2 Large Amplitude Oscillations

Oscillations with velocity amplitude greater than 20 km s–1 have been observed in filaments. It was suggested that their exciter was a wave, caused by a flare, which disturbs the filament and induces damped oscillations. This hypothesis was confirmed by Moreton and Ramsey (1960), who used a refined photographic technique that permitted the observation of the propagating perturbation, with velocities in the range 500 – 1500 km s–1. In some cases, during the course of the oscillations, the filament becomes visible in the Hα image when the prominence is at rest, but when its line-of-sight velocity is sufficiently large, the emission from the material falls outside the bandpass of the filter and the prominence becomes invisible in Hα. This process is repeated periodically and for this reason this type of event is called a “winking filament”. Ramsey and Smith (1965Jump To The Next Citation Point) and Hyder (1966Jump To The Next Citation Point) studied 11 winking filaments and derived oscillatory periods between 6 and 40 min, and damping times between 7 and 120 min. They reported that there seemed to be no correlation between the period and the filament dimensions, the distance to the perturbing flare or its size. In addition, a single filament perturbed by four flares during three consecutive days oscillated with essentially the same frequency and damping time in each event. As a consequence, it was suggested that prominences possess their own frequency of oscillation.

The oscillatory velocity of the winking filaments studied by Ramsey and Smith (1965Jump To The Next Citation Point, 1966) and Hyder (1966Jump To The Next Citation Point) is quite large compared with the relevant wave speeds in prominences (namely the sound and Alfvén speeds). For this reason, one usually refers to these events as large amplitude oscillations. Recently, and thanks to space- and ground-based instruments, new observations of large amplitude oscillations have been published. The exciters seem to be Moreton or EIT waves (Eto et al., 2002; Okamoto et al., 2004; Gilbert et al., 2008) or nearby jets and subflares (Jing et al., 2003Jump To The Next Citation Point, 2006Jump To The Next Citation Point; Vršnak et al., 2007Jump To The Next Citation Point), while in other cases the oscillations are associated to the eruptive phase of a filament (Isobe and Tripathi, 2006Jump To The Next Citation Point; Isobe et al., 2007; Pouget, 2007; Chen et al., 2008) or are produced by the Alfvénic vortex shedding mechanism recently developed by Nakariakov et al. (2009). In this last case, oscillations could be a signature of the transition from a stable to an unstable situation. Although in most of the observed flare-induced filament oscillations the material undergoes vertical oscillations, Kleczek and Kuperus (1969Jump To The Next Citation Point) and Hershaw et al. (2011Jump To The Next Citation Point) have also reported horizontal oscillations. Moreover, periodic motions along the longitudinal filament axis have also been observed (Jing et al., 2003Jump To The Next Citation Point, 2006Jump To The Next Citation Point; Vršnak et al., 2007Jump To The Next Citation Point).

The most recent interpretation of observations of large amplitude oscillations in a prominence has been given by Hershaw et al. (2011Jump To The Next Citation Point), who studied this kind of oscillations in an arched prominence observed with SoHO/EIT on 30 July 2005. The perturbations were produced by two consecutive trains of coronal waves coming from two different flares in an active region located far away from the prominence site. Both oscillatory trains had periods of around 100 min and excited prominence oscillations that lasted for about 18 h. During the oscillations, the displacement of the prominence was horizontal with respect to the solar surface. In the case of the first wave train, induced by a more energetic flare than the second one, the displacement in all the considered prominence locations shows a clear time damped oscillatory behaviour (see Figure 1View Image). The oscillatory period, the damping time, and the horizontal velocity at different heights along the two prominence legs were determined (see Table 1 in Hershaw et al., 2011Jump To The Next Citation Point). The prominence oscillatory periods seem to depend on the height at which they were measured and, for each wave train, they show some differences depending on the leg in which they were measured. Focussing on the first wave train, which seems to trigger a clearly damped oscillation, the periods range between 86 and 101 min in one leg, and between 92 and 104 min in the other. Furthermore, the velocity amplitude also changes with height and reaches a maximum value of 50 km s–1 in one leg and 33 km s–1 in the other. This growth of the velocity amplitude with height, together with the fact that the oscillation seems to start in phase for both legs, led the authors to suggest that the oscillatory behaviour is caused by a global kink mode. The approximate analytical relationship between the damping time and the period τ = (1.6 ± 0.2)P 0.9±0.1 was derived for the disturbance caused by the first wave train. This analytical fit suggests a linear dependence between the damping time and the period that could be compatible with resonant absorption as the damping mechanism (Ruderman and Roberts, 2002Jump To The Next Citation Point; Ofman and Aschwanden, 2002; Arregui et al., 2008bJump To The Next Citation Point). However, this interpretation must be taken with care since the use of scaling laws to discriminate between damping mechanisms is questionable, at least for resonant absorption (Arregui et al., 2008a). Furthermore, some observational features such as differences in the periods measured in both legs, in the velocity amplitudes at both legs, etc., enabled the authors to suggest that the prominence could be composed by separate oscillating filamentary threads. In summary, from the reported observations it seems that one of the wave trains was able to induce large amplitude oscillations in the prominence while the effect of the second wave train was not so strong. The reason for these different behaviours could be attributed to the different energy carried by the wave trains or, in spite of the wave train periods being apparently similar, to a resonance effect between the wave train frequency and the natural oscillatory frequency of the prominence. Also, it is worth to remark that the reported observation was made in EUV while other observations of large amplitude oscillations have been made in Hα. The correspondence between oscillations observed in EUV and in Hα remains to be ascertained. Probably, only simultaneous observations could cast light on this relationship.

View Image

Figure 1: Displacement versus time produced by two wave trains impacting on a prominence: (a) and (b) in the prominence apex; (c) and (d) in the NE leg; (e) and (f) in the SW leg (from Hershaw et al., 2011).

From the theoretical point of view, models that explain large amplitude filament oscillations are lacking. To explain the vertical motions, Anderson (1967) suggested that the disturbance coming from the flare propagates along the magnetic field and when it arrives to the filament, the material is pushed down. Hyder (1966Jump To The Next Citation Point) proposed a model which explains the vertical motions in terms of harmonically damped oscillations. The restoring force is provided by the magnetic tension, while the damping is due to coronal viscosity. Using this model, Hyder was able to calculate the strength of the vertical component of the magnetic field in the prominence. Later, Kleczek and Kuperus (1969) proposed a similar model to explain the horizontal oscillations, although in this case the damping is provided by the emission of acoustic waves. On the other hand, Sakai et al. (1987) developed a model for the formation of a prominence in a current sheet. One of the features of this model is the presence of non-linear oscillations of the current sheet. Bakhareva et al. (1992) considered a partially ionized plasma and developed a dynamical model for a solar prominence in which non-linear oscillations are present. Chin et al. (2010) have considered possible oscillatory regimes of non-linear thermal over-stability which can occur in prominences. Finally, numerical simulations (Chen et al., 2002) suggest that Moreton and EIT waves can be produced by CMEs. Then, the theoretical modelling of large amplitude oscillations excited by these events is a task that remains to be done. For a more extensive review about large amplitude prominence oscillations see Tripathi et al. (2009).

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