An Analysis of Magnetohydrodynamic Waves and Oscillations in the Solar Corona

Loops of hot plasma trace the invisible magnetic field lines of the Sun's dynamic corona. These structures are known to host various magnetohydrodynamic (MHD) waves and oscillations and are believed to play a crucial role in long-standing problems, such as the enigmatic heating of the solar corona. Recent high-resolution observations have galvanised interest in understanding the nature of these loops. Often, coronal loops are observed to oscillate in response to an expulsion of energy from solar flares, with frequencies of around 1-10 mHz. Current efforts are aimed at determining the physical parameters of the loops that are otherwise unmeasurable by virtue of seismology. However, before seismic inversions can be successfully executed, it is imperative to fully understand the observed wave signals emitted from these structures, including the techniques used for their analyses. Using multiple observations, we develop a novel technique for detecting periodicities within complex coronal arcades. Key applications include loop oscillations with a poor image contrast whereby the inherent waveform is revealed. Next, we ask whether it is possible to identify high-frequency wave signals from an oscillating loop using two common spectral methods. We demonstrate that the spectrum of a loop oscillating with a single frequency contains power at frequencies that are integer multiples of the fundamental frequency. These frequencies are completely artificial and arise owing to the inability of the methods to distinguish the periodic but non-sinusoidal signals from coronal waveforms. This study highlights simple cases where the methods themselves fail, and calls for more robust procedures, including 3D MHD simulations, to confidently reveal such high frequencies. Finally, we investigate a rare event of transverse oscillations induced by consecutive flares. We find evidence of a loop exhibiting a change in frequency during the observation and discuss the possibility of multiple simultaneous excitation sources.