Low-frequency oscillations of the solar atmosphere

This thesis presents detections of oscillations in multi- wavelength images of the quiet-Sun chromosphere, and interprets these oscillations in terms of various chromospheric wave-heating theories. Oscillations in network bright points (NBPs) are studied in the light of Ca II K₃ as a function of radial distance from the centre of each NBP. It is shown that low-frequencies (1 4 mHz) tend to dominate, especially in the central portions, suggesting a magnetic component in any waves present. Correlations between wavelet power density maps of light curves in four optical passbands is used to search for vertically propagating wave packets. In each NBP studied, observational evidence is found for transverse-mode magnetohydrodynamic (MHD) waves (1.3 mHz, 1.9 mHz) propagating upwards from the low- to mid-chromosphere, where they couple to longitudinal-mode MHD waves at twice the originally frequency (2.6 mHz, 3.8 mHz), which then shock in the high-chromosphere. There is also further evidence of other upward- and downward-propagating waves in the 1.3 4.6 mHz range. An automated wavelet analysis routine is developed to provide a comparison, in frequency and duration, between network and internetwork oscillations in ultraviolet images of the quiet Sun. The tendency of the network to contain lower frequencies (peak at 3.5 mHz, with an extended tail down to 1 mHz) is discussed in terms of transverse-mode MHD waves. In contrast, the internetwork contains frequencies around 4 mHz, with oscillations tending to contain a higher degree of spatial memory. These are interpreted as persistent flashers. The network tends to contain more oscillations below 3.3 mHz than the internetwork in the low-chromosphere, with this cross- over frequency increasing to 4 mHz in the upper chromosphere. However, below this cross-over frequency, the internetwork still contains a larger number of oscillations, but with short lifetimes. Both regions also contain a small number of non-recurring long-lived oscillations.