Outline of this Thesis

The aim of the work for this thesis was to use multi-wavelength narrowband images of the chromosphere to search for the possible oscillatory signals of AC heating mechanisms. There are several reasons for specifically studying the chromosphere. From a theoretical perspective, it is the boundary between the interior (dominated by fluid motions and gas pressure) and the magnetically dominated corona. As an introduction to this background theory, Chapter 2 contains a summary of the equations which explain the various modes of waves possible in a stratified, magnetised plasma such as the chromosphere. A brief overview of the fundamental equations is provided, along with a description of wave propagation in homogeneous and inhomogeneous plasmas.

From an observational aspect, the chromosphere provides several optical Fraunhofer lines, and UV continuum. Chapter 3 contains a description of the experimental set-up used in an observing run at Dunn Solar Telescope / National Solar Observatory, designed to observe the same region of the quiet Sun in as many optical wavelengths as possible, whilst still maintaining a sufficiently high cadence. As the earth's atmosphere absorbs UV continuum, it is necessary to use data from space-based instrument to observe the Sun in this part of the electromagnetic spectrum. Hence Chapter 3 also contains a description of the TRACE spacecraft, and imaging in the UV.

Chapter 4 discusses novel image segmentation and statistical analysis techniques used to study oscillations in the chromospheric network imaged in Ca II K₃. Specifically the `contour and contrast' technique is used to separate oscillations in network bright points from those of other solar (e.g., internetwork), or of non-solar (e.g., earth's atmosphere) origin. This work is extended to search for oscillations at several heights in the chromosphere in Chapter 5. Wavelet analysis routines are used to search for correlated oscillations across all frequencies, which may result from waves travelling through the chromosphere. This is used to test several chromospheric heating theories.

Chapter 6 uses an automated wavelet analysis technique to search for intensity oscillations in UV passband data. This extends previous Fourier analysis of this work into the time-localised domain. Differing oscillatory signals in the network and the internetwork at various atmospheric heights are found to be indicative of differing wave modes and heating mechanisms in these two regions. The recurrence of oscillations of various periodicities and lifetimes are discussed in terms of previous work.

Finally, Chapter 7 summarises the conclusions of this thesis, and suggests future directions of this project.