5.9
Discussion and Conclusions
An overview of the measured parameters determined using TRACE observations of longitudinal intensity oscillations in coronal loop footpoints is presented. 79data sub-cubes are examined, in which39loops
were found that show evidence of intensity oscillations. Wavelet analysis gave 25examples of periodic
intensity oscillations above a99%confidence level. The measured parameters from these25examples are
summarised in Table 5.5, and combined statistically with38other examples studied in De Moortel et al.
(2002b) in Table 5.6.
The data shows small amplitude periodic variations in intensity, suggestive of a compressional wave, with propagation speeds of the order of the coronal sound speed. The periods of these oscillations are much less than the coronal acoustic cutoff period and the disturbances are interpreted as slow propagating magnetoacoustic waves. Approximately half of the original datacubes analysed contain evidence of such oscillations, and approximately one third showed evidence of the oscillations above the99% confidence
level. Hence, these oscillations are commonplace in footpoints of large coronal loops.
The footpoints supporting the oscillations are observed to have length L ≈ 28.1±1.3 Mm, width
w ≈8.6±0.3Mm and a divergencewd ≈0.24±0.02. The oscillations are found to be of a small amplitude
A≈3.7±0.2%, propagating at a velocityv≈99.7±3.9km s−1, with a period ofP ≈281.5±10.3s.
All oscillations are observed to be outwardly propagating, and are usually only detected within the first15
Mm of the loop footpoint. The energy flux of the waves is found to be313±26erg cm−2s−1. This is only
a small percentage of the total energy required to heat coronal loops. However, as discussed in Tsiklauri and Nakariakov (2001) and Erd´elyi (1996), this estimate is a lower limit for the total energy flux as it only takes into account the contribution from a single harmonic.
The result that coronal loops embedded in regions of plage can oscillate, usually with a period of around
5minutes, is confirmed. However, no further examples of coronal loops embedded in a sunspot are found
and hence the statistic regarding coronal loops rooted in sunspot regions is not improved. Fig. 5.13(a) shows that despite the mean period being close to the expected 5 minutes, there is also a significant peak at around
3minutes, although no link to sunspots is observed in this study. Fig. 5.13(b) identifies that in general 3 or 5
minutes periodicities can be expected when observing longitudinal intensity oscillations along coronal loop footpoints. It is suggested that the peak at around200s in Fig. 5.13(a) could be related to the identification
of second harmonics, as described in Section 5.7.
Many of the loops observed in this study show evidence of filamentary behaviour. For example, in Fig. 5.1(a) there is a wide coronal loop footpoint in which many individual strands can be identified, but only one of these strands is observed to oscillate at that point in time. Indeed, most of the loops observed have shown some extent of filamentary behaviour (see Appendix A). The proximity of several of these os- cillations is used, particularly those occurring on June 13th 2001 and May 3rd 2003, to support the argument that the 5 minute coronal oscillations are driven by the leaking of the 5 minute globalp-mode oscillations.
De Pontieu et al. (2005) showed that the moss oscillations are driven by the leaking of the global 5 minute
p-modes along inclined magnetic fields into the lower solar atmosphere, and that subsequently the moss
oscillations become coronal shocks that drive the coronal loop oscillations. The coronal loop oscillations observed on June 13th 2001 and May 3rd 2003 are excited in regions of spatial extent of order 2 arcsec,
(a)
(b)
Figure 5.13: (a) Histogram showing the distribution of the periods measured in this study. The dominant period is clearly seen at around300s. There is a second peak at around200s, as was found by De Moortel
et al. (2002b). (b) The histogram showing the distribution for all 63 examples observed. Here, the dominant period at around300s with a second peak at around160s which is less pronounced.
5.9 Discussion and Conclusions 169
which matches the temporal and spatial properties ofp-mode driven moss oscillations very well. This sup-
ports the idea of the quasi-periodic globalp-modes exciting distinct strands of wide coronal loops over short
timescales. It also supports the simulations performed by De Pontieu et al. (2005). This has highlighted the need for higher spatial resolution instruments in the future, as the oscillations clearly indicate a filamentary structure in coronal loops.
Conclusions and Future Work
“Well it started badly, tailed off a bit in the middle and the less said about the ending the better!”, Rowan Atkinson.
6.1
Overview of Thesis
This Thesis investigates various effects of introducing inhomogeneities in models of coronal plasmas. This study concentrates on analytically deriving the dispersion relations for numerous models. This allows the effect of inhomogeneities, such as footpoint structure or a temperature profile, on the frequency of oscillation to be identified. Many of these findings are compared to the uniform models, allowing a fuller understanding of the processes involved in coronal loop oscillations to be built up.
This treatment is mainly theoretical, with a numerical determination of the solutions to the dispersion relations. The study is divided into two categories: the slow and fast magnetoacoustic modes. For the slow mode oscillations the effect of longitudinal density structuring on the frequency of oscillation is considered. This is done by studying various density profiles, caused by gravitational stratification or by structuring in the sound speed (or temperature). This models the coupling of the upper corona to the cooler, dense footpoint region.
Similarly, for the fast mode, a model in which gravity is perpendicular to the equilibrium magnetic field is introduced, and its effect on the frequency of oscillation is considered. During this analysis it is found that a new tool for coronal seismology can be developed, namelyP1/2P2analysis (Chapter 4) which uses
observed parameters to infer coronal properties, without the need for poorly constrained input parameters. Furthermore, TRACE observations are studied to identify evidence of propagating slow modes in coronal loops and evidence suggesting one possible mechanism for the upward driving of these oscillations is reported. Now follows a summary of the main results.