Structure and evolution of velocities in quiescent filaments

Abstract

Simultaneous observations of radial velocities in a ‘quiescent’ prominence seen in Hα on the disk and in the underlaying photosphere have been obtained in the Meudon Observatory: Doppler shifts in photospheric lines are weaker than in the surrounding regions (<0.3 km s^-1); the scale of velocity structures is smaller (<10⁴ km). The vertical component of velocities cannot be neglected. Hα Doppler shifts show that: (a) Highest velocities are often correlated with high brightness horizontal gradients, which suggests that filament and surrounding bright regions belong to the same geometrical and dynamical structure. (b) Fast motions (7 km s^-1) have short life-times (a few minutes). (c) Slow motions in dark regions (<3 km s^-1) are associated with blue shifts and may last several hours. This behaviour was confirmed in many other cases by filament observations with the 3-wavelength Hα patrol. This is consistent with EUV observations of the transition zone around prominences, but disagrees with ‘downward motions’ as seen at the limb, unless these motions do not refer to material velocities.