Oscillations in a solar filament: first observation of long periods in the HeI 584.33 Å line, modelling and diagnostic

Institut d'Astrophysique Spatiale, CNRS, Université Paris-Sud 11, Bâtiment 121, 91405 Orsay Cedex, France e-mail: guillaume.pouget@ias.u-psud.fr

Received: 22 July 2005
Accepted: 11 December 2005

Abstract

Three long observations of filaments were carried out in the 584.33 Å  HeI line during MEDOC campaigns in November 2003 and November 2004, by the Coronal Diagnostic Spectrometer onboard SOHO. Their duration was 15-16 h and the temporal resolution was 20 s, allowing measurements of the entire range of periodicities theoretically expected in a filament (particularly both the short periods, less than 10 min, and the long ones, more than 40 min). Fourier analysis of the Doppler velocities in the filament allows us to detect oscillations over a wide range, and particularly very slow velocity oscillations (5-6 h) that were not detected previously. We test an earlier interpretation of these velocity oscillations in the prominence model of Joarder & Roberts (1993, A&A, 277, 225), who treat the prominence as a slab. We used a systematic method to identify the six fundamental modes predicted by the model among the numerous frequencies detected. Identification of these modes – and especially the slowest one, the slow kink mode – provides a complete diagnostic of the filament in terms of the Alfvén speed, temperature, and angle between the magnetic field and the main axis of the filament.