Ixture of classical MHD discontinuities that advect with the wind [80,97]. This perspective stands in contrast to a number of features of interplanetary observations that are consistent with an active turbulence cascade, including, for example, reduction of Alfv icity, lowering of the Alfv ratio, development of anisotropy, evolution of the correlation scale [98,99], observed plasma heating [100,101] and the direct measurement of cascade rates [18,102,103]. The recent revival of the so-called spaghetti model of solar wind flux tubes [65,104,105] treats flux tubes and their boundaries, which are static discontinuities, as inert remnants of coronal processes. The interpretation goes so far as to separate the flux tubes and discontinuities from other fluctuations, the latter regarded as `turbulence’. The various studies alluded to in this section provide a growing body of evidence that the structure that is observed is a consequence of turbulence, either coronal or interplanetary, or both. It also seems difficult not to conclude that coherent structures are involved in the evolution of the solar wind turbulence. Intermittency and structure are therefore an integral part of the cascade, and not a separate inert component. There is still much to be understood about solar wind turbulence and dynamics even at MHD scales. We will now turn to an even more difficult topic, the nature of intermittency at kinetic scales, where the dissipation and heating in plasmas such as the solar wind must occur.rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 373:…………………………………………………6. Plasma intermittency at kinetic scalesIn hydrodynamics, examination of intermittency at scales smaller than the inertial range means characterizing the structures associated with viscous dissipation. MHD intermittency remains less well understood than hydrodynamics but maintains the clarity of well-defined resistive and viscous dissipation functions. An astrophysical plasma such as the solar wind however is only Lonafarnib site weakly collisional, and numerous plasma processes may influence what is seen at scales smaller than the ion kinetic scales. On the other hand dissipation and heating occupy central roles in LOXO-101 chemical information understanding the very existence of the solar wind, and therefore these processes are fundamental in a description of the geospace environment and the heliosphere. There are numerous questions that need to be addressed regarding the operative mechanisms and the relative roles of various plasma particle species–electrons, protons, alpha particles, minor species and suprathermalparticles of each type. Intermittency and structure may enter into this environment, but it is expected to be more complex than in the fluid models based on the multiplicity of plasma and electromagnetic variables and the numerous associated length and time scales. Here we briefly review some progress that plasma simulation has been able to make in revealing the complex physics of kinetic plasma intermittency and dissipation. In some sense one may view the collection of excellent work that has been done on collisionless or weakly collisional magnetic reconnection as a programme of study of kinetic-scale intermittency and coherent structures (e.g. [106?10]). Most of these studies regard reconnection and current sheet dynamics as isolated processes, which for the present purposes is a shortcoming given that coherent structures form and evolve in turbulence as an integral p.Ixture of classical MHD discontinuities that advect with the wind [80,97]. This perspective stands in contrast to a number of features of interplanetary observations that are consistent with an active turbulence cascade, including, for example, reduction of Alfv icity, lowering of the Alfv ratio, development of anisotropy, evolution of the correlation scale [98,99], observed plasma heating [100,101] and the direct measurement of cascade rates [18,102,103]. The recent revival of the so-called spaghetti model of solar wind flux tubes [65,104,105] treats flux tubes and their boundaries, which are static discontinuities, as inert remnants of coronal processes. The interpretation goes so far as to separate the flux tubes and discontinuities from other fluctuations, the latter regarded as `turbulence’. The various studies alluded to in this section provide a growing body of evidence that the structure that is observed is a consequence of turbulence, either coronal or interplanetary, or both. It also seems difficult not to conclude that coherent structures are involved in the evolution of the solar wind turbulence. Intermittency and structure are therefore an integral part of the cascade, and not a separate inert component. There is still much to be understood about solar wind turbulence and dynamics even at MHD scales. We will now turn to an even more difficult topic, the nature of intermittency at kinetic scales, where the dissipation and heating in plasmas such as the solar wind must occur.rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 373:…………………………………………………6. Plasma intermittency at kinetic scalesIn hydrodynamics, examination of intermittency at scales smaller than the inertial range means characterizing the structures associated with viscous dissipation. MHD intermittency remains less well understood than hydrodynamics but maintains the clarity of well-defined resistive and viscous dissipation functions. An astrophysical plasma such as the solar wind however is only weakly collisional, and numerous plasma processes may influence what is seen at scales smaller than the ion kinetic scales. On the other hand dissipation and heating occupy central roles in understanding the very existence of the solar wind, and therefore these processes are fundamental in a description of the geospace environment and the heliosphere. There are numerous questions that need to be addressed regarding the operative mechanisms and the relative roles of various plasma particle species–electrons, protons, alpha particles, minor species and suprathermalparticles of each type. Intermittency and structure may enter into this environment, but it is expected to be more complex than in the fluid models based on the multiplicity of plasma and electromagnetic variables and the numerous associated length and time scales. Here we briefly review some progress that plasma simulation has been able to make in revealing the complex physics of kinetic plasma intermittency and dissipation. In some sense one may view the collection of excellent work that has been done on collisionless or weakly collisional magnetic reconnection as a programme of study of kinetic-scale intermittency and coherent structures (e.g. [106?10]). Most of these studies regard reconnection and current sheet dynamics as isolated processes, which for the present purposes is a shortcoming given that coherent structures form and evolve in turbulence as an integral p.
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