Large-Amplitude Velocity Fluctuations in Coronal Loops: Flare Drivers?

Abstract

Recent space observations of coronal lines broadening during a flare occurrence suggest that unresolved nonthermal velocity rises well above the background level before the start of the flare, defined as the start of hard X-ray emission. Using a new shell model to describe the Alfvénic turbulence inside a coronal loop, it is shown that the occurrence of high values (of the order of 100 km s^-1) of the large-scale fluctuating velocity can represent an efficient trigger to a nonlinear intermittent turbulent cascade and then to the generation of a burst of dissipated energy. The numerical results of the model furnish a well-supported physical explanation for the reason why large velocity fluctuations represent the flare trigger rather than the result of the later energy deposition.