Years of Self-organized Criticality : Concepts and Controversies

Introduced by the late Per Bak and his colleagues, self-organized criticality (SOC) has been one of the most stimulating concepts to come out of statistical mechanics and condensed matter theory in the last few decades, and has played a significant role in the development of complexity science. SOC, and more generally fractals and power laws, have attracted much comment, ranging from the very positive to the polemical. The other papers (Aschwanden et al. in Space Sci. Rev., 2014, this issue; McAteer et al. in Space Sci. Rev., 2015, this issue; Sharma et al. in Space Sci. Rev. 2015, in preparation) in this special issue showcase the considerable body of observations in solar, magnetospheric and fusion plasma inspired by the SOC idea, and expose the fertile role the new paradigm has played in approaches to modeling and understanding multiscale plasma instabilities. This very broad impact, and the necessary process of adapting a scientific hypothesis to the conditions of a given physical system, has meant that SOC as studied in these fields has sometimes difB N.W. Watkins NickWatkins@mykolab.com G. Pruessner g.pruessner@imperial.ac.uk S.C. Chapman s.c.chapman@warwick.ac.uk N.B. Crosby Norma.Crosby@aeronomie.be H.J. Jensen h.jensen@imperial.ac.uk 1 Max Planck Institute for the Physics of Complex Systems, Dresden, Germany 2 London School of Economics and Political Science, London, United Kingdom 3 Open University, Milton Keynes, United Kingdom 4 University of Warwick, Coventry, United Kingdom 5 Imperial College London, London, United Kingdom 6 UiT The Arctic University of Norway, Tromso, Norway 7 Belgian Institute for Space Aeronomy, Brussels, Belgium