Theory of Kinetic Alfven Waves and Auroral Particle Acceleration

这一提议的重点是开发一种全新的极光弧形成模式，如果正确，将是极光物理学的突破性进展。出现在极地区域的动态极光是从近地空间肆虐的扰动流入大气的能量的视觉指标。为极光发射提供动力的电子和离子，在这些扰动下沿着磁力线行进，必须在此过程中进一步加速。几十年来，大部分加速被认为是电子和离子在沿磁力线移动的同时穿过准静态势降的结果。当两个平行的相反电荷层在等离子体中形成电场和相关的电压降时，就会产生这些电势降。极光电子通过电压降加速，产生单一的能量峰值。最近，动能Alfven波加速电子随时间变化的意义变得清晰起来。动力学阿尔芬波是在某些情况下形成的，当阿尔芬波(沿着磁场传播，传达磁场中其他地方的变化的影响)产生一个平行于背景磁场的电场时。与以前的机制不同，平行波电场在很大的能量范围内加速电子。然而，这两个过程都没有得到很好的理解。更复杂的是，这两个过程都受到与电离层的耦合和反馈的影响。PI发展了一个令人信服的新理论，即这两个过程是相关的(即，动态Alfven波转变为准稳态，形成平行的势降)。这里的重点是检验这一理论。这项工作的更广泛影响包括对研究生的培训和通过公众宣传交流成果。预计这项研究将引起其他等离子体物理界的广泛兴趣。从长远来看，这将有助于更好地了解空间环境，从而改进预测，并制定有效的缓解战略，以保护对社会有价值的脆弱技术。该方法结合了理论和模拟，其目标是包括从时间依赖条件向准稳定条件转变所涉及的大范围时间和空间尺度，大参数范围，通过阿尔芬波的电离层反射反馈，通过因电子沉淀而改变电离层电导率，以及通过沿极光场线密度空穴的形成和影响。主要工具是已经开发的线性化双流体模型，该模型将被修改为包括：沿磁场线的非线性电子动力学和真实的偶极磁场几何。这个模型与动能电子模型相结合，将被用来研究电子的加速及其与动能Alfven波的自洽相互作用。

This proposal is focused on developing an entirely new paradigm for auroral arc formation that, if correct, will be groundbreaking for auroral physics. The dynamic auroras that appear in the polar regions are visual indicators of energy flowing into the atmosphere from disturbances raging in near-Earth space. Electrons and ions, that power the auroral emissions, travel down magnetic field lines from these disturbances and must be further accelerated along the way. For several decades, the majority of the acceleration was thought to be the result of electrons and ions traversing quasi-static potential drops while moving down magnetic field lines. These potential drops are created when two parallel oppositely-charged layers form in the plasma setting up an electric field and an associated voltage drop. The auroral electrons are accelerated through the voltage drop creating a single energy peak. Recently the significance of time dependent acceleration of electrons by kinetic Alfven waves has become clear. Kinetic Alfven waves are formed under certain circumstances when Alfven waves (traveling along the magnetic field communicating the effects of changes elsewhere in the field) develop an electric field parallel to the background magnetic field. In contrast to the previous mechanism, the parallel wave electric fields accelerate electrons over a broad energy range. However, neither of these processes is well understood. To complicate things, both processes are influenced by coupling and feedbacks with the ionosphere. The PI has developed a compelling new theory that the two processes are related (i.e., that kinetic Alfven waves transition to a quasi-steady state forming parallel potential drops). The focus here is on testing this theory. Broader impacts of the work include the training of a graduate student and communication of results through public outreach. The research is expected to be of broad interest to other plasma physics communities. In the longer-term, it will contribute to better understanding of the space environment, resulting in improved forecasts, and the development of efficient mitigation strategies to protect vulnerable technologies of value to society. The approach combines theory and modeling with the goal of including a large range of temporal and spatial scales involved in the transition from time-dependent to quasi-steady conditions, a large parameter range, feedbacks through ionospheric reflection of the Alfven waves and by modifications of the ionospheric conductivity due to the precipitating electrons, as well as by the formation and effects of density cavities along the auroral field lines. The primary tool is an already developed linearized two-fluid model, which will be modified to include: nonlinear electron dynamics along the magnetic field line and a realistic dipole magnetic field geometry. This model combined with a kinetic electron model will be used to investigate the acceleration of electrons and their self-consistent interactions with the kinetic Alfven waves.