Nonlinear evolution of fluting oscillations in coronal flux tubes

Abstract

[eng] Magnetic flux tubes in the solar corona support a rich variety of transverse oscillations, which are theoretically interpreted as magnetohydrodynamic (MHD) modes with a fast and/or Alfvénic character. In the standard flux tube model made of a straight cylindrical tube, these modes can be classified according to their azimuthal wavenumber, m. Sausage m = 0 modes produce periodic expansion and contraction of the tube cross section and are observed during solar flares. Kink m = 1 modes laterally displace the tube axis and are related to, for example, post-flare global transverse oscillations of coronal loops. Fluting m ≥ 2 modes produce disturbances that are mainly confined to the tube boundary, but their observation remains elusive to date. We use 3D ideal MHD numerical simulations to investigate the nonlinear evolution of fluting modes in coronal flux tubes with transversely nonuniform boundaries. The simulations show that fluting modes are short-lived as coherent, collective motions of the flux tube. Owing to the process of resonant absorption, fluting oscillations become overdamped modes in tubes with wide enough nonuniform boundaries. During the nonlinear evolution, shear flows drive the Kelvin-Helmholtz instability at the tube boundary, which further disrupts the coherent fluting oscillation. For large-enough oscillation amplitudes, baroclinic instabilities of Rayleigh-Taylor type are also present at locations in the boundary where the plasma acceleration is normal to the boundary. The evolution of the instabilities drives turbulence in the flux tube, which may inhibit the resonant damping. However, the oscillations remain strongly damped even in this case. As a result of the combination of the strong damping and the induced instabilities, it is unlikely that coronal flux tubes can support fluting modes as sufficiently enduring coherent oscillations.