Experimental confirmation of the standard magnetorotational instability mechanism with a spring-mass analogue

Derek M H Hung,Hantao Ji,Erik P Gilson,Kyle J Caspary,Eric G Blackman

doi:10.1038/s42005-018-0103-7

Abstract

The magnetorotational instability (MRI) has long been considered a plausibly ubiquitous mechanism to destabilize otherwise stable Keplerian flows to support radially outward transport of angular momentum. Such an efficient transport process would allow fast accretion in astrophysical objects such as stars and black holes to release copious kinetic energy that powers many of the most luminous sources in the universe. But the standard MRI under a purely vertical magnetic field has heretofore never been directly measured despite numerous efforts over more than a decade. Here we report an unambiguous laboratory demonstration of the spring-mass analogue to the standard MRI by comparing motion of a spring-tethered ball within different rotating flows. The experiment corroborates the theory: efficient outward angular momentum transport manifests only for cases with a weak spring in quasi-Keperian flow. Our experimental method accomplishes this in a new way, thereby connecting solid and fluid mechanics to plasma astrophysics.

Highlights

The magnetorotational instability (MRI) has long been considered a plausibly ubiquitous mechanism to destabilize otherwise stable Keplerian flows to support radially outward transport of angular momentum
Luminous and jetted sources in the universe, including quasars, X-ray binaries[7,8,9], pre-planetary nebulae[10,11], and gammaray bursts[12] are likely powered by the conversion of gravitational potential energy into kinetic energy and radiation, as matter accretes onto central engines[13]
We appeal to the known result that the dispersion relation of the MRI for an initially vertical magnetic field characterizes the motion of two masses tethered by a weak spring[16,22]

Summary

Introduction

The magnetorotational instability (MRI) has long been considered a plausibly ubiquitous mechanism to destabilize otherwise stable Keplerian flows to support radially outward transport of angular momentum. Such an efficient transport process would allow fast accretion in astrophysical objects such as stars and black holes to release copious kinetic energy that powers many of the most luminous sources in the universe. We appeal to the known result that the dispersion relation of the MRI for an initially vertical magnetic field characterizes the motion of two masses tethered by a weak spring[16,22]. It has been speculated[16] that this analogue might be experimentally testable in the laboratory, distinct from multi-tethered configurations that have been previously theoretically explored[45,46,47]

Methods

Results

Conclusion