Large amplitude oscillatory motion along a solar filament

Abstract

Context: Large amplitude oscillations of solar filaments is a phenomenon that has been known for more than half a century. Recently, a new mode of oscillations, characterized by periodical plasma motions along the filament axis, was discovered.
Aims: We analyze such an event, recorded on 23 January 2002 in Big Bear Solar Observatory Hα filtergrams, to infer the triggering mechanism and the nature of the restoring force.
Methods: Motion along the filament axis of a distinct buldge-like feature was traced, to quantify the kinematics of the oscillatory motion. The data were fitted by a damped sine function to estimate the basic parameters of the oscillations. To identify the triggering mechanism, morphological changes in the vicinity of the filament were analyzed.
Results: The observed oscillations of the plasma along the filament were characterized by 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. We interpret the trigger in terms of poloidal magnetic flux injection by magnetic reconnection at one of the filament legs. The restoring force is caused by the magnetic pressure gradient along the filament axis. The period of oscillations, derived from the linearized equation of motion (harmonic oscillator) can be expressed as P=π√{2}L/v_Aϕ≈4.4L/v_Aϕ, where v_Aϕ =B_ϕ0/√μ_0ρ represents the Alfvén speed based on the equilibrium poloidal field B_ϕ0.
Conclusions: Combination of our measurements with some previous observations of the same kind of oscillations shows good agreement with the proposed interpretation.

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