On the connection between the magneto-elliptic and magneto-rotational instabilities

TL;DR

This paper unifies the magneto-elliptic instability (MEI) and magneto-rotational instability (MRI) as manifestations of a single Magneto-Elliptic-Rotational Instability (MERI), clarifying their relationship and growth dynamics in rotating magnetic flows.

Contribution

It reveals the connection between MEI and MRI, demonstrating they are different expressions of MERI, and analyzes their growth rates and physical mechanisms.

Findings

MEI and MRI are unified as MERI.

Growth rates of MEI and MRI coincide under certain conditions.

Magnetic and rotational effects influence instability ranges.

Abstract

It has been recently suggested that the magneto-rotational instability (MRI) is a limiting case of the magneto-elliptic instability (MEI). This limit is obtained for horizontal modes in the presence of rotation and an external vertical magnetic field, when the aspect ratio of the elliptic streamlines tends to infinite. In this paper we unveil the link between these previously unconnected mechanisms, explaining both the MEI and the MRI as different manifestations of the same Magneto-Elliptic-Rotational Instability (MERI). The growth rates are found and the influence of the magnetic and rotational effects is explained, in particular the effect of the magnetic field on the range of negative Rossby numbers at which the horizontal instability is excited. Furthermore, we show how the horizontal rotational MEI in the rotating shear flow limit links to the MRI by the use of the local shearing box model, typically used in the study of accretion discs. In such limit the growth rates of the two instability types coincide for any power-type background angular velocity radial profile with negative exponent corresponding to the value of the Rossby number of the rotating shear flow. The MRI requirement for instability is that the background angular velocity profile is a decreasing function of the distance from the centre of the disk which corresponds to the horizontal rotational MEI requirement of negative Rossby numbers. Finally a physical interpretation of the horizontal instability, based on a balance between the strain, the Lorentz force and the Coriolis force is given.