Survey of small-amplitude waves in solar prominences

The presence of small-amplitude oscillation or waves in solar prominences was investigated using archived data from a full disk Hα imaging spectroscopic observation. A total of 178 prominences on the limb were studied and it was found that there is plenty of evidence of oscillations or waves with preferential periods in 3–$5\:$min ($4\:$min) and 10–$20\:$min ($15\:$min). We found a very clear signature of a persistently propagating $4\:$min wave in 21 prominences and at least a hint of $4\:$min oscillation in 90 prominences in their Doppler signal. The $4\:$min waves are often found in prominences with a large horizontal size, i.e., long filaments seen from the side, and they propagate in the nearly horizontal direction with a phase speed of 140–400 km s⁻¹. On the other hand, very clear evidence of a $15\:$min wave was found in 14 prominences and at least a hint of oscillation in 62 prominences in the Doppler signal. The $15\:$min waves preferentially propagate in the vertical direction with a phase speed of 20–50 km s⁻¹. There is a positive correlation between the presences of $4\:$min and $15\:$min waves. The $4\:$min waves are interpreted as Alfvénic waves propagating along the coronal magnetic field supporting the prominences. Two scenarios are discussed for the $15\:$min wave, i.e., an Alfvénic wave propagating vertically in prominence bodies, and apparent cross-field propagation of an ensemble of Alfvénic waves in magnetic structures. Prominences that harbor the short-period waves or oscillations are distributed over the entire solar latitude, suggesting that these waves are excited by ubiquitous disturbance in the solar atmosphere. It is also true that a number of prominences do not show signatures of oscillations nor propagating waves, and the reason for this diversity remains open question. Our study revealed that persistently propagating short-period waves in prominences is not particularly rare but rather a common phenomena, and suggests that observing the $4\:$min waves in prominences will serve as a useful tool for tracing the coronal Alfvénic waves and the magnetic fields supporting the prominences. The result of this study implies that the magnetic fields are preferentially horizontal in long quiescent prominences.