On the relation of standard and helical magnetorotational instability

TL;DR

This paper explores the relationship between standard and helical magnetorotational instabilities, revealing how spectral exceptional points facilitate transitions and identifying new instability regimes in liquid metal experiments.

Contribution

It uncovers the mechanism linking SMRI and HMRI via spectral exceptional points and explores parameter regions with new HMRI instabilities.

Findings

Spectral exceptional points explain the transition from SMRI to HMRI.

HMRI can be excited in the inductionless limit.

New islands of HMRI are identified in parameter space.

Abstract

The magnetorotational instability (MRI) plays a crucial role for cosmic structure formation by enabling turbulence in Keplerian disks which would be otherwise hydrodynamically stable. With particular focus on MRI experiments with liquid metals, which have small magnetic Prandtl numbers, it has been shown that the helical version of this instability (HMRI) has a scaling behaviour that is quite different from that of the standard MRI (SMRI). We discuss the relation of HMRI to SMRI by exploring various parameter dependencies. We identify the mechanism of transfer of instability between modes through a spectral exceptional point that explains both the transition from a stationary instability (SMRI) to an unstable travelling wave (HMRI) and the excitation of HMRI in the inductionless limit. For certain parameter regions we find new islands of the HMRI.