The works mentioned in the previous paragraph use a spectrograph slit to detect oscillations; obviously, a two-dimensional data set is much more advantageous when it comes to ascertaining which part of a prominence is affected by oscillations. Terradas et al. (2002) reported on the propagation of waves over a large region (some 54,000 km by 40,000 km in size) in a limb prominence and high spatial resolution observations with Hinode/SOT (Berger et al., 2008) also show oscillations that affect a small area of a prominence. See also the discussion in Section 3.6.4 of the work by Lin et al. (2007) that gives evidence of coherent Doppler shift oscillations over a rectangular area 3.4 arcsec × 10 arcsec in size.
Other observations with high spatial resolution have shown that individual threads or small groups of threads may oscillate independently from the rest of the prominence with their own periods (Thompson and Schmieder, 1991; Yi et al., 1991). Figure 4 displays some of the results in Yi et al. (1991). It is clear that threads 1, 4, 13, and 14 oscillate in phase with their own period, which ranges from 9 to 14 min. In addition, Tsubaki et al. (1988) obtained successively two time series of spectra by placing the spectrograph slit first at a height of 30,000 km above the solar limb and next 40,000 km above the limb. A group of vertical threads detached from the prominence main body displayed 10.7-min periodic variations at both heights, which was a first indication that threads can oscillate collectively. After these preliminary studies, much attention has been given to the detection of thread oscillations (Lin et al., 2003; Lin, 2005; Lin et al., 2005, 2007, 2009; Okamoto et al., 2007; Ning et al., 2009b,a). Apart from reporting on thread oscillations, these works have also provided detailed information about wave features such as the period, wavelength, and phase speed. Because of the importance of these quantities in the seismology of prominences, these works are discussed in more detail in Section 3.6.4.
There is also some evidence that velocity oscillations are more easily detected at the edges of prominences or where the material seems fainter, while they sometimes look harder to detect at the prominence main body (Tsubaki and Takeuchi, 1986; Tsubaki et al., 1988; Suematsu et al., 1990; Thompson and Schmieder, 1991; Terradas et al., 2002). This result has occasionally been interpreted as the consequence of integrating the velocity signals coming from various moving elements along the line-of-sight. This explanation, however, would imply the presence of broader spectral lines at the positions showing periodic variations, which is not observed, so other explanations are also possible (Suematsu et al., 1990). Mashnich et al. (2009a,b) gave evidence that different parts of filaments may support different periodicities: short-period variations (with periods shorter than 10 min) had coherence scales shorter than 10 arcsec and were detected near the edges of filaments placed close to the Sun’s central meridian. These oscillations, hence, were characterized preferentially by vertical plasma displacements. On the other hand, variations with period around 1 h occured in different positions of the filament and the size of the oscillating area was not larger than 15 – 20 arcsec. In addition, these oscillations had an amplitude that increased by an order of magnitude or more in filaments far from the solar center compared to those near the center of the Sun’s disk. Then, these oscillations showed a mainly horizontal velocity.
More information about the spatial distribution of prominence oscillations comes from Ning et al. (2009b), who detected transverse oscillations of 13 threads in a quiescent prominence observed with Hinode/SOT. These authors found that prominence threads in the upper part of the prominence oscillate independently, whereas oscillations in the lower part of the prominence do not follow this pattern. Furthermore, the oscillatory periods were short (between 210 to 525 s), with the dominant one appearing at 5 min (more information is given in Section 3.6.4). In a subsequent work, Ning et al. (2009a) used the same data set to analyze the motions of two spines in the same quiescent prominence. The spine is synonymous with the horizontal fine structure along the filament axis and is the highest part of the prominence. In the observations of Ning et al. (2009a), the spines showed drifting motions that were removed by the subtraction of a linear trend, which allowed the authors to uncover the existence of oscillations with a very similar period (around 98 min) in both structures. Further insight into the behaviour of the spines comes from a fit of a function to the detrended data. Here is the oscillatory amplitude, the period, the oscillatory phase, and the damping time. The detrended signals and the function fits are displayed in Figure 5, which includes the fitted parameters, that give the following information: from the oscillatory amplitude, the peak velocities of the spines are 1 and 5 km s–1. The periods are almost identical (96.5 and 98.5 min) and the phase difference is 149°, which means that the spines oscillated almost in anti-phase. These results about the period and phase were taken by Ning et al. (2009a) as an indication of a collective behaviour of the two structures. These authors considered an analogy with the transverse MHD oscillations of two cylinders (a problem studied by Luna et al., 2008, and discussed in Section 4.4) and concluded that a coupling of kink-like modes can give the observed behaviour. In particular, the mode of the system has motions resembling the anti-phase oscillatory behaviour found by Ning et al. (2009a).
Living Rev. Solar Phys. 9, (2012), 2
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