Long-term evolution of decayless kink oscillations of solar coronal loops

Long-term evolution of instantaneous parameters of decayless kink oscillations of six solar coronal loops observed for longer than 2 h each is studied. The oscillations are analysed by processing sequences of 171 angstrom images obtained with the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) in the time interval from 2020 December till 2021 June, with the motion magnification technique. It is established that decayless kink oscillations could exist for more than 30 or 40 oscillation cycles. Neither the loop brightness nor instantaneous parameters of the oscillations show a monotonic increase or decrease during the oscillation. The observed instantaneous oscillation periods and amplitudes are found to vary randomly in time, with distributions around the mean values that resemble Gaussian profiles. Mean values of the oscillation periods and amplitudes are consistent with previous observations of this phenomenon. A power-law dependence of the oscillation period on the displacement amplitude is found, with the power-law index of 0.41 and with the 95 per cent confidence interval of [0.39, 0.71]. In general, we established the lack of correlation between instantaneous oscillation parameters and loop brightness. One exception is an event with relatively strong anticorrelation of the amplitude and the loop's brightness, with the cross-correlation coefficient of about -0.81, but this effect requires a further study. Fourier power spectra of the envelopes of the time-evolving instantaneous amplitudes and periods are white noise, indicating that consecutive values of the instantaneous parameters are independent of each other. The results obtained provide an empirical ground for validating and comparing existing and future theoretical models of decayless kink oscillations of coronal loops.