Swirling astrophysical flows – efficient amplifiers of Alfvén waves!?

¹ Dipartimento di Fisica Generale, Universitá degli Studi di Torino, Via Pietro Giuria 1, Torino 10125, Italy
² Osservatorio Astronomico di Torino, Strada dell'Osservatorio 20, 10025 Pino Torinese, Italy
³ Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, Trieste 34014, Italy
⁴ Institute for Fusion Studies, The University of Texas at Austin, Texas 78712, USA

Corresponding author: A. D. Rogava, andro@ictp.trieste.it

Received: 27 November 2001
Accepted: 25 November 2002

Abstract

We show that a helical shear flow of a magnetized plasma may serve as an efficient amplifier of Alfvén waves. We find that even when the flow is purely ejectional (i.e., when no rotation is present) Alfvén waves are amplified through the transient, shear-induced, algebraic amplification process. Series of transient amplifications, taking place sequentially along the flow, may result in a cascade amplification of these waves. However, when a flow is swirling or helical (i.e., some rotation is imposed on the plasma motion), Alfvén waves become subject to new, much more powerful shear instabilities. In this case, depending on the type of differential rotation, both usual and parametric instabilities may appear. We claim that these phenomena may lead to the generation of large amplitude Alfvén waves and the mechanism may account for the appearance of such waves in the solar atmosphere, in accretion-ejection flows and in accretion columns. These processes may also serve as an important initial (linear and nonmodal) phase in the ultimate subcritical transition to MHD Alfvénic turbulence in various kinds of astrophysical shear flows.