Date of Award

Fall 1996

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Martin A Lee

Abstract

This is a theoretical study of the acceleration of charged particles during solar flares. An attempt is made to trace the relationship between the processes of acceleration and primary flare energy release.

Motion of charged particles in a reconnecting current sheet (RCS) is considered, including both the electric field and the magnetic field with nonzero transverse (perpendicular to the RCS plane) and longitudinal (parallel to the electric current) components. An analytical technique is developed to calculate particle trajectories and energy gain. The solution predicts a critical value of the longitudinal field beyond which it counteracts the effect of the transverse field that serves to eject the particles out of the sheet rapidly.

A longitudinal component on the order of the reconnecting component is necessary to explain electron acceleration in RCSs up to 10-100 keV during the impulsive phase of solar flares. The acceleration time can be sufficiently short ($\approx$10$\sp{-6}$s) for the process to occur in the regime of impulsive, bursty reconnection. Particle escape turns out to be more efficient across the RCS rather than along it, placing strong requirements on the electric field necessary to accelerate the particles.

Protons can interact with the RCS more than once due to the transverse electric field outside the RCS. This field efficiently "locks" nonthermal ions in the RCS, allowing their acceleration by the direct electric field to an energy of up to a few GeV in less than 0.1 s. This mechanism explains the generation of relativistic ions in large gamma-ray/proton flares.

Electromagnetic ion-cyclotron waves are generated by the electrons in RCSs during impulsive flares. The resonant interaction with these waves is the most promising mechanism for selective acceleration of $\sp3$He ions. However, the observed break in the particle spectra at energies of about 1-10 MeV cannot be explained by the action of the acceleration mechanism alone. It is shown that Coulomb energy losses may be large enough to provide the observed spectral break. Its position is determined by the balance between energy gain by acceleration and the energy loss.

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