磁层ULF波影响辐射带电子获能过程的模拟研究

Ultra Low Frequency (ULF) waves are a powerful and ubiquitous agent within the magnetosphere, which has been causally linked to acceleration, transport and loss of energetic electrons from the radiation belts by a number of observation and modeling studies. The basis of this project is that a crucial factor affecting the energization of radiation belt electrons by ULF waves is the accessibility of ULF wave power to zones within the magnetosphere where power coupling between the waves and electrons is most efficient. For anti-sunward propagating waves launched along the dayside magnetopause, such a zone is the afternoon/evening sector, where the adiabatic orbital drift motion of electrons is also anti-sunward, allowing the possibility of drift-resonance. The accessibility of ULF waves to this region is determined by the local Alfvén speed (which is a function of the magnetic field strength and plasma mass density), and is highly inhomogeneous, causing strong wave refraction and reflection, particularly at the plasmapause. The plasma density and plasmapause structure in the afternoon sector of the magnetosphere is particularly dynamic during periods of magnetospheric activity and enhanced convection, as the plasmaspheric drainage plume produced in these circumstances passes through this region. This has a strong effect on the accessibility of ULF wave power to this region, and thereby on the potential for energization of radiation belt electrons by drift resonance. While global MHD simulations and ULF wave models have been employed to investigate ULF wave activity and electron energization by drift resonance, the plasma density models in these simulations have been highly idealized, with greater emphasis paid to inhomogeneity in the magnetic field. To date the effect of the plasmaspheric drainage plume in the context of ULF waves and radiation belt energization has remained unexplored. The purpose of this proposal is to fill this knowledge gap. This will be done by a systematic study using a 2D (equatorial) and 3D MHD wave models for ULF waves in the magnetosphere, and a model for the plasmasphere that includes drainage plume dynamics. The effect of the waves on radiation belt dynamics will be calculated using a test-kinetic model for equatorially mirroring and bounce-averaged electron trajectories. Validation of the model results will be sought using ground-based and satellite borne observations.

磁层超低频波(ULF波)携带能量，对辐射带电子的加速、输运和损失有重要影响。本项目针对ULF波是否可以进入磁层及其有效加速辐射带电子的区域展开模拟研究。磁层昏侧区域，电子背日漂移，是发生漂移共振的理想场所。ULF波能否进入该区域，取决于当地阿尔芬速度的大小。在等离子体层顶附近，密度梯度很大，波往往被折射和反射。在地磁活跃期，昏侧等离子体层的密度、结构高度变化，形成向日延伸的羽状结构（plume），使得ULF波进入该区域并有效加速电子成为可能。本项目将使用2D(赤道面) 模型进而发展为3D模型进行模拟，考虑包含等离子体层羽状结构的等离子层模型，系统研究羽状结构对ULF波进入并加速辐射带电子的影响，同时辅助以观测检验。通过本项目的研究，我们有望对磁层ULF波的分布以及辐射点电子来源等问题形成崭新的认识。

该项目旨在解决以下科学问题：近地空间内冷等离子体密度结构的演变如何影响整个磁层内超低频 (ULF) 波的分布，以及这种波动是如何与辐射带电子相互作用？通过相关的数值模拟与数据观测进行研究，最终这些工作在美国地球物理期刊上发表。 Degeling 等人（2018 年）使用最新开发的三维 MHD 波动模型进行的数值模拟，以研究不断演化的等离子体密度羽状结构是如何影响外部驱动的 ULF 波传播的。结果表明，由于等离子体层羽状结构在局部的共振结构激发，在磁层昏侧，波动能量进入内磁层的情况变得依赖于频率。这些特征模式随着羽状结构在空间尺度上变窄而演化，进而影响粒子的能量范围——波可以与之共振相互作用（手稿正在准备中）。 Zang 等人 (2019) 中给出了的观测示例显示了在晨侧扇区中，剩余狭窄的等离子体羽状结构内发生局部激发的极向模式 ULF 波，并详细说明了与高能粒子局部共振相互作用的观察结果。 Degeling 等人 (2019) 从理论上探讨了场线共振 (FLR) 结构中的漂移共振对粒子的激励，并且表明除此之外，强峰值 FLR 还可以产生除由标准理论预测新的漂移共振。这些可以增强 ULF 波与辐射带电子相互作用的能力，并增强随机传输过程（手稿正在准备中）。在 FLR 随时间演化的过程中，它们的空间尺度可能会缩小到单流体 MHD 理论失效的地步，此时必须应用双流体或全动理学理论。除此以外，还与 R. Rankin 教授（加拿大阿尔伯塔大学）合作，在 Space Science Reviews 上发表了一篇描述动理学尺度剪切 Alfven Waves、极光电子加速和 FLR 之间关系的综述文章。项目执行期间，标注本项目发表和接收SCI文章6篇，其中一作/通讯SCI论文3篇，共有6篇发表在Space science review、GRL、JGR等业内重要刊物上。