Observation of Persistent Propagating Waves in a Solar Prominence

Spectropolarimetric observation of a quiescent prominence was performed in He i 1083 nm. With the slit of the spectrograph fixed on the prominence, we obtained a time series of Stokes profiles for a duration of 108 minutes with a cadence of 8 s. Also using imaging spectroscopic data in H alpha and extreme-ultraviolet images at 193 & Aring;, we obtained the following results: (1) The prominence harbors persistent waves propagating across its body. (2) The waves are conspicuous in the line-of-sight (LOS) velocity with an amplitude of about 2 km s-1 but not detectable in the intensity of emission lines or magnetic field. (3) Noticeable periods are about 4 minutes (220 and 260 s) and 15 minutes (910 s), and they propagate with phase speeds of 140 similar to 200 and similar to 24 km s-1 preferentially in horizontal and vertical directions, respectively. (4) Magnetic field strength was estimated to be about 2.5 G with its LOS component of 0 +/- 1 G on average. We interpret the 4 minute oscillation as Alfv & eacute;nic waves propagating in the corona containing the prominence threads, and it is not a resonant wave trapped in the prominence body but excited by an external source. On the other hand, a 15 minute oscillation is interpreted as Alfv & eacute;nic waves propagating upward in the prominence body. The prominence was evolving in its height during our observation, and there is a hint that the 15 minute period gradually gets longer during the observation. These waves are thought to be taking place in layers with different field directions and strength superposed in the LOS.