Alfvén Waves: Coherent Phenomena in Coronal Loops and Open-Field Regions

Abstract

We analyze two situations where coherent properties of Alfvenic perturbations influence the behaviour of a turbulent system. The first case is a coronal loop where large scales are dominated by coherent fluctuations (eigenmodes) excited by resonance with motions at the loop bases. The input energy flux is mainly determined by the zero-frequency eigenmode which is independent of the background Alfven velocity profile; the resulting scaling law gives values compatible with the flux necessary to sustain the active-region corona. Nonlinear interactions are also influenced by coherence effects. From the resulting nonlinear flux a scaling law for the velocity perturbation is derived, which is compatible with measures of nonthermal velocities in corona. In second case we studied how monochromatic Alfven waves, propagating upward from the coronal base in a coronal hole region, develop small scales in form of a power-law spectrum when they go across a thin 3D inhomogeneous layer (∼104 km thick) located at the base of the corona. Though the obtained spectrum is steeper than what would be obtained by means of nonlinear interactions, it could influence the subsequent nonlinear dynamics of the system by reducing the time of heat deposition, with consequences on the acceleration of the solar wind.