On the resilience of helical magnetic fields to turbulent diffusion and the astrophysical implications

Understand

The extent to which large scale magnetic fields are susceptible to turbulent diffusion is important for interpreting the need for in situ large scale dynamos in astrophysics and for observationally inferring field strengths compared to kinetic energy.

• By solving coupled equations for magnetic energy and magnetic helicity in a system initiated with isotropic turbulence and an arbitrarily helical large scale field, we quantify the decay rate of the latter for a bounded or periodic system.
• The energy associated with the non-helical magnetic field rapidly decays by turbulent diffusion, but the decay rate of the helical component depends on whether the ratio of its magnetic energy to the turbulent kinetic energy exceeds a critical value given by M_{1,c} =(k_1/k_2)^2, where k_1 and k_2 are the wave numbers of the large and forcing scales.
• Turbulently diffusing helical fields to small scales while conserving magnetic helicity requires a rapid increase in total magnetic energy.