Joint inverse cascade of magnetic energy and magnetic helicity in MHD turbulence

TL;DR

This paper demonstrates that in MHD turbulence, magnetic energy and helicity undergo a joint inverse cascade, with small-scale helicity sources influencing large-scale magnetic energy dynamics, relevant for astrophysical dynamos.

Contribution

It reveals the simultaneous inverse cascade of magnetic energy and helicity in MHD turbulence with small-scale helicity sources, highlighting a new mechanism affecting dynamo processes.

Findings

Inverse cascade of magnetic energy linked to magnetic helicity.

Spectral slope approaches -3/2 with high relative helicity.

Small-scale helicity sources can drive large-scale magnetic energy growth.

Abstract

We show that oppositely directed fluxes of energy and magnetic helicity coexist in the inertial range in fully developed magnetohydrodynamic (MHD) turbulence with small-scale sources of magnetic helicity. Using a helical shell model of MHD turbulence, we study the high Reynolds number magnetohydrodynamic turbulence for helicity injection at a scale that is much smaller than the scale of energy injection. In a short range of scales larger than the forcing scale of magnetic helicity, a bottleneck-like effect appears, which results in a local reduction of the spectral slope. The slope changes in a domain with a high level of relative magnetic helicity, which determines that part of the magnetic energy related to the helical modes at a given scale. If the relative helicity approaches unity, the spectral slope tends to $-3/2$. We show that this energy pileup is caused by an inverse cascade of magnetic energy associated with the magnetic helicity. This negative energy flux is the contribution of the pure magnetic-to-magnetic energy transfer, which vanishes in the non-helical limit. In the context of astrophysical dynamos, our results indicate that a large-scale dynamo can be affected by the magnetic helicity generated at small scales. The kinetic helicity, in particular, is not involved in the process at all. An interesting finding is that an inverse cascade of magnetic energy can be provided by a small-scale source of magnetic helicity fluctuations without a mean injection of magnetic helicity.