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Transverse Oscillations of Coronal Loops: Statistics, Behaviour

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Transverse (kink) oscillations of coronal loops are intensively studied as one of the main tools for coronal seismology. Recent observational findings led to the major progress in our understanding of this phenomenon. The oscillations appear in two different regimes, the 169 decaying oscillations and 72 undamped oscillations allow us to demonstrate that in both regimes the oscillation periods are proportional the loop length. This finding, together with the independence of the oscillation amplitude on the period exclude the hypothesis that the oscillations are linked with the leakage of p-modes or other narrowband photospheric waves, and support the interpretation in terms of standing fast magnetoacoustic oscillations. In the decaying regime the loop displacement reaches several minor radii of the loop, and the oscillation decays in typically a few oscillation cycles. In the majority of cases, decaying kink oscillations are found to be excited by low coronal eruptions. The quality factor is found to be proportional to the oscillation amplitude, indicating the nonlinear nature of the damping. In the decayless regime the amplitude is usually lower than the loop’s minor radius, and remains constant or low-varying for a number of oscillation cycles. The same loop may oscillate in both these regimes, e.g. before and after some impulsive excitation. The oscillation period remains the same in both regimes. Low-amplitude undamped kink oscillations are observed continuously, without any visible source. We propose that undamped kink oscillations of coronal loops may be caused by the interaction of the loops with quasi-steady flows, and hence are self-oscillations, which is analogous to producing a tune by moving a bow across a violin string.

This talk is part of the DAMTP Astro Mondays series.

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