The aim of this proposal is to advance our understanding of large-amplitude oscillations on prominences by gathering an International Team, spanning a wide range of expertise. Based on the current knowledge about solar prominences, the team will produce new developments in the theory of prominence oscillations and analysis of high-resolution observations. Recent studies have shown that such oscillations open a new window on coronal connectivity, as well as novel diagnostics for hard-to-measure prominence properties such as magnetic field strength and geometry.
Prominences (also known as filaments on the disk) consist of mostly cool (10.000 K), dense material suspended in the million-degree corona within larger magnetic frameworks called filament channels. Filament channels are essential elements of major solar activity. These channels, (often existing without any cool material in them), form around polarity inversion lines (PILs) seen in the photospheric magnetic fields and are the birthplaces of all eruptive flares and coronal mass ejections (CMEs). Despite decades of observations and theoretical work, understanding magnetic structure and the source of prominence mass remain major unsolved problems in heliophysics, with profound implications for space weather.
Recently, solar physicists have realized that the prominence oscillations long observed by ground- and space-based instruments provide a powerful method for determining prominence parameters. By means of this technique, known as prominence seismology, one can determine physical properties that are difficult to measure directly by combining observations, theory, and modelling.
Prominence oscillations can generally be divided into small- and large-amplitude classes. According to Oliver & Ballester (2002), small-amplitude oscillations have velocities no greater than 2-3 km/s and are highly localized within a small portion of a filament. In contrast, large-amplitude oscillations (LAOs) involve motions with velocities above 20 km/s, and large portions of the filament that move in phase. While the literature about small-amplitude oscillations is extensive (see review by Arregui et al. 2012) the LAOs are less widely reported and their nature is poorly understood. The key point is that LAOs are just most appropriate motions for the purpose of determining the large-scale filament structure. Therefore our proposed team investigation will be focused on this phenomenon. Our long-term goal is to understand the nature and the excitation mechanisms of LAOs, the implications for prominence structure, and their relationship with energetic solar activity.
Explaining the LAOs is very challenging because the accelerations involved are huge, and therefore the restoring force must be very large. The energy contained in the oscillation is also enormous, because the filament is massive and moves with large velocities. However, the motions damp quickly in a few periods, implying a very effective damping mechanism. Motions are found in both the transverse and longitudinal directions with respect to the filament axis, therefore it is important to differentiate between the two kinds of LAOs. The excitation mechanisms and probably the underlying physics of both types of oscillations are different. Large-amplitude transverse oscillations (LATOs) are typically excited by energetic disturbances produced by distant flares and CMEs (Hyder, 1960; Ramsey & Smith 1966; Eto et al. 2002; Okamoto et al. 2004; Gilbert et al. 2008; Hershaw et al. 2011; and Liu et al. 2013). In contrast to the transverse motions, large-amplitude longitudinal oscillations (LALOs) can be excited by nearby impulsive events, e.g., microflares (Jing et al. 2003; Jing et al. 2006; Vršnak et al. 2007; Zhang et al. 2012; Li & Zhang 2012; and Luna et al. 2014). In other cases the LAOs occur during the pre-eruptive activation of a filament (Isobe &Tripathi 2006; Isobe et al. 2007; Pouget 2007; Chen et al. 2008) that is subsequently expelled as part of a CME. The LAOs associated with the eruptive phase will not be addressed by this team, as we are most interested in using the oscillations to probe quiescent prominence structure and dynamics.
Several models have been proposed to explain the restoring force and damping mechanism of the transverse LAOs (see review by Tripathi et al. 2009), but most do not successfully describe the motions. However, Hyder (1966) and Kleczek & Kuperus (1969), who proposed that the restoring force is the tension of the magnetic field, and despite the simple formulation of these heuristic models, they found reasonable agreement with observed displacements and field strengths. Since then, surprisingly few efforts have been made to improve upon these models for the LATOs. Recently, Hershaw et al. (2011), who suggested that their observations of transverse prominence oscillations could be associated with a global kink mode of the filament channel structure and that the damping mechanism could be resonant absorption. Existing analyses of transverse oscillations in coronal loops will surely provide guidance for a renewed effort to elucidate the physics of LATOs in prominences, as we propose below.
More comprehensive theoretical and numerical studies of LALOs have been performed recently, by members of the proposed team and others (e.g., Luna & Karpen 2012; Luna et al. 2012a, 2012b; Xia et al. 2011; Zhang et al. 2012, 2013). Perhaps the most important result of these studies is the clear association between the existence of longitudinal oscillations and the requirement for dips in the supporting filament-channel magnetic structure. In LALO models, the restoring force could not have a magnetic origin because the motion is along the magnetic field (i.e., the filament magnetic field is almost aligned with its main axis). Luna & Karpen (2012) identified longitudinal LALOs as the motion of the prominence threads along the dipped magnetic field lines that support the cool mass. Gravity was pointed out as the restoring force for these oscillations, while the damping is most likely attributable to a combination of radiative losses and continuous mass accretion onto the prominence threads. A new diagnostic tool was revealed: by measuring the frequency of oscillations, the radius of curvature of dips and the magnetic-field strength can be derived from observations. The longitudinal motions appear to be triggered by microflares close to the filament: both observations and simulations show that jets emanating from the microflares can propagate along the filament-channel field lines and displace the prominence threads, initiating the oscillations (Zhang et al. 2013; Luna et al. 2014). The agreement between direct measurements and theoretical models indicates that prominence seismology with LAOs is a very powerful technique, with great promise for resolving long-standing, fundamental mysteries about prominences. With the wealth of existing and anticipated data coming from spaceborne missions such as Solar Dynamics Observatory (SDO), Hinode, and Solar Orbiter, the time is right for an international team of solar physicists to bring this novel technique to maturity.
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