Structure of MHD turbulence in large-Prandtl-number plasmas

In turbulent MHD systems where the ratio of fluid viscosity and magnetic diffusivity (the magnetic Prandtl number, Pr = ν/η) is very large, there exists a broad range of subviscous scales available to magnetic fluctuations, but not to hydrodynamic ones. This MHD regime is encountered, for example, in such astrophysical environments as the interstellar medium and protogalactic plasmas, where Pr can be as large as 10 to 10 [2]. Since the fluid is highly conducting, the magnetic-field lines are (nearly) perfectly frozen into the fluid flow. The fluid motions, even though restricted to the scales above the viscous cutoff, can excite magnetic fluctuations at much smaller scales via stretching and folding of the field lines. The resultant folding structure is characterized by very rapid transverse variation of the magnetic field B, which flips its direction at scales ultimately bounded from below only by the resistive length. However, the field lines remain largely unbent up to the scale of the flow [5] (cf. [4] and see more discussion below). Quantitatively, the intermittency of the field distribution and the field-line geometry can be studied in terms of the PDFs of the field strength B = |B| and of the field-line curvature K = |b̂ · ∇b̂| (here b̂ = B/B).