电导率。因此，有必要详细研究加速极光粒子的动力学过程以及该区域与电离层的电动力学耦合，以了解为极光提供动力的基本等离子体过程。特别是，该项目将研究电离层阿尔芬谐振器中的阿尔芬相互作用。这是阿尔芬速度沿极光场线大幅增加，在极光电离层上方形成的谐振腔。研究将侧重于影响磁层和电离层耦合的三个过程：(1)导致极光电子加速的动力学Alfven波的非定域理论，(2)Alfven波和捕获在电离层Alfven谐振器中的波包之间的非线性相互作用，该相互作用可导致极光电流系统的结构和通过向外磁层喷射等离子体而形成密度空洞，以及(3)由于电子降水改变电离层电导率而导致的电离层反馈相互作用。这些过程将在现有数值代码的基础上，通过适当的理论和数值模型进行研究。非线性模拟将使研究极光场线上的密度和电流结构成为可能。阿尔芬波的传播及其与电离层的相互作用的三维模型将被用来非常详细地研究反馈相互作用。这项工作也将服务于培养研究生开发和利用数值等离子体模型的方法的教育目的。它将有助于了解地球空间系统，这将有助于社会了解可能影响通信和电力系统的空间天气过程。

This project will investigate the theory of kinetic Alfven waves and wave packets along auroral field lines in the Earth's magnetosphere. Such waves play a major role in carrying energy from the outer magnetosphere to lower altitudes where the waves accelerate the precipitating electrons and ions that cause the aurora. Alfven waves also play an important role in generating the out-flowing plasma that populates the outer magnetosphere. While the theory of magnetohydrodynamics (MHD) is adequate to describe a wide variety of space plasma phenomena, the plasma processes that dominate the physics in the lower regions of auroral field lines are inherently kinetic and so are not well described by MHD theory. In addition, the auroral acceleration region is a strongly dynamic and inhomogeneous region, implying that standard local plasma kinetic theory is also limited in its ability to describe these phenomena. In particular, the auroral acceleration region is coupled to the collisional ionosphere, and in fact can change the conductivity of the ionosphere due to the precipitation of charged particles. Thus, a detailed study of the kinetic processes that accelerate auroral particles and the electrodynamic coupling of this region to the ionosphere is necessary to understand the basic plasma processes that power the aurora. In particular, the project will investigate Alfvenic interactions in the ionospheric Alfven resonator. This is the resonant cavity formed in the topside auroral ionosphere by the large increase of the Alfven speed along auroral field lines. The research will focus on three processes that affect the coupling of the magnetosphere and ionosphere: (1) Non-local theory of kinetic Alfven waves that lead to the acceleration of auroral electrons, (2) nonlinear interactions between Alfven waves and wave packets trapped in the ionospheric Alfven resonator that can lead to structuring of the auroral current system and the formation of density cavities by the ejection of plasma into the outer magnetosphere, and (3) Ionospheric feedback interactions due to modifications of the ionospheric conductivity by electron precipitation. These processes will be studied by appropriate theoretical and numerical models based on existing numerical codes. Nonlinear modeling will enable the investigation of density and current structuring on auroral field lines. A three-dimensional model of the propagation of Alfven waves and their interaction with the ionosphere will be used to study the feedback interaction in great detail. This work will also serve the educational purpose of training graduate students in methods of developing and utilizing numerical plasma models. It will contribute to the understanding of the geospace system, which will assist society in its understanding of space weather processes that can impact communication and power systems.