Emergence of magnetic structure in supersonic isothermal magnetohydrodynamic turbulence

Abstract

The inverse transfer of magnetic helicity is a fundamental process which may explain large scale magnetic structure formation and sustainement. Until very recently, direct numerical simulations (DNS) of the inverse transfer in magnetohydrodynamics (MHD) turbulence have been done in incompressible MHD or at low Mach numbers only. We review first results obtained through DNS of the isothermal MHD equations at Mach numbers ranging from subsonic to about 10. The spectral exponent of the magnetic helicity spectrum becomes flatter with increasing compressibility. When considering the Alfv\'en velocity in place of the magnetic field however, results found in incompressible MHD, including a dynamic balance between shear and twist, can be extended to supersonic MHD. In the global picture of an inverse transfer of magnetic helicity, three phenomena are at work: a local direct transfer mediated by the large scale velocity field, a local inverse transfer mediated by the intermediate scale velocity field and a nonlocal inverse transfer mediated by the small scale velocity field. The compressive part of the velocity field is geometrically favored in the local direct transfer and contributes to the nonlocal inverse transfer, but plays no role in the local inverse transfer.Comment: Chapter in Helicities in Geophysics, Astrophysics and Beyond (AGU Books, Wiley, 2023 or 2024