## Dissertations

2016

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

Space Science

Gang Li

Gary P. Zank

S.T. Wu

Gary Webb

#### Subject(s)

Solar flares., Particle accelerators., Electron accelerators., Sun--Corona.

#### Abstract

Particle acceleration is a central topic of solar flare research. In this thesis, I study two topics concerning particle acceleration during solar flares. The first topic is on the acceleration of charged particles by a time-dependent chaotic magnetic filed. We developed one model based on the solar observations of complex electric current system in solar corona and solar active regions. In this model, self-consistent chaotic magnetic field and electric field are generated by a time-dependent electric current system. Through test-particle simulations, we show that the low energy particles can be efficiently accelerated to nonthermal energies in this system. The results provide a possible mechanism for the efficient particle acceleration during solar flares as well as a pre-acceleration mechanisms for the diffusive shock acceleration by the CME-driven shocks. The second topic is on particle acceleration by magnetic reconnection. We have carried out kinetic simulations of magnetic reconnection with and without a guide field. The results show that, in the low-$\beta$ regime, both electrons and ions are efficiently accelerated to nonthermal energies. These nonthermal particles contain more than half of the total electrons, and their distribution resembles a power-law energy distribution with spectral index $p\sim 1$ in a closed boundary simulation. This is in contrast to the high-$\beta$ cases, where no obvious power-law spectrum is obtained. By ensemble averaging the particle guiding center drift motions, we reveal the main acceleration mechanism as a \textit{Fermi}-type acceleration accomplished by the particle curvature drift along the electric field induced by the reconnection outflows. We then performed a series of simulations with different guide field. The results show that the energy conversion becomes less efficient as the guide field increases. An interesting finding is that reconnection with no guide field preferentially accelerate ions, but reconnection with a strong guide field preferentially accelerate electrons. Both electrons and ions develop power-law energy distributions, which become steeper as the guide field gets stronger. Perpendicular acceleration is dominant for electrons in the cases with a weak guide field, and the parallel acceleration gets more important as the guide field increases. However, the perpendicular acceleration is always dominant for ions. The drift-current analysis shows that the dominant acceleration mechanism for ions is the polarization drift along the motional electric field.

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