Long-term decaying evolution of MHD turbulence

TL;DR

This study uses numerical shell models to analyze the long-term decay of MHD turbulence, revealing two distinct evolution scenarios influenced by initial conditions and magnetic helicity conservation.

Contribution

It demonstrates the existence of two different decay scenarios in MHD turbulence and highlights the role of magnetic helicity in long-term energy dissipation behavior.

Findings

Two decay scenarios identified: rapid cross-helicity accumulation and sustained cascade.

Magnetic helicity conservation leads to highly helical magnetic fields.

Energy dissipation behavior varies significantly between scenarios.

Abstract

The free decay of MHD turbulence at large Reynolds numbers is studied numerically using a shell model. We study the statistical properties based on representative sample of realisations (128 realisations for each type of initial conditions) over the period of $10^5$ large-scale turnover times. The performed simulations show that the force-free non-helical MHD turbulence can demonstrate two different scenarios of evolution in spite of similar initial conditions. Within the first scenario, the cross-helicity accumulation is so fast that the energy cascade vanishes before significant magnetic energy dissipates. Then the system approaches the state of maximal cross-helicity. Within the second scenario, the cascade process continues to remain active until time $10^4$ in units of large-scale turnover time. Then the magnetic field becomes vastly helical due to magnetic helicity conservation. Thus the magnetic energy does not dissipate with kinetic energy.