Decoupled Cascades of Kinetic and Magnetic Energy in Magnetohydrodynamic Turbulence

Magnetic energy (ME) and kinetic energy (KE) in ideal incompressible magnetohydrodynamics are not global invariants and, therefore, it has been justified to discuss only the cascade of their sum total energy. We provide a physical argument based on scale locality, along with compelling evidence that ME and KE budgets statistically decouple beyond a transitional "conversion" range. This arises because magnetic field-line stretching is a large-scale process which vanishes on average at intermediate and small scales within the inertial-inductive range, thereby allowing each of the mean ME and KE to cascade conservatively and at an equal rate, yielding a turbulent magnetic Prandtl number of unity over these scales.