CAREER: A Laboratory Test of Radiation Belt Electron Acceleration and Diffusion by Whistler Chorus

在地球大气层之外是一个等离子体比固体、液体和气体丰富得多的宇宙。在太空时代的黎明，探索者1号意外地发现了空间等离子体中的离子和电子以接近光速的速度运动。这些高速粒子是在地球周围被称为范艾伦辐射带的两个甜甜圈形状的区域中发现的。了解辐射带的变化仍然具有紧迫性，因为需要可操作的空间天气预报来保护空间和地面上的关键基础设施。对外带等离子体的测量通常会探测到被称为哨声模波的强烈电磁波，以及高速电子数量的急剧变化。电子有时会在不到一秒的时间内迅速加速，有时会在数小时内加速。模型表明，哨子波可以产生这些不同的结果。口哨模式波的振幅和波峰与地球磁场之间的角度被预测为决定与电子相互作用的性质。这项拨款的科学目标包括在与外带有关的条件下进行一系列实验室实验，以研究哨声模式波和电子之间的相互作用。工作将包括开发一种新的天线阵列，以产生哨声模式波，同时控制波峰与磁场之间的角度。教育目标侧重于增加对空间等离子体物理研究的参与，通过早期本科途径进行研究，使学生能够从更多样化的群体中招募。研究学生将获得介绍性研讨会的支持，研讨会材料将免费分发，以帮助培养整个空间等离子体物理社区的早期本科生研究。由于频率低于电子回旋频率，共振速度通常与特征电子速度相当，哨声模式波与电子具有强大的波粒相互作用，这是描述辐射带动力学的核心。理论和仿真表明，哨声模波的振幅和波法向角是决定与电子相互作用特性的重要变量。早期实验室工作的一个限制是缺乏高分辨率和高精度的电子速度分布函数（evdf）测量，需要诊断哨声模式波电子相互作用。这项工作将把实验室实验与最近的诊断进展结合起来，对与地球辐射带有关的哨子模式波电子相互作用进行明确的测量。实验将使用汤姆逊散射evdf测量和哨声模式波天线阵列来定义发射波矢量。实验室测试将回答三个问题：条件如何决定哨声模式波是产生快速的电子加速还是较慢的扩散；非线性波粒相互作用的起始点随波法向角的变化规律以及导致扩散的拟线性相互作用如何随波法向角的变化而变化。这些问题的答案需要有效地模拟关键辐射带过程，包括捕获粒子的寿命，在数值模型中普遍存在的扩散系数，以及导致脉动极光和相关相对论性电子微爆发的快速变化。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Beyond Earth's atmosphere is a universe where plasma is far more abundant than solid, liquid, and gas. At the dawn of the space age, Explorer 1 unexpectedly discovered ions and electrons in space plasma traveling at nearly the speed of light. These high-velocity particles are found in two donut-shaped regions around Earth called the Van Allen Radiation Belts. Understanding variations of the radiation belts have continued urgency as actionable space weather predictions are needed to protect critical infrastructure in space and on the ground. Measurements in the plasma of the outer belt commonly detect intense electromagnetic waves called whistler-mode waves, along with dramatic changes in the number of high-velocity electrons. Electrons are sometimes accelerated rapidly, in less than a second, and sometimes over hours. Models suggest that whistler-mode waves can produce these varied outcomes. The amplitude of whistler-mode waves and the angle between wave crests and Earth's magnetic field are predicted to determine the nature of interactions with electrons. The scientific objectives of this grant include a series of laboratory experiments in conditions relevant to the outer belt to study the interaction between whistler-mode waves and electrons. Work will consist of developing a new antenna array to generate whistler-mode waves while controlling the angle between wave crests and the magnetic field. Educational objectives focus on increasing participation in space plasma physics research with an early-undergraduate pathway to research that will allow students to be recruited from more diverse groups. Research students will be supported by an introductory seminar, and seminar materials will be distributed freely to help cultivate early undergraduate research across the space plasma physics community.With frequencies below the electron cyclotron frequency and resonant velocities often comparable to characteristic electron velocities, whistler-mode waves have potent wave-particle interactions with electrons that are central to descriptions of radiation belt dynamics. Theory and simulation indicate the amplitude of whistler-mode waves and the wave-normal angle are important variables in determining the character of interactions with electrons. A limitation of earlier lab work was the absence of high-resolution and high-precision measurements of electron velocity distribution functions (evdf's) needed to diagnose whistler-mode wave-electron interactions. This work will combine laboratory experiments with recent diagnostic advances to make definitive measurements of whistler-mode wave-electron interactions relevant to Earth's radiation belts. Experiments will use Thomson scattering evdf measurements and a whistler-mode wave antenna array to define the launched wave vector. Laboratory tests will answer three questions: how conditions determine whether whistler-mode waves produce rapid electron acceleration or slower diffusion; how the onset of nonlinear wave-particle interactions varies with wave-normal angle; and how quasilinear interactions responsible for diffusion vary with wave-normal angle. Answers to these questions are required to effectively model critical radiation belt processes, including the lifetime of trapped particles, the diffusion coefficients prevalent in numerical models, and the rapid variations leading to pulsating auroras and associated microbursts of relativistic electrons.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.