2020/01/30 by A. Pouquet, J. E. Stawarz, Pouquet, Annick +3
Physics and Astronomy · Biochemistry, Genetics and Molecular Biology · Earth and Planetary Sciences · #Solar and Space Plasma Dynamics #Geomagnetism and Paleomagnetism Studies #Geophysics and Gravity Measurements
paper · pdf · doi:10.48550/arxiv.2001.11625
In turbulence, for neutral or conducting fluids, a large ratio of scales is\nexcited because of the possible occurrence of inverse cascades to large, global\nscales together with direct cascades to small, dissipative scales, as observed\nin the atmosphere and oceans, or in the solar environment. In this context,\nusing direct numerical simulations with forcing, we analyze scale dynamics in\nthe presence of magnetic fields with a generalized Ohm's law including a Hall\ncurrent. The ion inertial length epsilonH serves as the control parameter at\nfixed Reynolds number. Both the magnetic and generalized helicity -- invariants\nin the ideal case -- grow linearly with time, as expected from classical\narguments. The cross-correlation between the velocity and magnetic field grows\nas well, more so in relative terms for a stronger Hall current. We find that\nthe helical growth rates vary exponentially with epsilonH, provided the ion\ninertial scale resides within the inverse cascade range. These exponential\nvariations are recovered phenomenologically using simple scaling arguments.\nThey are directly linked to the wavenumber power-law dependence of generalized\nand magnetic helicity, k^(-2), in their inverse ranges. This illustrates and\nconfirms the important role of the interplay between large and small scales in\nthe dynamics of turbulent flows.\n