EDP Sciences
Free access
Volume 376, Number 1, September II 2001
Page(s) 292 - 301
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20010972

A&A 376, 292-301 (2001)
DOI: 10.1051/0004-6361:20010972

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

S. Régnier, J. Solomon and J. C. Vial

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

(Received 13 October 2000 / Accepted 18 June 2001)

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 ÅHeI 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 Joarder & Roberts (1993), we treat the filament as a plasma slab embedded in a uniform magnetic field inclined at an angle $\phi$ 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$^{\circ}$, and the magnetic field strength B (G) is proportional to the square root of the density $\rho_{\rm o}$ ( $\rm cm^{-3}$) in the slab, $B \sim 2.9 \times 10^{-5}
\sqrt{\rho_{\rm o}}$.

Key words: Sun: filaments -- Sun: oscillations -- MHD -- waves

Offprint request: S. Régnier, stephane.regnier@ias.u-psud.fr

© ESO 2001