Semester

Summer

Date of Graduation

2014

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Paul Cassak

Committee Co-Chair

Duncan Lorimer

Committee Member

Daniel Pisano

Committee Member

Arun Ross

Committee Member

Earl Scime

Abstract

Magnetic reconnection is a plasma process in which stored magnetic energy is converted into thermal and kinetic energies of the surrounding plasma. Oppositely directed magnetic field lines break and cross connect due to a dissipative mecha- nism. The now bent, reconnected field lines retreat from the X-line (the location of reconnection) at the Alfven speed due to the magnetic tension in the reconnected magnetic field, therefore generating outflows. This dissertation addresses three fundamental properties of magnetic reconnection.;Solar flares are explosive events in the solar corona in which magnetic reconnection mediates the rapid release (on the order of minutes) of energy stored in magnetic fields into the surrounding plasma. The Sweet-Parker (collisional) model was the first self-consistent theory to explain magnetic reconnection, but is far too slow to explain observations. The formation of secondary islands make Sweet-Parker reconnection faster, but is it fast enough to explain energy release rates? Collisionless (Hall) reconnection leads to energy release rates fast enough to explain observations. Large-scale resistive Hall-Magnetohydrodynamics simulations of the transition from Sweet-Parker to Hall reconnection are presented; the first to separate secondary islands from collisionless effects. Three main results are described. There exists a regime with secondary islands but without collisionless effects entering, and the reconnection rate is faster than Sweet-Parker, but significantly slower than Hall reconnection. This implies that secondary islands do not cause the fastest reconnection rates. The onset of Hall reconnection ejects secondary islands from the vicinity of the X-line, implying that energy is released more rapidly during Hall reconnection.;Early models of magnetic reconnection have treated reconnection as two- dimensional. However, naturally occurring magnetic reconnection often begins in a localized region and spreads in the direction perpendicular to the plane of reconnection. Theoretical arguments and large-scale two fluid simulations are used to study the spreading of reconnection X-lines localized in the direction of the current as a function of the strength of the out-of-plane (guide) magnetic field. It is found that the mechanism causing the spreading is different for weak and strong guide fields. In the weak guide field limit, spreading is due to the motion of the current carriers. However, spreading for strong guide fields is bidirectional and is due to the excitation of Alfven waves along the guide field. In general, we suggest that the X-line spreads bidirectionally with a speed governed by the faster of the two mechanisms for each direction. A prediction of the strength of the guide field at which the spreading mechanism changes is formulated and verified with three-dimensional simulations.;In the solar wind, magnetic reconnection exhausts measuring 600 [Gosling et al. (2007)] and 390 [Phan et al. (2006)] Earth radii in length have been observed. The authors assumed that the extended exhaust was caused by an extended X-line. If this is the case, what mechanism is responsible for these large scale structures? It has been suggested these structures are formed by a small X-line forming near the sun and spreading as the X-line convects away from the sun. Another possibility is the X-line is localized in a small region and the exhaust expands into the out-of-plane direction. Theoretical arguments and large-scale simulations are used to study localized (not spreading) magnetic reconnection, and its three-dimensional structure. Localized reconnection may also be vital to the formation of supra-arcade downflows (SADs) in the corona. Both solar wind and coronal applications are discussed.

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