  To better understand the formation and decay of sunspot penumbra, we studied
the evolution of sunspots in the three regions of the active-region NOAA 12673
in detail. The evolution of sunspots in the three regions were all involved in
the interaction of two magnetic field systems: the pre-existing magnetic field
system and the later emerging magnetic field system. Through analyzing the
photospheric magnetic field properties, it is found that the formation of the
penumbra originated from the newly emerging magnetic bipole that were trapped
in the photosphere. The change of magnetic fields in penumbra from horizontal
to vertical can cause the disappearance of penumbra. The transformation of the
magnetic field between the umbra and the penumbra is found and the outward moat
flow around sunspot gradually decreased and vanished during the decay of
sunspot. In addition, we found that the mean longitudinal magnetic strength in
penumbra decreased and the mean transverse magnetic strength in penumbra
increased with the increasing penumbral area during the formation of sunspots.
However, during the decay of sunspots, the mean longitudinal magnetic strength
in penumbra increased and the mean transverse magnetic strength in penumbra
decreased with the decreasing penumbral area. Comparatively, the dependence of
the area and the mean transverse/longitudinal magnetic field strength in umbra
is not remarkable. These results reveal that the formation and decay process of
umbra are different with penumbra.
