Probing the Progression, Properties, and Progenies of Magnetic Reconnection

Citation

Yoon, Young Dae (2020) Probing the Progression, Properties, and Progenies of Magnetic Reconnection. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hqwx-r892. https://resolver.caltech.edu/CaltechTHESIS:04302020-151019178

Abstract

Magnetic reconnection is a plasma phenomenon in which opposing magnetic fields annihilate and release their magnetic energy into other forms of energy. In this thesis, various aspects of collisionless magnetic reconnection are studied analytically and numerically, and an experimental diagnostic for magnetic fields in a plasma is described.

The progression of magnetic reconnection is first illustrated through the formulation of a framework that revolves around canonical vorticity flux, which is ideally a conserved quantity. The reconnection instability, electron acceleration, and whistler wave generation are explained in an intuitive manner by analyzing the dynamics of canonical vorticity flux tubes. The validity of the framework is then extended down to first principles by the inclusion of the electron canonical battery effect. The importance of this effect during reconnection determines the overall structure and evolution of the process.

A crucial property of magnetic reconnection is its accompaniment by anomalous ion heating much faster than conventional collisional heating. Stochastic heating is a mechanism in which, under a sufficiently strong electric field, particles undergo chaotic motion in phase space and heat up dramatically. Using the previously established canonical vorticity framework, it is demonstrated that the Hall electric fields that develop during reconnection satisfy the stochastic ion heating criterion and that the ions involved indeed undergo chaotic motion. This mechanism is then kinetically verified via exact analyses and particle simulations and is thus ultimately established as the main ion heating mechanism in magnetic reconnection.

An important progeny of magnetic reconnection is whistler waves. These waves interact with energetic particles and scatter their pitch-angles, triggering losses of magnetic confinement. A previous study demonstrated via exact relativistic analyses that if a particle undergoes a "two-valley" motion, it undergoes drastic changes in its pitch-angle. This analysis is extended to a relativistic thermal distribution of particles. The condition for two-valley motion is first derived; it is then shown that a significant fraction of the particle distribution meets this condition and thus undergoes large pitch-angle scatterings. The scaling of this fraction with the wave amplitude suggests that relativistic microburst events may be explained by the two-valley mechanism. It is also found that the widely-used second-order trapping theory is an inaccurate approximation of the theory presented.

A new method of probing the magnetic field in a plasma is described and developed to some extent. It utilizes the two-photon Doppler-free laser-induced fluorescence technique, where two counter-propagating laser beams effectively cancel out the Doppler effect and excite electron populations. The fluorescence resulting from the subsequent de-excitation is then measured, enabling the resolution of Zeeman splitting of the spectral lines from which the magnetic field information can be inferred. A high-power, repetitively-pulsed radio-frequency plasma source was developed as the subject of diagnosis, and preliminary results are presented.

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