Three-dimensional simulations of turbulent spectra in the local interstellar medium

Abstract

Abstract. Three-dimensional time dependent numerical simulations of compressible magnetohydrodynamic fluids describing super-Alfvénic, supersonic and strongly magnetized space and laboratory plasmas show a nonlinear relaxation towards a state of near incompressibility. The latter is characterized essentially by a subsonic turbulent Mach number. This transition is mediated dynamically by disparate spectral energy dissipation rates in compressible magnetosonic and shear Alfvénic modes. Nonlinear cascades lead to super-Alfvénic turbulent motions decaying to a subAlfvénic regime that couples weakly with (magneto)acoustic cascades. Consequently, the supersonic plasma motion is transformed into highly subsonic motion and density fluctuations experience a passive convection. This model provides a self-consistent explaination of the ubiquitous nature of incompressible magnetoplasma fluctuations in the solar wind and the interstellar medium.

Theory and simulations of principle of minimum dissipation rate

“…This can be elucidated as follows. For instance, an initial large value of plasma beta essentially characterizes the predominance of hydrodynamiclike effects in MHD, 24,25 where the nonlinear processes are governed predominantly by the eddy interactions, i.e., hydrodynamic convective force V · ١V, instead of magnetic field forces ͑J ϫ B͒. In such a case, the final state is dominated possibly by the hydrodynamic invariants.…”

Turbulent Disks Are Never Stable: Fragmentation and Turbulence-Promoted Planet Formation

“…In fact, while they apply strictly to only isothermal (non-intermittent) turbulence (as discussed in Section 4), the analytic derivation of lognormal PDFs actually assumes small (local) Mach numbers (see Nordlund & Padoan 1999). The lognormal model (with the higher-order intermittency corrections we include in Section 5) and the simple assumptions we use for the scaling of the density power spectrum with velocity power spectrum have been tested in the sub-sonic limit in both simulations (see Shaikh & Zank 2007;Kowal et al 2007;Burkhart et al 2009;Schmidt et al 2009;Konstandin et al 2012) and also in experimental data from the solar wind (Burlaga 1992;Forman & Burlaga 2003;Leubner & Vörös 2005) and laboratory MHD plasmas (Budaev 2008), as well as jet experiments (Ruiz-Chavarria et al 1996;Warhaft 2000;Zhou & Xia 2010). The power spectrum predictions in this limit, in fact, generically follow the well-known and tested weakly compressible Kolmogorov-like scaling (Montgomery et al 1987;.…”