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?
