The formation of large-scale structures in supersonic magnetohydrodynamic flows

In this paper we investigate the existence of an inverse cascade of magnetic helicity for statistically steady compressible magnetohydrodynamic flows with rms Mach numbers that go up to unity. This has been done by performing a set of three-dimensional numerical simulations that were carried out using the RIEMANN code for astrophysical fluid and magnetofluid dynamics [D. S. Balsara, Astrophys. J. Suppl. 116, 133 (1988)]. The random forcing is made up of ABC flows corresponding to helical wave patterns at an intermediate scale. Both time histories of integral quantities and Fourier spectra are shown, illustrating the inverse cascade process. The observed build-up of large-scale magnetic helical structures for a wide range of Mach numbers indicates that, in the context of the generation of the magnetic field of the Galaxy where supersonic flows are common, a dynamo mechanism based on the nonlinear dynamics of magnetic helicity may be at work. (C) 1999 American Institute of Physics. [S1070-664X(99)00901-5].