Modeling of the turbulent cross-helicity dissipation rate: Comparison using the solar-wind observations

Details Modeling of the turbulent cross-helicity dissipation rate: Comparison using the solar-wind observations Modeling of the turbulent cross-helicity dissipation rate: Comparison using the solar-wind observations

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh31a1664y&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #SH31A-1664

Physics

Plasma Physics

7500 Solar Physics, Astrophysics, And Astronomy, 7544 Stellar Interiors And Dynamo Theory, 7800 Space Plasma Physics, 7859 Transport Processes, 7863 Turbulence (4490)

Scientific paper

The turbulent cross helicity (velocity--magnetic-field correlation of turbulence) W ≡ , as well as the turbulent magnetohydrodynamic (MHD) energy K ≡ < u'2 + b'2 > / 2, is a quantity of primary importance which represents statistical properties of turbulence. The presence of the cross helicity in turbulence may alter the transport properties of turbulence, then it affects the magnitude and configuration of large-scale fields much. A typical example is the turbulent dynamo. If the cross helicity exists in turbulence accompanied by the large-scale vortical motions, electromotive force parallel to the vorticity is induced. This may counterbalance a huge magnetic diffusivity due to turbulence, and work for the magnetic-field generation. Although spacecraft observations of solar-wind turbulence have provided precious information on the turbulent cross helicity, their results have not been fully utilized in the studies of the MHD turbulence modeling. As for the dissipation rate of the turbulent cross helicity, ɛW, very little is known. This is in marked contrast with the dissipation rate of the turbulent energy, ɛ, whose model equation has long been discussed. We propose a few models for the turbulent cross-helicity dissipation rate ɛW: an algebraic model, a model equation for ɛW evolution, etc. Using comparison with the large-scale behavior of the cross helicity obtained by several solar-wind observations, we evaluate these models. The detailed observations by Roberts et al. (1987) inferred that in the absence of flow shear the turbulent cross helicity W remains to be relatively large value as the heliocentric distance increases. We will show that a turbulence model simulation with the algebraic model of ɛW can reproduce this W behavior with a reasonable model constant. Further discussions including the model equation for the ɛW evolution will be also presented.

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