# Dissertation/Thesis Abstract

Non-linear development of streaming instabilities in magnetic reconnection with a strong guide field
by Che, Haihong, Ph.D., University of Maryland, College Park, 2009, 162; 3372963
Abstract (Summary)

Magnetic reconnection has been recognized as a dominant mechanism for converting magnetic energy into the convective and thermal energy of particles, and has been thought as the driver of explosive events in nature and laboratory, such as solar and stellar flares, magnetic substorms and disruptions in fusion experiments. Magnetic reconnection (Sweet-Parker and Petscheck model) is often modeled using resistive magnetohydrodynamics, in which collisions play the key role in facilitating the release of energy in the explosive events. However, in space plasma the collisional resistivity is far below the required resistivity to explain the observed energy release rate. Turbulence is common in plasmas and the anomalous resistivity induced by the turbulence has been proposed as a mechanism for breaking the frozen-in condition in magnetic reconnection. Turbulence-driven resistivity has remained a poorly understood, but widely invoked mechanism for nearly 50 years. The goal of this project is to understand what role anomalous resistivity plays in fast magnetic reconnection.

Turbulence has been observed in the intense current layers that develop during magnetic reconnection in the Earth's magnetosphere. Electron streaming is believed to be the source of this turbulence. Using kinetic theory and 3D particlein-cell simulations, we study the nonlinear development of streaming instabilities in 3D magnetic reconnection with a strong guide field. Early in time an intense current sheet develops around the x-line and drives the Buneman instability. Electron holes, which are bipolar spatial localized electric field structures, form and then self-destruct creating a region of strong turbulence around the x-line. At late time turbulence with a characteristic frequency in the lower hybrid range also develops, leading to a very complex mix of interactions. A major challenge is to investigate what occurs after the saturation of Buneman instability and how the momentum and energy are exchanged among the waves and particles by the turbulence.

The difficulty we face in this project is how to address a long-standing problem in nonlinear kinetic theory: how to treat large amplitude perturbations and the associated strong wave-particle interactions. In my thesis, I address this long-standing problem using particle-in-cell simulations and linear kinetic theory.

The kinetic process of 3D magnetic reconnection is complicated. To separate problem of turbulent driven drag from reconnection, we carry out 3D simulations in which we specify the initial streaming velocities of particles to mimic the configuration of the x-line during magnetic reconnection. The geometry is chosen so that reconnection does not develop. Some important physics have been revealed. (1) At late time the lower hybrid instability (LHI) dominates the dynamics in low β plasma in combination with either the electron-electron two-stream instability (ETS) or the Buneman instability (BI), depending on the parallel phase speed of the LHI. If its parallel phase speed is sufficiently large and leaves sufficient velocity space for the ETS to grow, the ETS takes over the BI and interacts with the LHI to slow the streaming electrons. If not, the BI acts with the LHI to slow the high speed electrons. (2) An instability with a high phase speed is required to tap the energy of the high velocity electrons. The BI with its low phase speed, can not do this. The ETS and the LHI, both have high phase speed. (3) The condition for the formation of stable electron holes requires |vp – vg| < $2ef/me\left[/itex\right], where |&phis;| is the amplitude of the electric potential, and$vp and vg are the phase and group velocity of the relevant waves. Like the BI and ETS the LHI can form electron holes. (4) The overlapping resonance in phase space is the dominant mechanism for transporting the momentum and energy from high velocity electrons to low velocity electrons, which then couple to the ions. These resonances also lead to the chaotic motion of electrons in phase space and finally to the destruction of the electron holes.

Indexing (document details)
 Advisor: Drake, James F. Commitee: Antonsen Jr., Thomas M., Dorland, William, Guzdar, Parvez N., Hassam, Adil, Ostrike, Eve School: University of Maryland, College Park Department: Physics School Location: United States -- Maryland Source: DAI-B 70/09, Dissertation Abstracts International Source Type: DISSERTATION Subjects: Astronomy, Theoretical physics, Plasma physics Keywords: Chaotic process, Instability, Kinetic theory, Magnetic reconnection, Nonlinear dynamics, Turbulence Publication Number: 3372963 ISBN: 978-1-109-38323-2