# Inverse cascade of hybrid helicity in $B \Omega$-MHD turbulence

## TL;DR

This paper explores how solid-body rotation influences large-scale magnetic turbulence, revealing an inverse cascade of hybrid helicity that depends on the alignment of rotation and magnetic field, with implications for planetary and stellar magnetic fields.

## Contribution

It demonstrates the role of hybrid helicity in inverse magnetic transfer in rotating MHD turbulence and how alignment affects this process, providing new insights into rotating dynamo mechanisms.

## Key findings

- Inverse magnetic transfer occurs when rotation aligns with magnetic field.
- Hybrid helicity acts as an invariant driving inverse cascade.
- Alignment angle influences the strength of inverse transfer.

## Abstract

We investigate the impact of a solid-body rotation $\Omega_0$ on the large-scale dynamics of an incompressible magnetohydrodynamic turbulent flow in presence of a background magnetic field $\bf B_0$ and at low Rossby number. Three-dimensional direct numerical simulations are performed in a periodic box, at unit magnetic Prandtl number and with a forcing at intermediate wavenumber $k_f=20$. When $\Omega_0$ is aligned with $\bf B_0$ (i.e. $\theta \equiv \widehat{\left(\Omega_{0}, \bf B_0 \right)} = 0$), inverse transfer is found for the magnetic spectrum at $k<k_f$. This transfer is stronger when the forcing excites preferentially right-handed (rather than left-handed) fluctuations; it is smaller when $\theta>0$ and becomes weak when $\theta \ge 35^o$. These properties are understood as the consequence of an inverse cascade of hybrid helicity which is an inviscid/ideal invariant of this system when $\theta=0$. Hybrid helicity emerges, therefore, as a key element for understanding rotating dynamos. Implication of these findings on the origin of the alignment of the magnetic dipole with the rotation axis in planets and stars is discussed.