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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.
A new paradigm is suggested for 3D magnetic reconnection where the interaction of reconnection processes with current aligned instabilities plays an important role. According to the new paradigm, the initial equilibrium is rendered unstable by current aligned instabilities (lower-hybrid drift instability first, drift-kink instability later) and the non-uniform development of kinking modes Leads to a compression of magnetic field lines in certain locations and a rarefaction in others. The areas where the flow is compressional are subjected to a driven reconnection process on the time scale of the driving mechanism (the kink mode). In the present paper we illustrate this series of events with a selection of simulation results.
Presents a comprehensive review of physical processes in astrophysical plasmas. This title presents a review of the detailed aspects of the physical processes that underlie the observed properties, structures and dynamics of cosmic plasmas. An assessment of the status of understanding of microscale processes in all astrophysical collisionless plasmas is provided. The topics discussed include turbulence in astrophysical and solar system plasmas as a phenomenological description of their dynamic properties on all scales; observational, theoretical and modelling aspects of collisionless magnetic reconnection; the formation and dynamics of shock waves; and a review and assessment of microprocesses, such as the hierarchy of plasma instabilities, non-local and non-diffusive transport processes and ionisation and radiation processes. In addition, some of the lessons that have been learned from the extensive existing knowledge of laboratory plasmas as applied to astrophysical problems are also covered. This volume is aimed at graduate students and researchers active in the areas of cosmic plasmas and space science. Originally published in Space Science Reviews journal, Vol. 278/2-4, 2013.
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.
Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
A new paradigm is emerging for 3D magnetic reconnection where the interaction of reconnection processes with current aligned instabilities plays an important role. According to the new paradigm, the initial equilibrium is rendered unstable by current aligned instabilities (lower-hybrid drift instability first, drift-kink instability later) and the non-uniform development of kinking modes leads to a compression of magnetic field lines in certain locations and a rarefaction in others. The areas where the flow is compressional are subjected to a driven reconnection process on the time scale of the driving mechanism (the kink mode). In the present paper we illustrate this series of event with a selection of simulation results.
The Physics of Solar and Stellar Coronae provides the first comprehensive summary of the physical processes and phenomena occurring in solar and stellar coronae as observed at X-ray and other wavelengths. The book provides an early summary of the spectacular new solar X-ray observations being obtained with the Yohkoh satellite that are dramatically changing our understanding of the dynamics of the solar corona. With the perspective of two years' observations at X-ray and extreme ultraviolet wavelengths by the ROSAT satellite, many authors present new insights into the basic physical processes occurring in the coronae of stars across the Hertzsprung--Russell Diagram including both pre-main sequence and post-main sequence stars. Detailed models for the hot plasmas typically contained in magnetic loops in both stellar and solar coronae are presented to explain X-ray data obtained with the earlier X-ray instruments on Skylab, SMM, Einstein, and EXOSAT. The book includes papers on coronal observations obtained at other wavelengths and papers of the history of Palermo Astronomical Observatory. The Physics of Solar and Stellar Coronae is intended for researchers in the fields of solar physics and stellar astrophysics and will be a useful resource book for graduate level astrophysics courses. (ABSTRACT) This is the first comprehensive summary of the physical processes and phenomena occurring in solar and stellar coronae. Spectacular new solar X-ray observations by the Yohkoh satellite and stellar observations by ROSAT are highlighted, together with theoretical papers and detailed analyses of earlier data from Skylab, SMM, Einstein, and EXOSAT. Included are papers on coronal observations at other wavelengths and on the history of Palermo Astronomical Observatory.
Reconnection and turbulence frequently occur in plasmas, including laboratory, astrophysical and space plasmas. Over the past few decades, the interrelationship between turbulence and magnetic reconnection has been the focus of increasing scrutiny with significant research on the statistics of reconnection as an element of turbulence, the generation of turbulence due to instabilities associated with reconnection, and the effect of reconnection on the turbulent cascade. However, none of these studies directly address the universal properties that may relate reconnection and turbulence, i.e., ``What are the turbulent-like features of laminar magnetic reconnection?'' and ``Is magnetic reconnection fundamentally an energy cascade?'' To answer these questions, we analyze 2.5D fully kinetic particle-in-cell (PIC) simulations of reconnection and turbulence. First, we start with a laminar anti-parallel reconnection with no initial turbulent fluctuations. Even with no secondary instabilities, the reconnection process in its quasi-steady phase is found to generate a magnetic energy spectrum with a spectral index of -5/3 at scales larger than the ion-inertial length, in accordance with Kolmogorov turbulence. Further, the scale-to-scale energy transfer process is studied using the von-Karman Howarth equation generalized to Hall Magnetohydrodynamic (MHD) systems. Most notably, the energy transfer in laminar magnetic reconnection is also found to be similar to that of a turbulent system suggesting that reconnection involves an energy cascade. Consistent with this similarity, the reconnection rate is positively correlated to both the magnetic energy spectrum in the ion-scales and the cascade of energy. Next, we investigate how robust the antiparallel reconnection results are by generalizing them to reconnection with smaller magnetic shear (guide field reconnection). The guide field does not fundamentally change the energy transfer properties of reconnection. Finally, we study how the electric field spectrum and its constituent physical terms in Ohm's law are modified by changing the guide field in reconnection. Unlike the magnetic spectrum, the electric field spectrum drastically changes with guide field. At MHD length scales, there is a flat spectrum for the antiparallel case and a gradual steepening to a -5/3 slope with increasing guide field. In summary, the striking similarities found between the reconnection and turbulence simulations imply that both processes are fundamentally energy cascades, and raises the intriguing possibility of a fundamental universality linking the two.
This book, first published in 2000, is a comprehensive introduction to this major topic in plasma physics; for graduates and researchers.