Large scale dynamos with ambipolar diffusion nonlinearity

TL;DR

This paper demonstrates that ambipolar diffusion nonlinearities produce large-scale turbulent dynamo behavior similar to full MHD, with large-scale field growth via inverse cascade and alpha-effect, influenced by magnetic Reynolds number.

Contribution

It shows that ambipolar diffusion can serve as a simplified model to replicate large-scale dynamo behavior seen in full MHD simulations.

Findings

Large-scale magnetic fields develop through inverse cascade.

Growth time increases with magnetic Reynolds number.

Ambipolar diffusion mimics full MHD dynamo behavior.

Abstract

It is shown that ambipolar diffusion as a toy nonlinearity leads to very similar behaviour of large scale turbulent dynamos as full MHD. This is demonstrated using both direct simulations in a periodic box and a closure model for the magnetic correlation functions applicable to infinite space. Large scale fields develop via a nonlocal inverse cascade as described by the alpha-effect. However, because magnetic helicity can only change on a resistive timescale, the time it takes to organize the field into large scales increases with magnetic Reynolds number.