A notion of vortex is important in the plasma physics as much as in the fluid physics because of its indispensable roles in various phenomena. For example, vortices on the Sun advect magnetic lines, to twist magnetic lines, process of which was modeled and investigated by Amo et al.[1] from a point of view of self-organization of plasmas. In MHD simulations of rotating spherical shells in which fluid motions generate dipolar magnetic field structures, vortical columns are most dominant coherent structure as has been studied by Kageyama and Sato[2] and Kitauchi and Kida.[3] Especially, a complex topology of the velocity field was observed by Kitauchi et al.[4]. Vortical motions bring complex structures and strong mixing of fluids. Hence, it is important to identify and visualize vortex structures in order to understand complex behavior of plasmas.
Despite of such an easy use of the word ``vortex'' in everyday life, we must admit that there is no objective definition of vortex in fluid mechanics. Streamlines or vorticity lines are sometimes used to identify vortices. However, they often fail because the former is not Galilean invariant and the latter can not distinguish the shearing motions from swirling. Since coherent structures were found in fully developed turbulence, the definition and the identification of vortex structures have been one of hot and tough topics in research of fluid mechanics. Many schemes have been proposed so far, but unfortunately, none of them seem to have become ``de facto standard'' method of vortex identification. For discussion of various definition of a vortex, see Robinson[5], Lugt[6], Jeong and Hussain[7], or Kida and Miura[8].
Recently, we have developed a sectional-pressure-minimum-and-swirl (SPMS) method[8] which identifies central axis (skeletons) of swirling motions and cores. In this paper, we apply this method to a homogeneous and isotropic turbulence of a neutral fluid to investigate its statistics and dynamics.