  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.
