3.3 Detected periods

Early observational studies of small amplitude prominence oscillations revealed a wide range of characteristic periods, ranging from a few minutes (Harvey, 1969Jump To The Next Citation Point; Wiehr et al., 1984Jump To The Next Citation Point; Tsubaki and Takeuchi, 1986Jump To The Next Citation Point; Balthasar et al., 1986Jump To The Next Citation Point), to 15 – 25 min (Harvey, 1969Jump To The Next Citation Point; Landman et al., 1977Jump To The Next Citation Point), to 40 – 90 min (Bashkirtsev et al., 1983; Bashkirtsev and Mashnich, 1984Jump To The Next Citation Point; Wiehr et al., 1984Jump To The Next Citation Point; Balthasar et al., 1986). The apparent tendency of periods to group below 10 min or in the range 40 – 90 min led to the distinction between short- and long-period oscillations to refer to these two period ranges. Later, more reports of periods in the range 10 – 40 min were published (e.g., Yi et al., 1991Jump To The Next Citation Point; Suetterlin et al., 1997Jump To The Next Citation Point; Blanco et al., 1999Jump To The Next Citation Point; Régnier et al., 2001Jump To The Next Citation Point) and the intermediate-period class emerged. However, this classification (solely based on the period value) is far from complete: Balthasar et al. (1993Jump To The Next Citation Point) observed a prominence simultaneously with the GCT and VTT telescopes in Tenerife to remove doubts about the instrumental or atmospheric origin of prominence oscillations and obtained strong power in the Doppler shift from both telescopes with period around 30 s; hence, very short-period small amplitude oscillations also exist. Furthermore, a few works in which prominences have been observed from space during extended time intervals show that very long-period oscillations also exist: Pouget et al. (2006Jump To The Next Citation Point) detected periodicities of 5 – 6 h, while Foullon et al. (2004Jump To The Next Citation Point) and Foullon et al. (2009) have observed variations in EUV filaments with periods around 12 h, and 10 – 30 h, respectively. Although the classification in terms of short-period, long-period, etc. oscillations is still in use, it does not cast any light nor gives any help with regard to the nature, origin, or exciter of the oscillations.

In some occasions, a given prominence has been observed over a few consecutive days and the outcome is that the same period seems to be recovered (Bashkirtsev and Mashnich, 1984; Mashnich and Bashkirtsev, 1990; Suetterlin et al., 1997Jump To The Next Citation Point). This seems to indicate that the overall properties of this prominence did not change much over this time interval. Similar studies have not been later carried out.

Some authors have tried to find correlations of the periods of small amplitude oscillations with other parameters. Harvey (1969Jump To The Next Citation Point) reported a correlation of the period with the unperturbed longitudinal magnetic field, such that long periods are associated with strong field strengths (Figure 3View Image). This dependence is difficult to understand since, other parameters being equal (density, magnetic field line length, etc.), one expects just the inverse behaviour for fast MHD waves, and no dependence of the period on the magnetic field strength for slow MHD waves. Bashkirtsev and Mashnich (1993) claimed that the period of oscillation depends on solar latitude. Only periods above 40 min were included in this study and some 40 observations gathered along more than eight years were taken into account. The question then is whether this latitudinal dependence, if real, is related to the solar activity cycle or not. In a subsequent work by Mashnich and Bashkirtsev (1999) a similar latitudinal dependence was obtained for the quasi-hourly oscillations of the photosphere and chromosphere. The implications of these findings are profound and further checks are essential before their reality is firmly demonstrated.

View Image

Figure 3: Period of prominence Doppler velocity oscillations as a function of the line-of-sight magnetic field strength. The top and bottom panels correspond to active region and non-active region prominences, respectively (from Harvey, 1969).

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