Abstract
Spatial structure of small-scale plasma turbulence is studied under different conditions of plasma parameter beta directly in the three-dimensional wave vector domain. Two independent approaches are taken: observations of turbulent magnetic field fluctuations in the solar wind measured by four Cluster spacecraft, and direct numerical simulations of plasma turbulence using the hybrid code AIKEF, both resolving turbulence on the ion kinetic scales. The two methods provide independently evidence of wave vector anisotropy as a function of beta. Wave vector anisotropy is characterized primarily by an extension of the energy spectrum in the direction perpendicular to the large-scale magnetic field. The spectrum is strongly anisotropic at lower values of beta, and is more isotropic at higher values of beta. Cluster magnetic field data analysis also provides evidence of axial asymmetry of the spectrum in the directions around the large-scale field. Anisotropy is interpreted as filament formation as plasma evolves into turbulence. Axial asymmetry is interpreted as the effect of radial expansion of the solar wind from the corona.
Highlights
Plasmas in astrophysical systems often evolve into turbulence
While astrophysical plasma turbulence inherits some physical properties from fluid turbulence, the collisionless nature and the coupling with electromagnetic field introduce a larger degree of freedom in physical processes and make the picture of turbulence considerably different from that of hydrodynamic one
We find studying small-scale structures of plasma turbulence interesting for the reasons that there is a larger degree of freedom in the Fourier domain when spatial scales are small and that physical processes are diverse as dissipation of fluctuation energy and dispersion of wave packets may co-exist on the small scales
Summary
Plasmas in astrophysical systems often evolve into turbulence. Random and stochastic motions of plasmas and magnetic fields can be found, for example, in the solar atmosphere, interplanetary space, interstellar space, and accretion disks.
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