Random excitation of decay-less transverse oscillations of coronal loops

Andrei Afanasev, KU Leuven

Some of the observed quasi-periodic pulsations of solar flare emission are interpreted as a result of the impact of external perturbations on a flare process. Such perturbations can be generated by transverse loop oscillations detected abundantly in the solar corona. We study the excitation mechanism of transverse oscillations of coronal loops to get insight into the nature of quasi periodic pulsations. The relatively large-amplitude decaying regime of transverse oscillations of coronal loops has been known for two decades. They decay fast (in several oscillation cycles) and are interpreted in terms of MHD kink modes of cylindrical plasma waveguides. Analysis of SDO/AIA data has revealed also decay-less small-amplitude oscillations, with a multi-harmonic structure being detected. Several models were proposed to explain them. In particular, decay-less oscillations were interpreted in terms of standing kink waves driven with continuous monoperiodic motions of the loop footpoints, in terms of a simple oscillator model of forced oscillations due to harmonic external force, and as a self-oscillatory process due to the interaction of loops with quasi-steady flows. We consider the model of standing waves driven by random motions of the loop footpoints. The footpoint motions are modelled by broad-band coloured noise with the Kolmogorov exponent. We analyse the excitation of eigenmodes, frequency ratios of eigenmodes, and spatial distribution of the oscillation energy along the loop, taking into account effects of damping and varying kink speed. Our findings can be of importance for interpretation of the fine structure of solar emission. They can also contribute into seismological techniques applied to determine coronal plasma parameters.