Physics
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Item The Effects of Turbulence on Magnetic Reconnection at the Magnetopause(2017) Price, Lora; Drake, James F; Swisdak, M M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic reconnection facilitates the conversion of magnetic energy to thermal energy and plasma flows. Reconnection occurs at the magnetopause, the magnetic boundary between the plasmas of the terrestrial magnetosphere and the heliosphere. Turbulence is known to develop at this boundary, but its influence on reconnection, particularly on small scales, is unknown. In light of this, an important goal of NASA's Magnetospheric Multiscale (MMS) Mission is to understand the role turbulence plays in the development of reconnection. We present two- and three-dimensional particle-in-cell simulations of the 16 October 2015 MMS magnetopause reconnection event. While the two-dimensional simulation is laminar, turbulence develops at both the x-line and along the magnetic separatrices in the three-dimensional simulation. This turbulence is electromagnetic, is characterized by a wavevector $k$ given by $k\rho_e\sim(m_e/m_i)^{0.25}$ with $\rho_e$ the electron Larmor radius, and appears to have the ion pressure gradient as its source of free energy. Taken together, these results suggest the instability is a variant of the lower-hybrid drift instability. The turbulence produces electric field fluctuations in the out-of-plane direction with an amplitude of around $\pm 10$ mV/m, which is much greater than the reconnection electric field of around $0.1$ mV/m. Such large values of the out-of-plane electric field have been identified in the MMS data. The turbulence in the simulation controls the scale lengths of the density profile and current layers, driving them closer to $\sqrt{\rho_e\rho_i}$ than the $\rho_e$ or $d_e$ scalings seen in 2D reconnection simulations, where $d_e$ is the electron inertial length. The turbulence produces both anomalous resistivity and anomalous viscosity. Each contribute significantly to breaking the frozen-in condition in the electron diffusion region. The crescent-shaped features in velocity space seen both in MMS observations and in two-dimensional simulations survive. We compare and contrast these results to a three-dimensional simulation of the 8 December 2015 MMS magnetopause reconnection event in which the reconnecting and out-of-plane guide fields are comparable. LHDI is still present in this event, although its appearance is modified by the presence of the guide field. The crescents also survive although, in agreement with MMS, their intensity decreases. Nevertheless, the developing turbulence remains strong.Item ELECTRON ACCELERATION IN MAGNETIC RECONNECTION(2015) Dahlin, Joel Timothy; Drake, James F; Swisdak, Michael M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic reconnection is a ubiquitous plasma physics process responsible for the explosive release of magnetic energy. It is thought to play a fundamental role in the production of non-thermal particles in many astrophysical systems. Though MHD models have had some success in modeling particle acceleration through the test particle approach, they do not capture the vital feedback from the energetic particles on the reconnection process. We use two and three-dimensional kinetic particle-in-cell (PIC) simulations to self-consistently model the physics of electron acceleration in magnetic reconnection. Using a simple guiding-center approxima- tion, we examine the roles of three fundamental electron acceleration mechanisms: parallel electric fields, betatron acceleration, and Fermi reflection due to the re- laxation of curved field lines. In the systems explored, betatron acceleration is an energy sink since reconnection reduces the strength of the magnetic field and hence the perpendicular energy through the conservation of the magnetic moment. The 2D simulations show that acceleration by parallel electric fields occurs near the mag- netic X-line and the separatrices while the acceleration due to Fermi reflection fills the reconnection exhaust. While both are important, especially for the case of a strong guide field, Fermi reflection is the dominant accelerator of the most energetic electrons. In a 3D systems the energetic component of the electron spectra shows a dramatic enhancement when compared to a 2D system. Whereas the magnetic topology in the 2D simulations is characterized by closed flux surfaces which trap electrons, the turbulent magnetic field in 3D becomes stochastic, so that electrons wander over a large region by following field lines. This enables the most energetic particles to quickly access large numbers of sites where magnetic energy is being released.Item The Kinetic Structure of Collisionless Slow Shocks and Reconnection Exhausts(2011) Liu, Yi-Hsin; Drake, James F; Swisdak, Michael M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A 2-D Riemann problem is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Simulations are carried out for various ratios of normal magnetic field to the transverse upstream magnetic field (i.e., propagation angle with respect to the upstream magnetic field). When the angle is sufficiently oblique, the simulations reveal a large firehose-sense (Pparallel>Pperpendicular) temperature anisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a non-coplanar rotational mode. In the downstream region the firehose stability parameter epsilon=1-$mu0(Pparallel-Pperpendicular)/B2 tends to plateau at 0.25. This balance arises from the competition between counterstreaming ions, which drives epsilon down, and the scattering due to ion inertial scale waves, which are driven unstable by the downstream rotational wave. At very oblique propagating angles, 2-D turbulence also develops in the downstream region. An explanation for the critical value 0.25 is proposed by examining anisotropic fluid theories, in particular the Anisotropic Derivative Nonlinear-Schrodinger-Burgers equations, with an intuitive model of the energy closure for the downstream counter-streaming ions. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes, and corresponds to the lower bound of the transition point in a compound slow shock(SS)/rotational discontinuity(RD) wave. This work implies that it is a pair of compound SS/RD waves that bounds the reconnection outflow, instead of a pair of switch-off slow shocks as in Petschek's model.Item Non-linear Development of Streaming Instabilities in Magnetic Reconnection with a Strong Guide Field(2009) Che, Haihong; Drake, James F.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetic reconnection is recognized as a dominant mechanism for converting magnetic energy into the convective and thermal energy of particles, and the driver of explosive events in nature and laboratory. Magnetic reconnection 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 particle-in-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. 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.Some important physics have been revealed. 1: The lower hybrid instability (LHI) dominates the dynamics in low $beta$ 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. 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 $|v_p -v_g|< sqrt{2e|phi|/m_e}$, where $|phi|$ is the amplitude of the electric potential, and $v_p$ and $v_g$ are the phase and group velocity of the relevant waves. Like ETS and BI, LHI all 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.