Physics – Plasma Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm42b..04b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SM42B-04
Physics
Plasma Physics
[2149] Interplanetary Physics / Mhd Waves And Turbulence, [7863] Space Plasma Physics / Turbulence
Scientific paper
Three-dimensional numerical simulations of incompressible magnetohydrodynamic (MHD) turbulence are performed in the decaying and balanced cases and with a strong uniform magnetic field B0. The main property observed is a bi-dimensionalisation of the dynamics with an energy transfer towards small scales which mainly occurs in planes perpendicular to the B0 direction. For such flows it is convenient to study their properties in different wavenumber k⊥ - planes (where ⊥ and ∥ define respectively the perpendicular and parallel directions to B0). This study reveals the coexistence of two different regimes: Alfvén wave turbulence for the wave modes i.e. {k⊥,k∥>0} and strong turbulence for the 2D state i.e. {k⊥,k∥=0}. These two dynamics are connected together via the Alfvén wave resonance condition. For wave modes, an equipartition between the kinetic and magnetic energies is observed via oscillations around unity of the spectral Alfvén ratio. This behavior - predicted in Alfvén wave turbulence - may be seen as a first signature of wave turbulence. In the meanwhile, the 2D state dynamics is characterized by a smaller spectral Alfvén ratio with a value around 0.5. The nonlinear dynamics is also characterized by different 2D k⊥ - energy spectra (which are plotted at fixed k∥). The 2D state is characterized by spectra close to k⊥-3/2 similar to what it is generally found in 2D isotropic MHD turbulence. Spectra for wave modes are significantly different with, for example at k∥=1, spectra slightly steeper than k⊥-2 and close to k⊥-7/3 which may coincide with the intermediate regime before the formation of the exact solution found in wave turbulence. Finally, the 1D energy spectrum E(k⊥) exhibits a reduced inertial range with a scaling law apparently close to k⊥-5/3. It is interpreted as an average effect which may hide the true nature of the dynamics where wave and strong turbulence coexist. It could also explain the spectra f - 5/3 in the solar wind which is still not completely understood (Bigot et al., PRE 78, 066301, 2008).
Bigot Barbara
Galtier Sebastien
Politano Helene
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