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Magnetic arcades in the solar atmosphere, or coronal loops, are commonstructures known to host magnetohydrodynamic (MHD) waves and oscillations. Ofparticular interest are the observed properties of transverse looposcillations, such as their frequency and mode of oscillation, which havereceived significant attention in recent years because of their seismologicalcapability. Previous studies have relied on standard data analysis techniques,such as a fast Fourier transform (FFT) and wavelet transform (WT), to correctlyextract periodicities and identify the MHD modes. However, how these methodscan lead to artefacts requires investigation. We assess whether these twocommon spectral analysis techniques in coronal seismology can successfullyidentify high-frequency waves from an oscillating coronal loop. We examineextreme ultraviolet images of a coronal loop observed by the AtmosphericImaging Assembly in the 171 \AA waveband on board the Solar DynamicsObservatory. We perform a spectral analysis of the loop waveform and compareour observation with a basic simulation. The spectral FFT and WT power of theobserved loop waveform is found to reveal a significant signal with frequency2.67 mHz superposed onto the dominant mode of oscillation of the loop (1.33mHz), that is, the second harmonic of the loop. The simulated data show thatthe second harmonic is completely artificial even though both of these methodsidentify this mode as a real signal. This artificial harmonic, and severalhigher modes, are shown to arise owing to the periodic but non-uniformbrightness of the loop. We further illustrate that the reconstruction of the2.67 mHz component in the presence of noise yields a false perception ofoscillatory behaviour that does not otherwise exist. We suggest that additionaltechniques such as a forward model of a 3D coronal arcade are necessary toverify such high-frequency waves.

The need for new techniques to identify the high-frequency MHD waves of an oscillating coronal loop