PhD Seminar: Torsional Alfvén waves can(not) trigger turbulence in nonuniform coronal loops

Title: Torsional Alfvén waves can(not) trigger turbulence in nonuniform coronal loops

Abstract: High-resolution observations have shown the ubiquity of Alfvénic waves in the solar atmosphere. Recently, torsional Alfvén waves have been reported at coronal heights. Here, we perform numerical simulations of torsional Alfvén waves. The model consists of a cylindrical, radially inhomogeneous, straight coronal flux tube permeated by a twisted magnetic field where the twist is a free parameter of the model. The flux tube is line-tied at two rigid walls representing the photosphere. Standing torsional Alfvén waves are excited by perturbing the component of velocity perpendicular to the magnetic field lines. The nonlinear evolution of these waves is obtained with the PLUTO code, which solves the ideal three-dimensional MHD equations using a finite-volume discretization and uses the adaptive mesh refinement technique. Torsional Alfvén waves undergo the process of phase-mixing owing to the nonuniform density and magnetic field. This phenomenon generates small scales. After few periods of torsional waves, the perpendicular-to-magnetic-field shear flows can trigger the Kelvin-Helmholtz instability (KHi), if the magnetic twist is absent or weak. The nonlinear evolution of KHi naturally leads to turbulence, which speeds up the energy transport to small scales. Nonetheless, if the magnetic twist is strong enough, the nonlinear evolution of KHi is suppressed by the magnetic tension and turbulence is absent. Consequently, phase-mixing remains as the only mechanism of energy transport to small scales in strongly twisted tubes.