6.1 Seismology of large amplitude prominence oscillations

Several studies have made use of the observed characteristics of large amplitude oscillations in prominences to deduce physical parameters of these objects. The classic example is the interpretation by Hyder (1966) of the winking filament phenomenon in terms of a global mode of the prominence. This author modeled the eleven winking filament events reported by Ramsey and Smith (1965) as damped harmonic oscillators and obtained estimates of the vertical magnetic field strength in the range 2 – 30 G. More recent studies have also used large amplitude oscillations in filaments to deduce the magnetic field strength in these objects.

Vršnak et al. (2007Jump To The Next Citation Point) reported on Hα observations of periodical plasma motions along the axis of a filament. The motions were both large amplitude and large scale, with an initial displacement of 24 Mm, an initial velocity amplitude of 51 km s–1, a period of 50 min, and a damping time of 115 min. Oscillations were interpreted as a global mode of the system and the driver was thought to be the magnetic flux injection by magnetic reconnection at one of the filament legs. Although oscillatory motions along the prominence axis were also reported by Jing et al. (2003, 2006), the study by Vršnak et al. (2007Jump To The Next Citation Point) proposes an explanation for the triggering process and the restoring force, and performs diagnostics based on these interpretations.

The seismology analysis by Vršnak et al. (2007Jump To The Next Citation Point) is based on the fitting of the oscillation properties to a mechanical analogue model in terms of the classic damped harmonic oscillator equation. This analogue is first used to discard gas pressure as the restoring force, since it leads to sound speed values one order of magnitude larger than those corresponding to the typical temperature of prominence plasmas, and no signature of plasma at those temperatures was observed in TRACE EUV images. In this work a twisted flux rope model with both axial and azimuthal magnetic field components was considered and an excess azimuthal field at one of the prominence legs was assumed. This gives rise to a magnetic pressure gradient and a torque, which in turn drive a combined axial and rotational motion of the plasma. Next, an expression that relates the azimuthal Alfvén speed, vAφ, and the oscillatory period was obtained. From this relation, the Alfvén speed vAφ ∼ 100 km s–1 was inferred. By further assuming that the number density of the prominence plasma is in the range 1010 – 1011 cm–3, the azimuthal magnetic field strength results in the range 5 – 15 G. By measuring the pitch angle, Vršnak et al. (2007Jump To The Next Citation Point) additionally determined the internal structure of the flux rope helical magnetic field, from which the axial magnetic field strength was estimated to be in the range 10 – 30 G.

The twisted flux rope model was also invoked by Pintér et al. (2008Jump To The Next Citation Point) in their analysis of SoHO EUV observations of large amplitude transverse oscillations in a polar crown filament previously studied by Isobe and Tripathi (2006). Oscillations were present along a foot belonging to a larger prominence structure and occurred prior to the eruption of the full structure. Wavelet analysis tools were used to shed light into the temporal and spatial behaviour of oscillations. The filament oscillated as a rigid body with a period of 2.5 h, that was constant along the filament, but decreased in time. The line-of-sight velocity was estimated to be about a few tens of km s–1. The analysis of the spatial properties of the oscillations shows evidence of a global standing transverse oscillation, although some small scale oscillations within the structure cannot be discarded. Using the twisted flux rope model for the filament and based on the same scenario and analysis as Vršnak et al. (2007), the azimuthal Alfvén speed component was estimated to be v A φ = 49 km s–1 and the axial magnetic field strength in the range 2 – 10 G. In this case, the pitch angle could not be measured. By assuming a mean value of 65°, Pintér et al. (2008) estimated that the axial component of the magnetic field must be in the range 1 – 5 G.

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