Magnetic Helicity In Space And Laboratory Plasmas

Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 111. Using the concept of magnetic helicity, physicists and mathematicians describe the topology of magnetic fields: twisting, writhing, and linkage. Mathematically, helicity is related to linking integrals, which Gauss introduced in the 19th century to describe the paths of asteroids in the sky. In the late 1970s the concept proved to be critical to understand laboratory plasma experiments on magnetic reconnection, dynamos, and magnetic field relaxation. In the late 1980s it proved equally important in understanding turbulence in the solar wind and the interplanetary magnetic field. During the last five years interest in magnetic helicity has grown dramatically in solar physics, and it will continue to grow as observations of vector magnetic fields become increasingly sophisticated.

Pedagogical in style, this book provides insights into plasma behavior valid over twenty orders of magnitude in both time and space. The book assumes that the reader has a basic knowledge of magnetohydrodynamics and explains topics using detailed theoretical analysis supported by discussion of relevant experiments. This comprehensive approach gives the reader an understanding of the essential theoretical ideas and their application to real situations.The book starts by explaining the topological concept of magnetic helicity and then develops a helicity-based model that predicts the ultimate state towards which magnetically-dominated plasmas evolve. The model predicts that no matter how messy or complicated the dynamics, a great range of plasma configurations always self-organize to a unique, simple final state. This self-organization, called relaxation, is a fundamental concept that unifies understanding of spheromaks, solar corona loops, interplanetary magnetic clouds, and astrophysical jets.After establishing why relaxation occurs, the book then examines how relaxation occurs. It shows that relaxation involves a sequence of complex non-equilibrium dynamics including fast self-collimated plasma jets, kink instabilities, magnetic reconnection, and phenomena outside the realm of magnetohydrodynamics.

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.

This volume is devoted to the dynamics and diagnostics of solar magnetic fields and plasmas in the Sun’s atmosphere. Five broad areas of current research in Solar Physics are presented: (1) New techniques for incorporating radiation transfer effects into three-dimensional magnetohydrodynamic models of the solar interior and atmosphere, (2) The connection between observed radiation processes occurring during flares and the underlying flare energy release and transport mechanisms, (3) The global balance of forces and momenta that occur during flares, (4) The data-analysis and theoretical tools needed to understand and assimilate vector magnetogram observations and (5) Connecting flare and CME phenomena to the topological properties of the magnetic field in the Solar Atmosphere. The role of the Sun’s magnetic field is a major emphasis of this book, which was inspired by a workshop honoring Richard C. (Dick) Canfield. Dick has been making profound contributions to these areas of research over a long and productive scientific career. Many of the articles in this topical issue were first presented as talks during this workshop and represent substantial original work. The workshop was held 9 – 11 August 2010, at the Center Green campus of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. This volume is aimed at researchers and graduate students active in solar physics, solar-terrestrial physics and magneto-hydrodynamics. Previously published in Solar Physics journal, Vol. 277/1, 2012.

The present research studies the fundamental physics occurring during the magnetic flux and magnetized plasma compression by plasma implosion. This subject is relevant to numerous studies in laboratory and space plasmas. Recently, it has attracted particular interest due to the advances in producing high-energy-density plasmas in fusion-oriented experiments, based on the approach of magnetized plasma compression. The studied configuration consists of a cylindrical gas-puff shell with pre-embedded axial magnetic field that pre-fills the anode-cathode gap. Subsequently, axial pulsed current is driven through the plasma generating an azimuthal magnetic field that compresses the plasma and the axial magnetic field embedded in it. A key parameter for the understanding of the physics occurring during the magnetized plasma compression is the evolution and distribution of the axial and azimuthal magnetic fields. Here, for the first time ever, both fields are measured simultaneously employing non-invasive spectroscopic methods that are based on the polarization properties of the Zeeman effect. These measurements reveal unexpected results of the current distribution and the nature of the equilibrium between the axial and azimuthal fields. These observations show that a large part of the current does not flow in the imploding plasma, rather it flows through a low-density plasma residing at large radii. The development of a force-free current configuration is suggested to explain this phenomenon. Previously unpredicted observations in higher-power imploding-magnetized-plasma experiments, including recent unexplained structures observed in the Magnetized Liner Inertial Fusion experiment, may be connected to the present discovery.

Magnetic helicity is approximately conserved by the turbulence associated with resistive instabilities of plasmas. To generalize the application of the concept of helicity, the helicity content of an arbitrary bounded region of space will be defined. The definition has the virtues that both the helicity content and its time derivative have simple expressions in terms of the poloidal and toroidal magnetic fluxes, the average toroidal loop voltage and the electric potential on the bounding surface, and the volume integral of E-B. The application of the helicity concept to tokamak plasmas is illustrated by a discussion of so-called MHD current drive, an example of a stable tokamak q profile with q less than one in the center, and a discussion of the possibility of a natural steady-state tokamak due to the bootstrap current coupling to tearing instabilities.

Leading experts present a unique, invaluable introduction to the study of the geometry and typology of fluid flows. From basic motions on curves and surfaces to the recent developments in knots and links, the reader is gradually led to explore the fascinating world of geometric and topological fluid mechanics. Geodesics and chaotic orbits, magnetic knots and vortex links, continual flows and singularities become alive with more than 160 figures and examples. In the opening article, H. K. Moffatt sets the pace, proposing eight outstanding problems for the 21st century. The book goes on to provide concepts and techniques for tackling these and many other interesting open problems.

This book, first published in 2000, is a comprehensive introduction to this major topic in plasma physics; for graduates and researchers.