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.
