Contributed Talk - Splinter ISM

Thursday, 24 September 2020, 11:49   (virtual room F)

Inverse transfer of magnetic helicity in supersonic isothermal MHD turbulence

J.-M. Teissier, W.-C. Müller
TU Berlin

Magnetic helicity, which quantifies topological aspects of the magnetic field, is an ideal invariant of the three-dimensional magnetohydrodynamic (MHD) equations that is dynamically important in many astrophysical situations, where helical fields occur naturally. Contrary to kinetic energy which cascades to ever smaller scales where dissipation dominates, magnetic helicity exhibits an inverse transport in spectral space, forming large-scale magnetic structures. Up to the present day, this inverse transfer has been mostly studied in the incompressible or the subsonic range, which is often far from realistic. We investigate the inverse transfer of magnetic helicity in supersonic isothermal turbulence through direct numerical simulations by injecting small-scale magnetic helical fluctuations in hydrodynamic turbulent steady-states generated and sustained through a large-scale mechanical forcing. The turbulent Mach number range explored is 0.1--10, which covers typical values encountered in the interstellar medium (ISM). The two extreme cases of a purely solenoidal and a purely compressive mechanical driving are compared. For a solenoidal driving, the changes in the spectral scaling laws are relatively low, even at Mach numbers of the order of 10, whereas they are significant already at around Mach 3 for compressive driving. This means that compressive effects in the ISM can be relevant for the magnetic field dynamics at relatively low Mach numbers if the turbulence drivers are rather compressive. A dynamical balance in spectral space found in the incompressible case is extended to compressible MHD by an appropriate change of variables.