Session DI2: Reconnection, Gamma-Ray Bursts, and Angular Momentum Transport

Towards a New Understanding of Collisionless Magnetic Reconnection

A central cornerstone of modern concepts regarding fast magnetic reconnection has been the expectation that the non-ideal electron region remains localized on the electron scale. Based on this understanding, it has been widely argued that the reconnection rate is controlled by the ions in a manner that is insensitive to the specific details of the electron physics. This picture implies that a single x-line will lead to steady reconnection with an open geometry similar to the Petscheck model. These expectations were largely based on two-fluid simulations and small-scale kinetic simulations with periodic boundary conditions. Recently, this problem was re-examined using 2D fully kinetic simulations with open boundary conditions \footnote{Daughton, Scudder and Karimabadi, {\it Phys.~Plasmas} {\bf 13}, 072101,2006} as well as the largest periodic simulations ever considered. In contrast to previous expectations, both of these approaches demonstrate that the length of the electron diffusion region expands in time to form a highly elongated current layer with a width on the electron scale but a length that can exceed tens of ion inertial lengths. These non-ideal electron layers exhibit multiple scales in the outflow direction\footnote{Karimabadi, Daughton and Scudder, {\it Geophys.~Res.~Lett.} {\bf 34}, L13104, 2007} with an inner region characterized by strong out-of-plane current and an outer region marked by a collimated electron jet. The formation of these highly elongated layers involves a competition between the outward convection of magnetic flux with the non-ideal terms arising from the divergence of the electron pressure tensor. Although it is possible to setup a balance over limited durations, the resulting layers are always highly elongated. Over longer time scales, these layers are unstable to the formation of secondary magnetic islands leading to a reconnection process that is inherently time-dependent. These results point to a very different picture regarding the essential physics of reconnection since both the reconnection rate and time dependence are sensitive to the details of the electron physics.   

Three-dimensional magnetic reconnection in Earth's magnetosphere

Magnetic reconnection is thought to be the primary mode by which the solar wind couples to the terrestrial magnetosphere, driving phenomena such as magnetic storms and aurorae. While the theory of two-dimensional reconnection is well developed, and has been applied with great success to axisymmetric and toroidal systems such as laboratory plasma experiments and fusion devices, it is difficult to justify the application of two-dimensional theory to nontoroidal plasma systems such as Earth's magnetosphere. Unfortunately, the theory of three-dimensional magnetic reconnection is much less well developed, and even defining magnetic reconnection has turned out to be problematic. In this talk, we review recent progress in the use of MHD to address the physics of three-dimensional reconnection in Earth's magnetosphere. The talk consists of two parts. In the first part, we review the various definitions of three-dimensional reconnection which have appeared in the literature in the last twenty years. Our goal here is to map these definitions to sets of physical phenomena which have been identified as ``reconnection'' in various three-dimensional contexts. In the second part of the talk, we present our latest magnetosphere MHD simulation results and indentify two qualitatively distinct types of reconnection phenomena (organized by the orientation of the Interplanetary Magnetic Field): 1) steady separator reconnection under generic northward IMF conditions, involving plasma flow across magnetic separatrix boundaries, and 2) time-dependent reconnection under generic southward IMF conditions, involving a global change in the topology of the magnetic field. While neither of these types of reconnection is well described by two-dimensional theory (indeed, we argue that attempts to apply two-dimensional ideas to the magnetopause have resulted in more confusion than clarification), both can be easily categorized according to existing definitions of three-dimensional reconnection.   

Jitter radiation mechanism --- a diagnostic tool of Weibel turbulence and Gamma-Ray Bursts

The Weibel instability is common in laser-produced plasmas and, as one has recently realized, it plays a major role in the formation and dynamics of astrophysical shocks of gamma-ray bursts (GRBs) and, perhaps, supernovae. Thanks to technological advances in the recent years, experimental studies of the Weibel instability are now possible at Petawatt- scale laser plasma facilities (such as NIF, Omega, etc) and in direct particle-in-cell (PIC) numerical simulations. We, thus, have a unique opportunity to model and study astrophysical conditions in numerical and laboratory experiments. At this stage, accurate diagnostic techniques are of great demand. In this presentation, we will discuss the physics of radiation, referred to as the {\it Jitter Radiation}, emitted by relativistic electrons (e.g., an electron beam or a thermal distribution) moving through the Weibel-generated magnetic fields, to which we refer as the {\it Weibel turbulence}. The similarity of Jitter radiation and the newly introduced ``diffusive synchrotron radiation'' is stressed. We'll show that the Jitter radiation field is anisotropic with respect to the direction of the Weibel current filaments and that its spectral and polarization characteristics are determined by microphysical plasma parameters. With the present computing capabilities, it is feasible to obtain radiation from plasma with Weibel-generated fields directly from PIC simulations, for the conditions relevant to laboratory laser- plasma experiments and relativistic astrophysical shocks of GRBs. Synergy of computer modeling, laboratory experiments and astrophysical observations will provide unique possibilities to diagnose plasma conditions in wide range of systems, thus putting {\it Plasma High-Energy Astrophysics} on the firm quantitative basis.   

Experimental Study of Angular Momentum Transport in Astrophysically Relevant Flows

Rapid angular momentum transport in accretion disks is a longstanding astrophysical puzzle. Transport by molecular viscosity is inadequate to explain observationally inferred accretion rates. Since Keplerian flows are linearly stable in hydrodynamics, there exist only two main viable mechanisms for the required turbulence: nonlinear hydrodynamic instability or linear magnetorotational instability (MRI). The latter is regarded as a dominant mechanism for rapid angular momentum transport in hot accretion disks ranging from quasars and X-ray binaries to cataclysmic variables. The former is proposed mainly for cooler protoplanetary disks, whose Reynolds numbers are typically large. Despite their popularity, however, there is limited experimental evidence for either mechanism. In this talk, I will describe \urllink{a laboratory experiment at Princeton}{http://mri.pppl.gov} in a short Taylor-Couette flow geometry intended to study these mechanisms. Based on the results from prototype experiments and simulations, the apparatus contains novel features for better controls of the boundary-driven secondary flows. The experiments in water have shown\footnote{H.~Ji, M.~Burin, E.~Schartman, \& J.~Goodman, Nature {\bf 444}, 343-346 (2006).} that nonmagnetic quasi-Keplerian flows at Reynolds numbers as large as $2\times 10^6$ are essentially laminar, through means of direct measurements of Reynolds stress via a synchronized, dual Laser Doppler Velocimetry. Scaled to accretion disks, rates of angular momentum transport lie far below astrophysical requirements. By ruling out hydrodynamic turbulence, our results indirectly support MRI as the likely cause of turbulence even in cool disks. The experiments in liquid gallium eutectic alloy have recently begun, and initial results on MRI as well as other related phenomena including numerical predictions will be also discussed if available.