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Three dimensional magnetic null points are now accepted as important topological features at which magnetic reconnection occurs. However, the understanding of the processes involved is still far behind the well developed field of 2D X-point reconnection. Therefore, the aim of this thesis is to present realistic extensions of the known ways in which 3D null point reconnection occurs. The Torsional (twisting) regimes of 3D null point reconnection are investigated using analytical models with, for the first time, localised current structures that qualitatively match those seen in simulation studies. These solutions show a wealth of possible scenarios in which new connections can form as a result of twisting perturbations near 3D nulls. Analytical solutions for fan and spine reconnection are presented with asymmetric current sheets as this scenario is thought to be commonplace in astrophysical plasmas. The asymmetry in each solution has a profound and rather different effect in each case. This analysis is then complimented by a series of numerical experiments studying the self consistent formation of similar current structures for the spine-fan mode in response to transient driving. Time dependent effects, such as the movement of the null position and the applicability of scaling laws derived from analyses with symmetric current sheets, are discussed. These results suggest that, in typical astrophysical plasmas, 3D null points may be continuously shifting position with a flow of plasma at the null point itself. Lastly, as instabilities are thought to play an important role in astrophysical reconnection dynamics, a series of numerical experiments investigating the self consistent formation and subsequent instability of a current-vortex layer at the fan plane of a 3D null point is presented. The results suggest that separatrix surfaces are great potential sites for current-vortex sheet formation and, therefore, the additional heating and connection change associated with instabilities of this layer.
Magnetic reconnection is at the core of many dynamic phenomena in the universe, such as solar flares, geomagnetic substorms and tokamak disruptions. Written by two world leaders on the subject, this volume provides a comprehensive overview of this fundamental process. Coverage gives both a pedagogical account of the basic theory and a wide-ranging review of the physical phenomena created by reconnection--from laboratory machines, the Earth's magnetosphere, and the Sun's atmosphere to flare stars and astrophysical accretion disks. It also includes a succinct account of particle acceleration by electric fields, stochastic fields and shock waves, and how reconnection can be important in these mechanisms. Clearly written and highly accessible, this volume serves as an essential introduction for graduate students in solar physics, astrophysics, plasma physics and space science. Researchers in these fields also will find Magnetic Reconnection an authoritative reference.
The aim of this book is twofold: to provide an introduction for newcomers to state of the art computer simulation techniques in space plasma physics and an overview of current developments. Computer simulation has reached a stage where it can be a highly useful tool for guiding theory and for making predictions of space plasma phenomena, ranging from microscopic to global scales. The various articles are arranged, as much as possible, according to the - derlying simulation technique, starting with the technique that makes the least number of assumptions: a fully kinetic approach which solves the coupled set of Maxwell’s equations for the electromagnetic ?eld and the equations of motion for a very large number of charged particles (electrons and ions) in this ?eld. Clearly, this is also the computationally most demanding model. Therefore, even with present day high performance computers, it is the most restrictive in terms of the space and time domain and the range of particle parameters that can be covered by the simulation experiments. It still makes sense, therefore, to also use models, which due to their simp- fying assumptions, seem less realistic, although the e?ect of these assumptions on the outcome of the simulation experiments needs to be carefully assessed.
These are the Proceedings of Colloquium No. 153 of the International Astro nomical Union, held at Makuhari near Tokyo on May 22 - 26, 1995, and hosted by the National Astronomical Observatory. This meeting was intended to be an interdisciplinary meeting between re searchers of solar and stellar activity, in order for them to exchange the newest information in each field. While each of these areas has seen remarkable advances in recent years, and while the researchers in each field have felt that information from the other's domain would be extremely useful in their own work, there have not been very many opportunities for intensive exchanges of information between these closely related fields. We therefore expected much from this meeting in pro viding stellar researchers with new results of research on the counterparts of their targets of research, spatially and temporarily resolved, as observed on the Sun. Likewise we hoped to provide solar researchers with new results on gigantic ver sions of their targets of research under the very different physical circumstances on other active stars. It was our greatest pleasure that we had wide attendance of experts and active researchers of both research fields from all over the world. This led to extremely interesting talks and very lively discussions, thereby stimulating the exchange of ideas across the fields.
We consider the transition to turbulence in three dimensional reconnection of a magnetic neutral sheet. We find that the transition can occur via a three-step process. First, the sheet undergoes the usual tearing instability. Second, the tearing mode saturates to form a two-dimensional quasi-steady state. Third, this secondary equilibrium is itself unstable when it is perturbed by three-dimensional disturbances. Most of this paper is devoted to the analysis and simulation of the three-dimensional linear stability properties of the two-dimensional saturated tearing layer. The numerical simulations are performed with a semi-implicit, pseudospectral-Fourier collocation algorithm. We identify a three-dimensional secondary liner stability which grows on the ideal timescale. An examination of the modal energetics reveals that the largest energy transfer is from the mean field to the three-dimensional field, with the two-dimensional field acting as a catalyst.
The essential introduction to magnetic reconnection—written by a leading pioneer of the field Plasmas comprise more than 99 percent of the visible universe; and, wherever plasmas are, magnetic reconnection occurs. In this common yet incompletely understood physical process, oppositely directed magnetic fields in a plasma meet, break, and then reconnect, converting the huge amounts of energy stored in magnetic fields into kinetic and thermal energy. In Magnetic Reconnection, Masaaki Yamada offers an illuminating synthesis of modern research and advances on this important topic. Magnetic reconnection produces such phenomena as solar flares and the northern lights, and occurs in nuclear fusion devices. A better understanding of this crucial cosmic activity is essential to comprehending the universe and varied technological applications, such as satellite communications. Most of our knowledge of magnetic reconnection comes from theoretical and computational models and laboratory experiments, but space missions launched in recent years have added up-close observation and measurements to researchers’ tools. Describing the fundamental physics of magnetic reconnection, Yamada links the theory with the latest results from laboratory experiments and space-based observations, including the Magnetic Reconnection Experiment (MRX) and the Magnetospheric Multiscale (MMS) Mission. He concludes by considering outstanding problems and laying out a road map for future research. Aimed at advanced graduate students and researchers in plasma astrophysics, solar physics, and space physics, Magnetic Reconnection provides cutting-edge information on a vital area of scientific investigation.
This book presents an introduction to, and modern account of, magnetohydrodynamic (MHD) turbulence, an active field both in general turbulence theory and in various areas of astrophysics. The book starts by introducing the MHD equations, certain useful approximations and the transition to turbulence. The second part of the book covers incompressible MHD turbulence, the macroscopic aspects connected with the different self-organization processes, the phenomenology of the turbulence spectra, two-point closure theory, and intermittency. The third considers two-dimensional turbulence and compressible (in particular, supersonic) turbulence. Because of the similarities in the theoretical approach, these chapters start with a brief account of the corresponding methods developed in hydrodynamic turbulence. The final part of the book is devoted to astrophysical applications: turbulence in the solar wind, in accretion disks, and in the interstellar medium. This book is suitable for graduate students and researchers working in turbulence theory, plasma physics and astrophysics.