\thesischapter{Concluding Remarks and Future Work}
\label{future}
This thesis has concentrated on the detections of low-frequency (1--8~mHz) oscillations in the chromosphere, and the possible role of these oscillations in solving the chromospheric heating problem. This chapter will summarise the main results of the work carried out for this thesis and suggest possible extensions of each study. 

\section{Optical Datasets}

Chapters~4 \&~5 discussed the detection and evolution of oscillations in NBPs in three dimensions - two spatial and one temporal. The importance of separating the NBP contribution from that of the internetwork and other non-solar sources is addressed using a contour and contrast method. The {\em contour} ensures only NBP pixels are studied, whereas the {\em contrast} ensures that only intensity oscillations above that of the background are included. A study of oscillatory power against radial distance showed that oscillations in the 1--4~mHz range are much stronger in the centre of NBPs.  
 
The need for high-cadence, two-dimensional images for this type of study shows the importance of using narrow-band optical filters on ground-based telescopes. Space-based telescopes suffer from the fact that high-cadence imaging will be necessarily combined with short duration sequences and a problem of telemetry rate in transferring the data. Ground-based are also cheaper to run, and easier to upgrade. Recent post-acquisition software also makes it possible to remove a lot of seeing effects from the data. 

\subsection{Mode-Coupling in the Network}

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 {\em 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. 

\subsection{Spectroscopy}

Spectroscopy of quiet-Sun regions may be one solution to the problems mentioned above. Although spectroscopy requires a scan across an area to make an image, this can be neglected by using a sit and stare study, and provides information at a range of heights throughout the chromosphere. There is also the possibility of confirming the presence of waves via the Doppler shift of the line profile. It is also well known that many oscillations are stronger in a study of the Doppler shift, rather than the intensity of the line. Ideally a combined spectroscopic and imaging campaign, with high resolution magnetograms, and involving support from {\cds} and {\trace} would provide a definitive dataset. With the onset of solar minimum over the next few years it will be easier to select in advance a quiet target region, far from active regions.  

\section{Automated Wavelet Analysis}

Chapter~6 presents an automated wavelet analysis approach to searching for oscillations in intensity in a large number of light curves from both the network and internetwork. This permits a comparison study, in terms of both periodicity and lifetime, of oscillations present in these two regions of the quiet Sun. The network displays a higher peak periodicity ($\sim$280~s) than the internetwork ($\sim$250~s). In the lower chromosphere the network also displays a large number of oscillations at higher periods, which may be the signature of kink-mode waves. In both regions, oscillations around these peak values (and with a lifetime of 2--3 cycles) show a tendency to recur in the same spatial position, suggesting a good degree of spatial memory. On the other hand, oscillations with a long lifetime ($>$8 cycles) do not recur, suggesting the driving source may be expending all its energy in the creation of the initial oscillation. 

A study of oscillations in each UV passband shows upwardly-propagating waves in both the network and internetwork. The sudden decrease in oscillation strength (and number) in the 1550~{\AA} band is attributed to waves dissipating, shocking or moving away in the high chromosphere. The cross-over period (i.e., the minimum period at which the network exhibits more oscillatory power than the internetwork) is shown to increase from $\sim$250~s in the low chromosphere to $\sim$300~s in the high chromosphere. The time-localised nature of wavelet analysis is used to show that even in these high-period regimes the internetwork can still exhibit more oscillatory power, but only for oscillations with short lifetimes.

A study into the spatial position of pixels displaying different periodicities and lifetimes may answer some of the questions posed in this chapter. It may be useful to apply this method to higher cadence {\trace} quiet-Sun datasets and other instruments. It can also be extended into a third spatial region between the network and internetwork (classified as `intermediate' in Krijger {\etal} 2001). In a wider context, this technique can be used in other areas of solar physics (e.g., active regions) or astronomy in general where a search for transient, spatially-localised, oscillatory power is required (e.g., multi-target wide-field imaging). 

The automated aspect of the wavelet analysis in Chapter~6 is worth discussing. With the large datasets expected from future space missions (e.g., Solar Dynamics Observatory may produce up to 1 terabyte per day), it will impossible to manually inspect every light curve for periodicity. Automated routines must be created to filter through datasets and detect interesting features. In this respect an automated search for both periodicity and possible propagating waves is essential. A search for horizontal flows can also be carried out by detecting the starting time of any oscillations. This may make near real-time detections of solar phenomena a possibility.
