Do Periods of Decayless Kink Oscillations of Solar Coronal Loops Depend on Noise?

TL;DR

This study models decayless kink oscillations in solar coronal loops as a combination of natural modes and random external influences, finding that the oscillation period remains nearly unaffected by noise, supporting seismological estimations.

Contribution

The paper introduces a low-dimensional, randomly driven Rayleigh oscillator model for decayless kink oscillations, demonstrating period independence from noise and explaining observed amplitude modulations.

Findings

Oscillation period is nearly independent of noise.

Oscillatory patterns are almost harmonic.

Amplitude modulations are caused by external flow fluctuations and footpoint motions.

Abstract

Decayless kink oscillations of solar coronal loops are studied in terms of a low-dimensional model based on a randomly driven Rayleigh oscillator with coefficients experiencing random fluctuations. The model considers kink oscillations as natural modes of coronal loops, decaying by linear resonant absorption. The damping is counteracted by random motions of the loop footpoints and the interaction of the loop with external quasi-steady flows with random fluctuations. In other words, the model combines the self-oscillatory and randomly driven mechanisms for the decayless behaviour. The random signals are taken to be of the stationary red noise nature. In the noiseless case, the model has an asymptotically stationary oscillatory solution, i.e., a kink self-oscillation. It is established that the kink oscillation period is practically independent of noise. This finding justifies the seismological estimations of the kink and Alfv\'en speeds and the magnetic field in an oscillating loop by kink oscillations, based on the observed oscillation period. The oscillatory patterns are found to be almost harmonic. Noisy fluctuations of external flows modulate the amplitude of the almost monochromatic oscillatory pattern symmetrically, while random motions of the loop footpoints cause antisymmetric amplitude modulation. Such modulations are also consistent with the observed behaviour.