The Role of Ultra Low Frequency (ULF) Field Fluctuations in Radial Transport in the Outer Radiation Belt Electrons

超低频 (ULF) 场涨落在外辐射带电子径向传输中的作用

• 批准号: 0540121
• 负责人: Aleksandr Ukhorskiy
• 金额: --
• 依托单位: Johns Hopkins University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-01-01 至 2010-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0540121&HistoricalAwards=false
• 关键词: Role; Ultra; Low; Frequency; ULF

During geomagnetic storms relativistic electrons in Earth's outer radiation belt exhibit complex behavior. Electron fluxes vary by many orders of magnitude over time scales from minutes to days. In spite of a growing volume of in situ observations and a number of proposed theories, we do not yet understand the transport and energization mechanisms responsible the dynamic behavior of the belt. This project is focused on radial transport of energetic electrons, a fundamental mechanism both of electron losses as well as acceleration of lower energy electrons to relativistic energies. Radial transport requires violation of the third adiabatic invariant. The adiabaticity of electron drift motion can be broken in the process of wave-particle interactions of the electrons with electric and magnetic field fluctuations in the ULF (Ultra Low Frequency) frequency range. As a result, particles gain or lose energy corresponding to their inward or outward transport. Thus, to describe transport in the belt, it is essential to identify and quantify ULF phenomena which can exhibit resonance with the drift motion of energetic electrons. This study will combine different kinds of ULF phenomena and investigate their relative impact on radial transport in the belt. To quantify the impact of ULF waves on the electron transport, the first step will be to determine their occurrence characteristics. Energetic electrons can exhibit drift resonance only with ULF waves which have low values of the azimuthal wave numbers ( 10). In the inner magnetosphere such waves are observed in the form of field line resonances (FLR) which is the only ubiquitous ULF feature with narrow band frequency spectrum and predominantly toroidal polarization. FLRs can be identified based on a single spacecraft measurements. The spatio-temporal profile of FLRs will be quantified based on electric and magnetic field data from the combined release and radiation effects satellite (CRRES) spacecraft. Unlike FLRs, global ULF fields cannot be quantified using single spacecraft observations, since they do not resolve spatial structure of the observed disturbances. For this purpose observations have to be complemented by a global model of the inner magnetospheric field. A time-dependant Tsyganenko 04 magnetic field model with self-consistent inductive electric fields will be used. Independent control parameters of the model will be adjusted to fit the various global ULF phenomena. After the ULF fields capable of resonant scattering of energetic electrons are quantified, their impact on the radiation belt electrons will be analyzed with the use of a test particle modeling. Both analytical theory and detailed numerical simulations will be used. Several fundamental questions of transport theory, not discussed by previous studies, will be considered. (1) Can radial transport be described by diffusion or is a more detailed treatment required? In the framework of radial diffusion what are the diffusion coefficients corresponding to various ULF drivers? (2) If particles exhibit stochastic motion, what is its origin? Is it caused by strong nonlinearity in the system (deterministic chaos) or a random character of ULF fluctuations? (3) Do the relative roles of different ULF drivers change with geomagnetic activity? (4) What is the role of off-equatorial dynamics in realistic time varying fields?

在地磁风暴期间，地球外辐射带中的相对论电子表现出复杂的行为。电子通量随着时间尺度从几分钟到几天的变化有多个数量级。尽管现场观测数量不断增加，并提出了许多理论，但我们仍不了解导致该带动态行为的传输和供能机制。该项目的重点是高能电子的径向传输，这是电子损失以及低能电子加速到相对论能量的基本机制。径向传输需要违反第三个绝热不变量。电子漂移运动的绝热性可以在电子与ULF（超低频）频率范围内的电场和磁场波动的波粒相互作用过程中被打破。结果，粒子获得或损失与其向内或向外传输相对应的能量。因此，为了描述带中的传输，有必要识别和量化超低频现象，这种现象可以与高能电子的漂移运动产生共振。本研究将结合不同类型的超低频现象，并研究它们对带内径向输运的相对影响。为了量化超低频波对电子传输的影响，第一步是确定它们的发生特征。高能电子只能与方位波数较低的 ULF 波表现出漂移共振 (10)。在内磁层中，这种波以场线共振 (FLR) 的形式观察到，这是唯一普遍存在的 ULF 特征，具有窄带频谱和主要是环形极化。 FLR 可以根据单个航天器的测量来识别。 FLR 的时空分布将根据组合释放和辐射效应卫星 (CRRES) 航天器的电场和磁场数据进行量化。与 FLR 不同，全球 ULF 场无法使用单个航天器观测来量化，因为它们不能解析观测到的扰动的空间结构。为此目的，必须通过内部磁层场的全局模型来补充观测结果。将使用具有自洽感应电场的时变 Tsyganenko 04 磁场模型。模型的独立控制参数将被调整以适应各种全局 ULF 现象。在量化能够共振散射高能电子的超低频场后，将使用测试粒子模型来分析它们对辐射带电子的影响。将使用分析理论和详细的数值模拟。将考虑以前的研究未讨论的几个运输理论的基本问题。 (1) 径向传输可以用扩散来描述还是需要更详细的处理？在径向扩散的框架中，各种 ULF 驱动因素对应的扩散系数是多少？ (2) 如果粒子表现出随机运动，其起源是什么？它是由系统中的强非线性（确定性混沌）还是 ULF 波动的随机特性引起的？ (3) 不同ULF驱动因素的相对作用是否会随着地磁活动的变化而变化？ (4) 离赤道动力学在现实时变场中的作用是什么？