Amplification of MHD waves in swirling astrophysical flows

¹ Dipartimento di Fisica Generale, Universitá degli Studi di Torino, Via Pietro Giuria 1, Torino 10125, Italy
² Abdus Salam International Centre for Theoretical Physics, Trieste 34014, Italy
³ Osservatorio Astronomico di Torino, Strada dell'Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Centre for Plasma Astrophysics, Abastumani Astrophysical Observatory, Tbilisi–380060, Georgia

Corresponding author: A. D. Rogava, rogava@ph.unito.it

Received: 2 January 2003
Accepted: 18 March 2003

Abstract

Recently it was found that helical magnetized flows efficiently amplify Alfvén waves ([CITE]). This robust and manifold nonmodal effect was found to involve regimes of transient algebraic growth (for purely ejectional flows), and exponential instabilities of both usual and parametric nature. However the study was made in the incompressible limit and an important question remained open – whether this amplification is inherent to swirling MHD flows per se and what is the degree of its dependence on the incompressibility condition. In this paper, in order to clear up this important question, we consider full compressible spectrum of MHD modes: Alfvén waves (AW), slow magnetosonic waves (SMW) and fast magnetosonic waves (FMW). We find that helical flows inseparably blend these waves with each other and make them unstable, creating the efficient energy transfer from the mean flow to the waves. The possible role of these instabilities for the onset of the MHD turbulence, self-heating of the flow and the overall dynamics of astrophysical flows are discussed.