Theory of Kinetic Alfven Waves and Auroral Particle Acceleration

运动阿尔文波和极光粒子加速理论

• 批准号: 1558134
• 负责人: Robert Lysak
• 金额: $ 52.02万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-05-15 至 2019-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1558134&HistoricalAwards=false
• 关键词: Theory; Kinetic; Alfven; Waves; Auroral

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.

这项建议的重点是开发一个全新的极光弧形成范例，如果正确的话，将是极光物理学的突破性进展。 出现在极地地区的动态极光是从近地空间肆虐的扰动流入大气层的能量的视觉指标。 为极光发射提供动力的电子和离子从这些扰动沿着磁场线行进，并且必须在沿途进一步加速。几十年来，大部分加速被认为是电子和离子在沿着磁场线移动时穿过准静态电位降的结果。 当等离子体中形成两个平行的带相反电荷的层时，会产生这些电位降，从而建立电场和相关的电压降。 极光电子通过电压降被加速，产生一个单一的能量峰。近年来，动力学阿尔芬波对电子的随时间变化的加速的重要性已变得清楚。当阿尔芬波（沿着磁场传播，传递磁场中其他地方变化的影响）产生平行于背景磁场的电场时，在某些情况下形成动力学阿尔芬波。 与先前的机制相反，平行波电场在宽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，这两个过程都受到电离层的耦合和反馈的影响。 PI开发了一个引人注目的新理论，即这两个过程是相关的（即，动力学Alfven波过渡到形成平行电位降的准稳态）。 这里的重点是测试这个理论。 这项工作的更广泛影响包括培训一名研究生和通过公共宣传传播成果。这项研究预计将引起其他等离子体物理学界的广泛兴趣。从长远来看，它将有助于更好地了解空间环境，从而改进预测，并制定有效的减缓战略，以保护对社会有价值的脆弱技术。 该方法结合了理论和建模的目标，包括一个大范围的时间和空间尺度的过渡所涉及的时间依赖的准稳定的条件下，一个大的参数范围内，反馈通过电离层反射的阿尔芬波和电离层电导率的修改，由于沉淀电子，以及密度腔沿着极光场线的形成和影响。 主要的工具是一个已经开发的线性化双流体模型，这将被修改，包括：非线性电子动力学沿着磁场线和一个现实的偶极磁场几何形状。 该模型结合动力学电子模型将用于研究电子的加速及其与动力学Alfven波的自洽相互作用。