Charged Particle Energization and Transport in the Magnetotail during Substorms
This dissertation addresses the problem of energization of particles (both electrons and ions) to tens and hundreds of keV and the associated transport process in the magnetotail during substorms. Particles energized in the magnetotail are further accelerated to even higher energies (hundreds of keV to MeV) in the radiation belts, causing space weather hazards to human activities in space and on ground. We develop an analytical model to quantitatively estimate flux changes caused by betatron and Fermi acceleration when particles are transported along narrow high-speed flow channels from the magnetotail to the inner magnetosphere. The model shows that energetic particle flux can be significantly enhanced by a modest compression of the magnetic field and/or shrinking of the distance between the magnetic mirror points. We use coordinated spacecraft measurements, global magnetohydrodynamic (MHD) simulations driven by measured upstream solar wind conditions, and large-scale kinetic (LSK) simulations to quantify electron local acceleration in the near-Earth reconnection region and nonlocal acceleration during plasma earthward transport. Compared to the analytical model, application of the LSK simulations is much less restrictive because trajectories of millions of test particles are calculated in the realistically determined global MHD fields and the results are statistical. The simulation results validated by the observations show that electrons following a power law distribution at high energies are generated earthward of the reconnection site, and that the majority of the energetic electrons observed in the inner magnetosphere are caused by adiabatic acceleration in association with magnetic dipolarizations and fast flows during earthward transport. We extend the global MHD+LSK simulations to examine ion energization and compare it with electron energization. The simulations demonstrate that ions in the magnetotail are first nonadiabatically accelerated in the weak field region close to the reconnection site, and then adiabatically accelerated in the high-speed flow channels as they catch up with and ride on the earthward propagating dipolarization structures. The nonlocal adiabatic acceleration mechanism for ions is very similar to that for electrons. However, the motion of energetic electrons is adiabatic except in very limited regions near the reconnection site while the motion of energetic ions is marginally adiabatic in the dipolarization regions. The simulations also show that the earthward transport of both species is controlled by the high-speed flows via the dominant ExB drift in the magnetotail. To understand how the power law electrons are initially produced in the magnetotail, we use an implicit particle-in-cell (PIC) code to model the processes in the near-Earth reconnection region. We find that the power law electrons are produced not in the reconnection diffusion region, but in the immediate downstream of the reconnection outflow in the course of dipolarization formation and intensification. Our study illustrates that during substorms, particles are accelerated via a multi-step process, including local acceleration in the reconnection region and nonlocal acceleration during the earthward transport, and the multi-step acceleration occurs on multiple spatial scales ranging from a few kilometers (the scale of electron diffusion region) to more than ten Earth radii (the transport scale).