Reliable values of the damping time, , have been derived by Molowny-Horas et al. (1999) after fitting the function to Doppler velocity time series recorded simultaneously in different positions of a polar crown prominence (Figure 6). The values of thus obtained are usually between 1 and 4 times the corresponding period, in agreement with previous observational reports. In addition, there is one particular case for which the line-of-sight velocity grows in time, but no interpretation of this result is given by these authors.
Terradas et al. (2002) performed a deeper investigation of the data of Molowny-Horas et al. (1999). After fitting the same sinusoidal function to all points in the two-dimensional field of view, Terradas et al. (2002) generated two-dimensional maps of various oscillatory parameters, such as the period, damping time and velocity amplitude (Figure 7). Terradas et al. (2002) stressed that there is a region near the prominence edge (54,000 km by 40,000 km in size) in which the correlation coefficient of the fit is rather large and in which the period and damping time are very uniform. The mentioned region is around position x = 80, y = 50 of Figure 7. In Section 3.6.3 we discuss other aspects of this work, which is unique since it is one of a few in which coherent wave behaviour has been found in a large area of a prominence and the only one in which the wave parameters in a two-dimensional prominence area have been computed.
Very often the presence of a periodic signal in the data is not obvious under a visual scrutiny and the FFT or periodogram simply provide the period of such signal, but not its duration. Dividing the time series into shorter intervals and calculating the Fourier spectrum of each of them allows to narrow down the epoch of occurrence of the oscillation. Wiehr et al. (1984) followed this procedure and determined that a 3-min oscillation found in a 2-h Doppler velocity record only existed in the last 40 min of the sample. The wavelet technique, however, is much better suited for the calculation of lifetimes since it can be used to precisely determine the beginning and end of the time interval in which a periodicity, previously detected in the Fourier spectrum, takes place. This approach was used by Molowny-Horas et al. (1997), who obtained a period around 7.5 min in 16 consecutive points, spanning a distance of 7300 km, along the spectrograph slit. The time/frequency diagram of the corresponding 16 time signals indicates that the periodic perturbation is not present for the whole duration of the data and that it only operates for about 12 min (Figure 8). Molowny-Horas et al. (1998) performed a similar study by placing the slit on a filament, rather than on a limb prominence, with comparable results. Two oscillations with periods around 2.7 and 12.5 min were present at consecutive points covering some 2000 and 3300 km, respectively. From the wavelet analysis, the lifetimes of these two perturbations are of the order of 10 and 20 min, respectively. These results provide convincing evidence of the train-like character of some prominence oscillations. Further details of the work by Molowny-Horas et al. (1997) are provided in Section 3.6.2.
Oscillations of prominence threads also display fast attenuation. For example, Lin (2005) detected several periodicities over large areas of a filament, with maximum power at periods of 26, 42, and 78 min. Pronounced Doppler velocity oscillations with 26 min period could only be observed for 2 – 3 periods, after which they became strongly damped.
Living Rev. Solar Phys. 9, (2012), 2
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