Numerical simulations of turbulence in prominence threads induced by torsional oscillations

Abstract

Threads are the main constituents of prominences and are subjected to oscillations that might be interpreted as MHD waves. Moreover, the Kelvin-Helmholtz instability (KHI) has been reported in prominences. Both waves and KHI may affect the thermodynamic state of the threads. We investigate the triggering of turbulence in a thread caused by the nonlinear evolution of standing torsional Alfv\'en waves as well as possible observational signatures of this dynamics and the plasma heating. We modeled the thread as a radially and longitudinally nonuniform cylindrical flux tube with a uniform axial magnetic field embedded in a coronal environment. We perturbed the flux tube with the longitudinally fundamental mode of standing torsional Alfv\'en waves and numerically solved the 3D MHD equations to study the temporal evolution in both ideal and dissipative scenarios. We also performed forward modeling to calculate the synthetic H{\alpha} imaging. Standing torsional Alfv\'en waves undergo phase-mixing owing to the radially nonuniform density. The phase-mixing generates azimuthal shear flows that eventually trigger the KHI and, later, turbulence. If nonideal effects are included, plasma heating is localized in an annulus region at the thread boundary and does not increase the temperature in the cool core. Instead, the average temperature in the thread decreases owing to the mixing of internal and external plasmas. In the synthetic observations, first we find periodic pulsations in the H{\alpha} intensity caused by the integration of the phase-mixing flows along the line of sight. Later, we find fine strands that may be associated with the KHI vortices. Turbulence can be generated by standing torsional Alfv\'en waves in a thread after the onset of KHI, but this mechanism is not enough to heat globally the structure. The dynamics could be seen in high-resolution H{\alpha} observations.