Dynamo model for the inverse transfer of magnetic energy in a nonhelical decaying magnetohydrodynamic turbulence

Abstract

The inverse cascade of magnetic energy occurs when helicity or rotational instability exists in the magnetohydrodynamic (MHD) system. This well known phenomenon has been considered as a basis for the large scale magnetic field in universe. However nonhelical magnetic energy in a decaying MHD system also migrates toward the large scale, which holds vital clues to the origin of large scale magnetic field in a quiescent astrophysical system. Zeldovich's rope dynamo model is considered as a basic and symbolistic model of magnetic field amplification. However, the rope model assuming specific external forces like buoyancy or Coriolis force is not appropriate for a decaying turbulent system without any external force. So we suggest a new dynamo model based on magnetic induction equation excluding a forcing source. This model shows the expansion and growth of magnetic field (flux) is basically the redistribution of energy in the system. The transfer of magnetic energy is in fact a successive induction of magnetic field resulted from the interaction between the fluid motion and seed magnetic field. We also discuss about an analytic theorem based on the scaling invariant MHD equation.