Comment on "Helical MRI in magnetized Taylor-Couette flow"

TL;DR

This paper challenges previous claims by demonstrating that the helical magnetorotational instability (HMRI) can exist in Keplerian rotation profiles if radial boundary conditions are properly considered, especially with conducting boundaries.

Contribution

It reveals that boundary conditions are crucial for the existence of HMRI in Keplerian flows, contradicting earlier analyses that neglected these effects.

Findings

HMRI can occur in Keplerian profiles with conducting boundaries.

Radial boundary conditions are essential for HMRI existence.

Previous WKB analysis overlooked boundary effects.

Abstract

Liu et al. [Phys. Rev. E 74, 056302 (2006)] have presented a WKB analysis of the helical magnetorotational instability (HMRI), and claim that it does not exist for Keplerian rotation profiles. We show that if radial boundary conditions are included, the HMRI can exist even for rotation profiles as flat as Keplerian, provided only that at least one of the boundaries is sufficiently conducting.