Observations of oppositely directed umbral wavefronts rotating in sunspots obtained from the New Solar Telescope of BBSO

Abstract

We study the umbral waves as observed by chromospheric imaging observations of two sunspots with the New Solar Telescope at the Big Bear Solar Observatory. We find that the wavefronts (WFs) rotate clockwise and form a one-armed spiral structure in the first sunspot, whereas two- and three-armed structures arise in the second sunspot where the WFs rotate anticlockwise and clockwise alternately. All the spiral arms display propagation outwards and become running penumbral waves once they cross the umbral boundaries, suggesting that the umbral and penumbral waves propagate along the same inclined field lines. We propose that the one-armed spiral structure may be produced by the WF reflections at the chromospheric umbral light bridge, and the multi-armed spirals may be related to the twist of the magnetic field in the umbra. Additionally, the time lag of the umbral oscillations in between the data of He I 10830 Å and Ha - 0.4 Å is ∼17 s, and it is ∼60 s for that in between the data of 304 Å and Ha - 0.4 Å. This indicates that these disturbances are slow magnetoacoustic waves in nature, and that they propagate upward along the inclined lines with fast radial expansions causing horizontal velocities of the running waves.