3 Small Amplitude Oscillations: Observational Aspects

Quiescent prominences are also subject to small amplitude oscillations, a type of periodic events characterized by one or more of these features:

  1. Not related to flare activity.
  2. Small velocity amplitude.
  3. Only a restricted volume of the prominence displays periodic variations.

Regarding item 1, so far it has not been possible to identify the trigger of small amplitude oscillations. A popular conjecture about their excitation is that it lies in the periodic motions of magnetic fields caused by photospheric or chromospheric oscillations. The idea is that Alfvén waves ought to propagate upwards and that any prominence material threaded by the field should also be subject to periodic motions if there is enough energy available to overcome the inertia of the dense plasma (Harvey, 1969Jump To The Next Citation Point); this idea was later suggested by other authors too (e.g., Yi et al., 1991Jump To The Next Citation Point). Harvey made order-of-magnitude calculations to show that the ratio of prominence to photospheric oscillatory energy is around or smaller than 10–4, which indicates that this excitation mechanism is feasible. On the other hand, Harvey also noted that a few prominences in his study appear to oscillate with periods nearly twice as large as those of the photospheric oscillation. This appears to contradict the above hypothesis about the initiator of small amplitude oscillations. Much longer and shorter periods than those present in Harvey’s work have been detected afterwards (see Section 3.3), so probably this mechanism of energy transfer from the photosphere (or chromosphere), if correct, may not be the only one to cause these prominence oscillations. Mashnich et al. (2009aJump To The Next Citation Point,bJump To The Next Citation Point) studied the Doppler velocity field in some filaments and the underlying photosphere by means of simultaneous observations of the Hβ line and the neighbouring photospheric Fe i line at 4863 Å. They detected a quasi-hourly oscillation in certain areas of the filaments and photosphere and found a good spatial correlation between them. They also reported that the parts of the photosphere displaying this oscillation are often observed below filament barbs. The spatial coincidence of this periodicity and the relation of filament barbs and the photosphere led these authors to suggest that the photosphere was the origin of these particular prominence oscillations. From an observation of a limb prominence with Hinode, Ning et al. (2009bJump To The Next Citation Point) reported that the detected oscillatory behaviour only lasted about one period and that new oscillations appeared nearby simultaneously. These authors then concluded that the exciters or drivers of such oscillations are numerous and of small scale. The current understanding (see Section 4) is that periodic perturbations in prominences can be produced by an external impulsive agent that excites different eigenmodes of the structure or by a continuous agent, as may be the case with the 5-min photospheric and 3-min chromospheric oscillations whose influence could propagate along magnetic field lines and force motions of the prominence plasma. Recently, some evidence about the effect of the chromospheric 3-min oscillations on the corona has been found by Sych et al. (2009).

Regarding item 2, the detected Doppler velocity peak ranges from the noise level (down to 0.1 km s–1 in some cases) to 2 – 3 km s–1 (e.g., Harvey, 1969Jump To The Next Citation Point), although larger values have also been reported (e.g., Bashkirtsev and Mashnich, 1984Jump To The Next Citation Point; Molowny-Horas et al., 1999Jump To The Next Citation Point; Ning et al., 2009aJump To The Next Citation Point). This maximum value has to be compared with the typical speeds of the prominence plasma (the sound and Alfvén speeds), which are larger than 10 km s–1 for typical prominence conditions. The main purpose of studying prominence oscillations is to obtain insight into their physics via a seismological approach (see Section 6). Therefore, the information that observations should provide are the periods, wavelength, phase, and group velocity and damping time of these phenomena. In addition, observations should also determine whether these periodic variations are standing oscillations or propagating waves, whether they affect some prominence threads or larger areas of a prominence, whether threads oscillate independently from their neighbours or which physical variables are disturbed and by which amount.

The solar origin of prominence oscillations remained controversial until the beginning of the 1990s. For example, Engvold (1981) failed to detect oscillatory motions in the velocity field of a limb prominence, although the observational setup used prevented him from reliably distinguishing velocity amplitudes below 2 km s–1, the range in which many peak values are found. In addition, Malherbe et al. (1981, 1987) recognized no oscillatory pattern in time series of line-of-sight velocities obtained with the MSDP operating on the Meudon solar tower, although positive results were later achieved using the same instrument (Thompson and Schmieder, 1991Jump To The Next Citation Point). On the other hand, the lack of small amplitude, periodic variations in signals coming from solar prominences cannot be considered a proof against the existence of these kind of processes. Harvey (1969Jump To The Next Citation Point) noted that in a sample of 68 non-active region prominences, 31% of the objects presented no significant velocity change along the line-of-sight, 28% showed apparently random line-of-sight velocity variations, and 41% presented a definite oscillatory behaviour. Analogous results were obtained for a set of 45 active region prominences. There are several reasons that may lead to the absence of periodic variations in some prominences: the velocity amplitude or its projection along the line-of-sight may be too small to stand above the instrumental noise level; or the prominence material may actually not oscillate at the time the observations are performed; or the light emitted or absorbed by various plasma elements along the line-of-sight and having different oscillatory properties may result in a noisy signal.

 3.1 Detection methods
 3.2 Spectral indicators
 3.3 Detected periods
 3.4 Spatial distribution of oscillations
 3.5 Wave damping and oscillation lifetime
 3.6 Wavelength, phase speed, and group velocity
  3.6.1 Simple analyses
  3.6.2 An elaborate one-dimensional analysis
  3.6.3 A two-dimensional analysis
  3.6.4 Thread oscillations

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