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Astron. Astrophys. 354, 749-759 (2000)

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Shear-driven wave oscillations in astrophysical flux tubes

A.D. Rogava  *  1,2, S. Poedts  **  1 and S.M. Mahajan 2,3

1 Centre for Plasma Astrophysics, K.U.Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
2 Abdus Salam International Centre for Theoretical Physics, 34014 Trieste, Italy
3 Institute for Fusion Studies, The University of Texas at Austin, TX 78712, USA

Received 7 June 1999 / Accepted 19 November 1999


In plane-parallel flows velocity shear couples magnetohydrodynamic (MHD) wave modes and induces their mutual transformations. Since the majority of astrophysical flows are not plane-parallel it is important to clarify whether this nonmodal phenomenon also takes place in flows with a more complicated spatial geometry and kinematics. The recently devised local method for studying linear perturbation dynamics in flows with arbitrary kinematic complexity is tailor-made for this actual problem. In this paper we apply this new method to the study of velocity shear induced wave transformations in a cylindrical flux tube . We found that the MHD modes sustained by the flux tube flow-the Alfvén (AW), the slow magnetosonic (SMW), and the fast magnetosonic (FMW) waves - are efficiently coupled through the agency of the velocity shear. Based on this issue we argue that the individual wave transformation events, happenning perpetually and irregularly in the whole space occupied by the flux tube flow, establish the regime of shear induced wave oscillations throughout the flow. We claim that this previously overlooked linear phenomenon may be important for the generation of solar hydromagnetic waves, for the transmission of the waves through the transition region, for coronal heating and for the acceleration of the solar wind.

Key words: magnetohydrodynamics (MHD) – plasmas – waves – methods: analytical – methods: numerical

* On leave from Abastumani Astrophysical Observatory, Kazbegi avenue 2a, 380060 Tbilisi; and Department of Physics, Tbilisi State University, Chavchavadze avenue 2, 380028 Tbilisi, Republic of Georgia
** Research Associate of the Belgian National Fund for Scientific Research (NFWO).

Send offprint requests to: Andria D. Rogava

© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000