Semester 1, Week 7: Prof Valery Nakariakov, University of Warwick

Kink oscillations of the decayless class are detected as small amplitude (< 1 Mm) persistent transverse repetitive displacements of solar coronal plasma loops, occurring without any association with solar flares, eruptions, or other impulsive energy releases. The linear correlation of the oscillation periods with the lengths of the oscillating loops demonstrates that the oscillations are natural modes of the loops. The statistical distribution of oscillation amplitudes with the periods shows no pronounced peaks, suggesting the lack of a periodic driver. Together with the lack of an impulsive excitation, it suggests that the energy losses are compensated by the interaction of the loop with either quasi-steady or random external plasma flows. In this study, the decayless regime is associated with the energy supply from coronal plasma flows via the negative friction, i.e., self-oscillations, and/or random movements of footpoints of the oscillating loop.

The kink oscillation period is found to be practically independent of noise, which justifies seismological estimations of the coronal magnetic field. The transition from the large-amplitude rapidly-decaying regime to the low-amplitude decayless oscillations demonstrates that the decay pattern differs from the usually assumed exponential decay.

Implications of this finding for magnetohydrodynamic seismology of the solar corona, based on the effect of resonant absorption are discussed.