Chapter 5 extended the radial study of intensity oscillations in NBPs into the time-domain via wavelet analysis, and throughout the atmosphere using multi-wavelength imaging. A cross-correlation study of oscillatory power curves between light curves in four wavelengths showed the existence of many upward- and downward- propagating waves throughout the chromosphere. In particular, each NBP displayed propagating waves which satisfied observational tests predicted by theory. In each case a low-frequency wave (1.3 mHz, 1.9 mHz) travelled from the low- to mid- chromosphere. This is taken to be the signature of a kink-mode wave created by granular buffeting of magnetic flux tubes in the photosphere. In the higher chromosphere, higher frequency travelling waves are detected, and the low- frequency oscillations decrease in power. This agrees with the theoretical studies of Kalkofen (1997) and Hasan & Kalkofen (1999), whereby the low-frequency kink mode waves transfer their power to longitudinal mode waves at twice the original frequency of the kink-mode waves. These longitudinal waves can then shock, hence heating the surrounding plasma.
There are two main extensions to this work. Firstly co-temporal and co-spatial magnetic field information needs to be combined with a larger spatial sample of NBPs. It is believed that both the speed of the wave and frequency of oscillation will be directly influenced by the magnetic field strength. Indeed a subtle change in magnetic field may coincide with the initiation of a wave in a flux tube. Secondly the errors in the estimated velocities of these waves can be greatly reduced by observational and theoretical studies. Observationally, higher-cadence imaging provides better temporal constraints, and improved narrowband imaging provides a better estimate to the height of formation of the data. Theoretically a better understanding of the emission of the strong Fraunhofer lines in the presence of kilogauss magnetic field will also reduce the error in the estimated height of formation. A larger spatial sample of NBPs will also provide further evidence as to the global significance of these oscillations.