A simple thread model consists of an infinitely long cylinder filled with cold, dense plasma and embedded in the hotter and less dense corona; field line curvature is neglected. The magnetic field is parallel to the cylinder axis and uniform everywhere (Figure 31).
The MHD modes of this structure have been extensively studied in the context of coronal and photospheric magnetic tube oscillations (Spruit, 1982; Edwin and Roberts, 1983; Cally, 1986). The mode of interest here is the kink mode because it is the only one that produces a significant transverse displacement of the thread, which is the observed behavior of oscillating threads. In the absence of mass flows and assuming that the thread radius is much smaller than the wavelength, the kink frequency is given by
This formula for is based on some assumptions, namely that the thread is much longer than the wavelength, which in turn is much larger than the thread radius (this last approximation is also know as the thin tube limit). Short-wavelength propagating waves in threads have been detected by Lin et al. (2007) (see Section 3.6.4 and Figure 22). The length of the fine structure is around 20 arcsec, the reported wavelength is 3.8 arcsec, and the radius of threads is typically between 0.1 and 0.15 arcsec. We can appreciate that the assumptions made to derive Equation (23) are satisfied in this event.
Nakariakov and Roberts (1995) studied the magnetosonic modes of a magnetic slab when flows are present, while Soler et al. (2008) considered non adiabatic waves and included a mass flow parallel to the magnetic field in the thread model of Figure 32, which is identical to that of Figure 31 except for the inclusion of plasma flows. Without loss of generality the flow speed in the corona was neglected in this last work, while typical values observed in prominences were taken for the flow speed in the thread (namely ). In the absence of flow, the complex oscillatory frequencies for a fixed, real and positive wavenumber appear in pairs, and . The solution corresponds to a wave propagating towards the positive z-direction (parallel to magnetic field lines). The solution corresponds to a wave that propagates toward the negative z-direction (antiparallel to magnetic field lines). Both solutions have exactly the same physical properties in the absence of flows. In the presence of flow, the frequencies are Doppler shifted. In addition, the symmetry between parallel and antiparallel propagation is broken. For instance, for strong enough flows, slow waves can only propagate parallel to the flow direction, antiparallel propagation being forbidden. Figure 33 presents the period of the slow, fast and thermal modes as a function of the flow speed in the thread. For the fast and slow waves acquire different periods that diverge as is increased. For the antiparallel slow wave becomes a backward wave, which causes its period to grow dramatically near this flow velocity. The influence of the flow on the fast mode is not so severe, while the thermal mode has a finite period that takes very large values.
Some authors have reported that groups of threads oscillate in unison (e.g., Yi et al., 1991) and that large areas of a prominence present in-phase oscillations (e.g., Terradas et al., 2002; Lin et al., 2007), which may be also taken as a sign of collective thread behaviour (see Sections 3.4, 3.6.3, and 3.6.4). Similar collective oscillations have been observed in coronal loops (Verwichte et al., 2004) and their properties have been studied by, e.g., Murawski (1993), Luna et al. (2008), Van Doorsselaere et al. (2008b), and Robertson and Ruderman (2011). To model this situation, an equilibrium model made of two homogeneous and infinitely long prominence threads embedded in the coronal medium has been considered (see Figure 34).
When identical threads are considered, the system exhibits four kink-like transverse oscillatory modes (Luna et al., 2008; Soler et al., 2009a). These modes are denoted by , , and . The and denote symmetry or antisymmetry of the total pressure perturbation with respect to the yz-plane. The subscript describes the main direction of polarization of motions, that is in the xy-plane; the choice of the coordinate axes is shown in Figure 34 and the spatial distribution of the modes is displayed in Figure 35. In addition to the kink-like modes, Soler et al. (2009a) studied the collective slow modes and obtained only two fundamental collective solutions, one symmetric and the other antisymmetric with respect to the yz-plane, with motions mainly polarized along the z-direction (Figure 35).
A measure of the interaction between threads is the frequency of their normal modes. If the modes have frequencies similar to that of the isolated cylinder, then the threads oscillate independently from one another. If the frequencies are significantly different, the threads oscillate in a collective manner. The left panel of Figure 36 displays the real part of the frequency of the four kink-like solutions as a function of the distance between cylinders. For large separations, i.e., for a distance between threads larger than about 6 or 7 radii, the collective kink mode frequencies are almost identical to the individual kink frequency. This is a signature of a weak interaction between threads, which behave as independent structures. On the other hand, for short thread separations the four frequencies separate in two branches as a consequence of a strong interaction between the cylinders. Therefore, the collective behaviour of oscillations becomes stronger when the threads are closer. In the case of slow modes the interaction between threads is almost negligible and as a result the frequencies of the and modes are almost identical to the individual slow mode frequency (cf. right panel of Figure 36) in the whole range of thread separations. This is in agreement with the fact that transverse motions (responsible for the interaction between threads) are not significant for slow modes in comparison with their longitudinal motions. Therefore, the and modes essentially behave as individual slow modes, contrary to kink-like modes, which display a more significant collective behaviour.
Soler et al. (2009a) assessed the effect of material flows along two threads on the behaviour of collective modes (see Figure 37 for a sketch of the model). Arbitrary flows and were assumed in both cylinders, while coronal flows were neglected. The first main conclusion of this work is that the flows do not eliminate wave modes with collective dynamics (i.e., those that produce significant perturbations in the two threads), even in the case . Nevertheless, the requisite for retaining the collective dynamics is that the Doppler-shifted individual frequencies of the threads must be very similar. In the case of kink-like modes the Doppler-shifted frequencies are given byet al. (2009a) obtained
Soler et al. (2009a) extracted another conclusion from Equations (26) and (27): the difference between the Alfvén (sound) speed of the threads determines the difference of the flow speeds for the existence of collective behaviour of kink (slow) modes. Therefore, when flows are present in the equilibrium, collective motions can be found even in systems of non-identical threads for very specific combinations of the two flow velocities. These velocities are within the observed values in prominences if threads with not too different temperatures and densities are considered. However, since the flow velocities required for collective oscillations must take very particular values, such a special situation may rarely occur in prominences. This conclusion has important repercussions for future prominence seismological applications, given that if collective oscillations are observed in large areas of a prominence, threads in such regions should possess very particular combinations of temperatures, densities, magnetic field strengths and flows.
Living Rev. Solar Phys. 9, (2012), 2
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