Undamped transverse oscillations of coronal loops as a selfoscillatory process 

Valery Nakariakov Submitted: 20160518 13:18
Context. Standing transverse oscillations of coronal loops are observed to operate in two regimes, the rapidly decaying large amplitude oscillations, and undamped small amplitude oscillations. In the latter regime the damping should be compensated by energy supply, which allows the loop to perform almost monochromatic oscillations with almost constant amplitude. Different loops oscillate with
different periods. The oscillation amplitude does not show dependence on the loop length or the oscillation period.
Aims. We aim to develop a lowdimensional model explaining the undamped kink oscillations as a selfoscillatory process caused by the effect of negative friction. The source of energy is an external quasisteady flow, e.g. supergranulation motions near the loop footpoints or external flows in the corona.
Methods. We demonstrate that the interaction of a quasisteady flow with a loop can be described by a Rayleigh oscillator equation that is a nonlinear ordinary differential equation, with the damping and resonant terms determined empirically.
Results. Lowamplitude selfoscillatory solutions to the Rayleigh oscillator equation are harmonic signals of constant amplitude, which is consistent with the observed properties of undamped kink oscillations. The period of selfoscillations is determined by the frequency of the kink mode. The damping by dissipation and mode conversion is compensated by the continuous energy deposition
at the frequency of the natural oscillation.
Conclusions. We propose that undamped kink oscillations of coronal loops may be caused by the interaction of the loops with quasisteady flows, and hence are selfoscillations, in analogy with producing a tune by a stick moving across a violin string.
Authors: Nakariakov, V. M., Anfinogentov, S., Nistico, G., Lee, D.H.
Projects: SDOAIA

Publication Status: A&A, Letter, Accepted
Last Modified: 20160518 15:32


