Spatio-temporal behavior of magnetohydrodynamic fluctuations with cross-helicity and background magnetic field

LUGONES, R.; POUQUET, A.; MININNI, P.D.; DMITRUK, P.; MATTHAEUS, W.H.

PHYSICS OF PLASMAS

AMER INST PHYSICS

Lugar: New York; Año: 2019 vol. 26 p. 1223701 - 1223711

1070-664X

We study the spatiotemporal behavior of the Els?asser variables describing magnetic and velocity field fluctuations, using direct numericalsimulations of three-dimensional magnetohydrodynamic turbulence. We consider cases with relatively small, intermediate, and large valuesof a mean background magnetic field and with null, small, and high cross-helicity (correlations between the velocity and the magnetic field).Wavenumber-dependent time correlation functions are computed for the different simulations. From these correlation functions, the decor-relation time is computed and compared with different theoretical characteristic times: the local nonlinear time, the random-sweeping time,and the Alfvenic time. It is found that decorrelation times are dominated by sweeping effects for low values of the mean magnetic field andfor low values of the cross-helicity, while for large values of the background field or of the cross-helicity and for wave vectors sufficientlyaligned with the guide field, decorrelation times are controlled by Alfvenic effects. Finally, we observe counterpropagation of Alfvenic fluctua-tions due to reflections produced by inhomogeneities in the total magnetic field. This effect becomes more prominent in flows with largecross-helicity, strongly modifying the propagation of waves in turbulent magnetohydrodynamic flows.