Transverse oscillations in solar coronal loops induced by propagating Alfvénic pulses

¹ Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy e-mail: ldz@arcetri.astro.it
² Department of Astrophysics, University of Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands

Received: 23 August 2004
Accepted: 26 October 2004

Abstract

The propagation and the evolution of Alfvénic pulses in the solar coronal arcades is investigated by means of MHD numerical simulations. Significant transverse oscillations in coronal loops, triggered by nearby flare events, are often measured in EUV lines and are generally interpreted as standing kink modes. However, the damping times of these oscillations are typically very short (from one to a few periods) and the physical mechanism responsible for the decay is still a matter of debate. Moreover, the majority of the observed cases actually appears to be better modeled by propagating, rather than standing, modes. Here we perform 2.5D compressible MHD simulations of impulsively generated Alfvén waves propagating in a potential magnetic arcade (assumed as a simplified 2D loop model), taking into account the stratification of the solar atmosphere with height from the photosphere to the corona. The results show a strong spreading of the initially localized pulses along the loop, due to the variations in the Alfvén velocity with height, and correspondingly an efficient damping of the amplitude of the oscillations. We believe that simple explanations based on the effects of wave propagation in highly inhomogeneous media may apply to the majority of the reported cases, and that variations of the background density and Alfvén speed along the loop should be considered as key ingredients in future models.