Collaborative Research: GEM--The Excitation and Propagation of Fast Magnetosonic Waves and Their Effect on Radiation Belt Electrons

该项目将研究磁层内部磁声波的产生、传播和耗散，以及与之相互作用的辐射带电子的能量。在接近地球的太空环境中，等离子体是如此稀薄，以至于等离子体粒子(离子和电子)很少碰撞。然而，尽管如此，磁层中的一部分电子达到了危险的高能量，超过了100万电子伏特。这种能量高到足以对在磁层内部区域内运行的卫星构成风险，磁层内部区域是通信、导航、搜救、天气预报和国家安全卫星的首选地点。等离子体波主要负责将辐射带电子加速到这些高能，但它们也可以促进辐射带粒子向地球大气层的散射。在有磁场存在的无碰撞等离子体中产生许多类型的等离子体波，每种等离子体波都与等离子体中的带电粒子有自己的一组相互作用。这些波在内部磁层和辐射带的动力学中扮演着重要的角色。磁声波是拟议调查的重点，在磁层日侧部分附近的一大片磁赤道面附近发现了磁声波。沿着磁场线排列的波电场分量可以持续加速接近波速的电子，其方式与海浪推动冲浪者前进的方式大致相同。这被称为朗道衰减。磁声波被认为是由环流质子产生的，并在加速辐射带电子中消散，实际上在不同粒子群之间的无碰撞能量传输中起到了中介作用。作为更广泛的影响，该项目由一位职业生涯早期的科学家领导，从而有助于培训下一代科学家。这项研究的主要目标是使用线性理论和动力学粒子单元(PIC)模拟来探索由观测到的环状质子速度分布类型驱动的快速磁声波的激发和传播，并使用收集的见解来解释观测到的波测量，这两个数据集都来自双Van Allen探测器。此外，辐射带电子的散射将使用测试粒子计算来量化，从而提供关于是否以及在什么条件下可以应用传统的准线性方法的明确标准。将讨论两个基础科学问题：(1)快速磁声波的激发和传播如何产生观测到的波频谱的复杂模式？和(2)与拟线性理论中通常假定的冷等离子体色散相比，波的复杂动力学色散特性对辐射带电子散射的评估有多重要？推广完整的动力学PIC代码以解释非均匀介质中的偶极磁场，将是辐射带社区的又一宝贵资源，使以前不可能进行的一系列新研究成为可能。

This project will investigate the generation, propagation and dissipation of magnetosonic waves in the inner region of the magnetosphere as well as the energization of radiation belt electrons interacting with them. In the space environment near Earth, the plasma is so tenuous that plasma particles (ions and electrons) rarely collide. However despite this, some portion of the electrons in the magnetosphere reach dangerously high energies exceeding a million electron volts. This energy is high enough to pose a risk to satellites orbiting within the inner regions of the magnetosphere, a preferred location for communications, navigation, search-and-rescue, weather prediction and national security satellites. Plasma waves are mainly responsible for the acceleration of radiation belt electrons to these high energies but they can also contribute to the scattering of radiation belt particles into the Earth's atmosphere. Many types of plasma waves are generated in a collisionless plasma in the presence of a magnetic field and each has its own set of interactions with the charged particles in the plasma. These waves play a significant role in the dynamics of the inner magnetosphere and radiation belts. Magnetosonic waves, the focus of the proposed investigation, are found in the vicinity of the magnetic equatorial plane in a wide region near the dayside portion of the magnetosphere. The component of the wave electric field, which is aligned along a magnetic field line, can continuously accelerate electrons traveling near the speed of the wave in much the same way that a surfer is pushed forward by an ocean wave. This is termed Landau damping. Magnetosonic waves are thought to be generated by ring current protons, and dissipated in accelerating radiation belt electrons, in effect acting as intermediaries in the collisionless transport of energy between different particle populations. As a broader impact, this project is led by an early-career scientist thus contributing to the training of the next generation of scientists. The primary goal of this research is to use linear theory and kinetic particle-in-cell (PIC) simulations to explore the excitation and propagation of fast magnetosonic waves driven by observed types of ring-like proton velocity distributions and to use insights gathered to interpret the observed wave measurements, both datasets from the twin Van Allen Probes. Furthermore, scattering of radiation belt electrons will be quantified using test-particle computations, providing clear criteria as to whether and under what conditions the conventional quasi-linear approach can be applied. Two fundamental science questions will be addressed: (1) how does the excitation and propagation of fast magnetosonic waves produce the complex pattern of the observed wave frequency spectra? and (2) how important to the evaluation of radiation belt electron scattering are the complex kinetic dispersion properties of the waves compared to the commonly assumed cold plasma dispersion used in quasi-linear theory? The generalization of the full kinetic PIC code to account for the dipole magnetic field in an inhomogeneous medium will be an additional valuable resource for the radiation belt community enabling a whole spectrum of new studies not previously possible.