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

众所周知，频率在超低频范围内的电磁波是高能电子在地球辐射带中径向扩散和传输的主要原因之一。超低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。为了定量地描述径向扩散，人们导出了许多表达式，以便将它们纳入辐射带电子的全球模型中。然而，大多数径向扩散率的表达式只针对赤道镜像的电子，并且基于对从靠近赤道面的航天器或从地面获得的ULF波的功率的估计。最近使用Van Allen探测器和ARASE的研究表明，在远离磁赤道的地方，磁波动中的波功率显著增强，这与模拟磁层磁力线的自然振荡模式的模型是一致的。这对径向扩散速率的估计有重要意义，因为高俯仰角的电子将经历比赤道电子高得多的ULF波波动。该项目将推导出磁场和电场的波功率，并将其整合到3D测试粒子模拟中，以估计扩散系数。这种新的与螺距角相关的扩散速率将被引入到辐射带电子的全球模型中，以评估扩散系数的螺距角依赖对辐射带动力学的影响。这一结果可能会对目前估计的径向扩散速率产生重大影响。这将为在全球模式中引入与俯仰角相关的径向扩散系数以更好地预测近地辐射环境铺平道路。本项目的主要目的是量化赤道外辐射带(L~4~7)中相对论电子(100S keV到少数MeV)径向输运和扩散的作用，研究高能粒子与俯仰角相关的径向输运对辐射带全球动力学的影响。以下科学问题将得到解答：在变化的太阳和地磁条件下，超低频电场和磁场涨落在磁纬度和磁当地时间中是如何分布的？赤道外超低频波动对相对论电子在外辐射带中的径向扩散和输运有何作用(L~4至7)？赤道外的超低频波预计将如何影响目前的辐射带模型，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、星团和ARASE)、测试粒子跟踪模拟和全球辐射带模型来量化赤道外超低频波对辐射带径向扩散的贡献。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.