Oscillations in an active region filament: Observations and comparison with MHD waves

Institut d'Astrophysique Spatiale, Unité Mixte CNRS-Université Paris XI, bâtiment 121, 91405 Orsay Cedex, France

Corresponding author: S. Régnier, stephane.regnier@ias.u-psud.fr

Received: 13 October 2000
Accepted: 18 June 2001

Abstract

During the MEDOC Campaign 4, on October 1999, observations of a solar active region filament were carried out by the SUMER/SoHO spectrometer. A time sequence of this filament has been obtained with a duration of 7 h 30 min and with a temporal resolution of 30 s. The Fourier analysis of the line-of-sight Doppler velocities measured in the 584.33 Å HeI line allows us to detect oscillations in several ranges of periodicities (short periods: less than 5 min, intermediate periods: 6-20 min, and long periods: greater than 40 min). From a theoretical point of view, we consider the possible modes of oscillations of an active region filament. Following [CITE], we treat the filament as a plasma slab embedded in a uniform magnetic field inclined at an angle ϕ to the long axis of the slab. Solving the dispersion equations for Alfvén waves and magnetoacoustic waves, primary and secondary mode frequencies appear to be non-equidistant. For the comparison between the observed and calculated frequencies, we outline an identification method of the oscillation modes in the observed filament. This identification provides a diagnostic of the filament: the angle between the magnetic field and the long axis of the slab is estimated to be 18°, and the magnetic field strength B (G) is proportional to the square root of the density () in the slab, .