Collaborative Research: GEM--Quantifying the Contribution of Off-Equatorial Ultra-Low Frequency (ULF) Waves on Radial Diffusion in the Radiation Belts

频率在超低频（ULF）范围内的电磁波是地球辐射带中高能电子径向扩散和输运的主要原因之一。超高频波的频率与高能电子绕地球运行时的漂移频率范围重叠，导致共振相互作用。已经导出了许多定量描述径向扩散的表达式，以便将它们纳入辐射带电子的全局模型。然而，径向扩散速率的大多数表达式仅推导出赤道镜像电子，并且是基于从靠近赤道平面的航天器或从地面获得的极低频波功率的估计。最近使用Van Allen探测器和Arase进行的研究表明，在远离磁赤道的地方，磁波动中的波能显著增强，这与模拟磁层场线振荡的自然模式相一致。这对径向扩散速率的估计具有重要意义，因为高俯仰角电子将经历比赤道电子高得多的极低频波波动。本项目将推导磁场和电场波功率，并将其纳入三维测试粒子模拟中，以估计扩散系数。在辐射带电子的全局模型中引入新的、依赖于螺距角的扩散速率，以评估扩散系数依赖于螺距角对辐射带动力学的影响。该结果可能对目前估计的径向扩散速率具有重要意义。这将为在全球模型中纳入与俯仰角相关的径向扩散系数铺平道路，从而更好地预测近地辐射环境。本项目的主要目标是量化赤道外超低频波对辐射带外（L~4 ~ 7）相对论电子（100 keV ~几MeV）径向输运和扩散的作用，研究高能粒子径向输运随俯角的变化对辐射带整体动力学的影响。将回答以下科学问题：在不同的太阳和地磁条件下，极低频电场和磁场波动如何在磁纬度和磁地方时分布？离赤道极低频波波动对外辐射带（L~4 ~ 7）相对论电子的径向扩散和输运有何影响？预计赤道外的极低频波将如何影响当前的辐射带模式，它们对辐射带的全球动力学有何贡献？该团队将使用多个卫星数据集（THEMIS， Van Allen探测器，Cluster和Arase），测试粒子追踪模拟，以及全球辐射带模型来量化赤道外超高频波对辐射带径向扩散的贡献。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。

Electromagnetic waves with frequencies in the ultra-low frequency (ULF) range are known to be among the leading causes of radial diffusion and transport of energetic electrons in Earth's radiation belts. The frequencies of ULF waves overlap with the range of drift frequencies of energetic electrons as they circle the Earth, leading to resonant interactions. Numerous expressions have been derived to quantitatively describe radial diffusion so that they can be incorporated into global models of radiation belt electrons. However, most expressions of the radial diffusion rates are derived only for equatorially mirroring electrons and are based on estimates of the power of ULF waves that are obtained either from spacecraft close to the equatorial plane or from the ground. Recent studies using the Van Allen Probes and Arase have shown that the wave power in magnetic fluctuations is significantly enhanced away from the magnetic equator, consistent with models simulating the natural modes of oscillation of magnetospheric field lines. This has significant implications for the estimation of radial diffusion rates, as higher pitch angle electrons will experience considerably higher ULF wave fluctuations than equatorial electrons. This project will derive the magnetic and electric field wave powers and incorporate them into the 3D test particle simulations to estimate the diffusion coefficient. The novel, pitch-angle-dependent diffusion rates will be introduced to a global model of radiation belt electrons to evaluate the effect of the pitch-angle dependence of the diffusion coefficient on radiation belt dynamics. The result could have significant implications for the radial diffusion rates as currently estimated. It will pave the way for incorporating pitch-angle-dependent radial diffusion coefficients in global models to predict the near-Earth radiation environment better.The main goal of this project is to quantify the role of off-equatorial Ultra-Low Frequency (ULF) waves on the radial transport and diffusion of relativistic electrons (100s keV to few MeV) in the outer radiation belt (L~4 to 7), investigating the effect of pitch-angle-dependent radial transport of energetic particles on global dynamics of the radiation belts. The following science questions will be answered: How are ULF electric and magnetic field fluctuations distributed in magnetic latitude and magnetic local time under varying solar and geomagnetic conditions? What is the role of off-equatorial ULF wave fluctuations on the radial diffusion and transport of relativistic electrons in the outer radiation belt (L~4 to 7)? How are off-equatorial ULF waves expected to impact current radiation belt models, and what is their contribution to the global dynamics of the radiation belts? The team will use multiple satellite datasets (THEMIS, Van Allen Probes, Cluster, and Arase), test particle tracing simulations, and a global radiation belt model to quantify the contribution of off-equatorial ULF waves on radial diffusion in the radiation belts.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.