Statistics – Computation
Scientific paper
May 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010phdt.........3s&link_type=abstract
PhD Thesis. Universitat de les Illes Balears
Statistics
Computation
5
Magnetohydrodynamics, Waves, Oscillations, Prominences, Filaments
Scientific paper
High-resolution observations of solar filaments and prominences reveal that these large-scale coronal structures are formed by a myriad of long and thin ribbons, here called threads, which are piled up to form the prominence body. Evidences suggest that these fine structures are magnetic flux tubes anchored in the solar photosphere, which are partially filled with the cool and dense prominence material. Individual and collective oscillations of prominence and filament fine structures are frequently reported by means of oscillatory variations in Doppler signals and spectral line intensity. Common features of these observations are that the reported oscillatory periods are usually in a narrow range between 2 and 10 minutes, that the velocity amplitudes are smaller than ˜3 km/s, and that the oscillations seem to be strongly damped after a few periods. Typically, the ratio of the damping time, tD, to the period, P, is tD/P < 10. While the oscillations have been interpreted
in the context of the magnetohydrodynamic (MHD) theory, i.e., in terms of the MHD normal modes supported by the filament thread body and/or propagating MHD waves, the mechanism or mechanisms responsible for the damping are not well-known and a comparative study between different damping mechanisms is needed. In this Thesis, we study the efficiency of several physical mechanisms for the damping of MHD oscillations in prominence fine structures. Both individual and collective oscillations of threads are analyzed. We model a filament thread as a straight cylindrical magnetic flux tube with prominence conditions, embedded in a magnetized environment representing the solar coronal medium. The basic MHD equations are applied to the model and contain non-ideal terms accounting for effects as, e.g., non-adiabatic mechanisms, magnetic diffusion, ion-neutral collisions, etc., that may be of relevance in prominence plasmas and whose role on the damping of the oscillations is assessed. Our method combines analytical treatments along with numerical computations to obtain the frequency and the perturbations of the linear MHD modes. Among the studied mechanisms, we find that the most efficient one for the damping of transverse thread oscillations, interpreted as kink MHD modes, is the process of resonant absorption in the Alfven continuum. The efficiency of resonant absorption is independent of the plasma ionization degree and is consistent with the reported values of tD/P. Thermal effects, as well as magnetic diffusion, are irrelevant for the damping of transverse oscillations. Regarding longitudinal oscillations, i.e., slow MHD modes, radiative losses from the prominence plasma and ion-neutral collisions are the processes that provide the smallest damping times. Their combined effect causes an efficient attenuation of slow modes in filament threads, with tD/P compatible with the observed values. Finally, Alfven waves are also investigated, and we obtain that they are damped by ion-neutral collisions. However, the damping of Alfven waves is not very efficient because the theoretical damping times are between one and two orders of magnitude larger than the corresponding periods. All these conclusions apply for both individual and collective oscillations of threads.
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