Sunspots are one of the most common solar activities on the sun's surface. Sunspots are also the strongest magnetic field in the solar active region. The magnetic field structure in the upper atmosphere of the Sun is closely related to the magnetic field evolution of sunspots. The vast majority of flare occurred in the atmosphere above the sunspots, the adjacent area of sunspot will mutate after the flare (sunspot penumbral disappearance, or umbral longitudinal magnetic field enhancement, etc.). The study of sunspots can not only further understand the local evolution of the solar magnetic field, but also understand the relationship between other solar activities and the evolution of sunspots. In this paper, the formation and decay of sunspot penumbra are studied in detail by using the high resolution data of space and ground-based telescopes, and explore the physical origin of the formation and decay of sunspot penumbra.The main research content includes two aspects.First, the formation of a sunspot penumbra sector in Active Region 12574.By using the data from the Solar Dynamics Observatory and Goode Solar Telescope at the Big Bear Solar Observatory, we studied the process of a penumbra sector formation in a developing sunspot located in active region 12574. We derived the fol-lowing results:(1) The area where the penumbra is about to form is always blueshift and the area and magnetic flux in sunspot umbra and penumbra all increase with time at the beginning of the formation of sunspot penumbra sector. These phenomena suggest that the sunspot penumbra sector was the result of the newly emerged magnetic flux tube that was trapped in the photosphere.(2) When the rudimentary penumbra formed, the total magnetic flux and area of the umbra gradually decreased, while the total magnetic flux and area of the penumbra was increasing. The penumbra developed at the cost of umbra.(3) After the formation of the rudimentary penumbra, the developing sunspot began to rotate counterclockwise. The counterclockwise rotatin of sunspot may stretch the magnetic field lines of penumbra and help the rapid formation of penumbra.Secondly, the formation and decay of sunspot penumbrae in three regions of Active Region 12673.In this case, we studied the formation and decay process of sunspot penumbra in three regions of active region 12673 by using the multi-band observation data from the Solar Dynamics Observatory.The main results are as follow:(1) During the formation of penumbra,the area and magnetic flux in umbra and penumbra all increase with time. In addition,the average longitudinal magnetic strength in penumbra decreased, the average transverse magnetic strength increased and the mean magnetic inclination angle increased with the increasing penumbral area. All observations show that the sunspot penumbra is formed by the newly emerged magnetic flux tube trapped in the photosphere.The formation of sunspot penumbra was involved in the interaction of two magnetic field systems,and the first emerging magnetic flux tubeinhibits the upward movement of the subsequent emerging magnetic flux tube.(2) When the decay of penumbra, the area and magnetic flux in umbra and penum-bra decrease with time. The average longitudinal magnetic strength in penumbra in-creased, the average transverse magnetic strength decreased and the mean magnetic inclination angle decreased with the increasing penumbral area. These results suggest that some of the horizontal magnetic lines in the penumbra became more vertical dur-ing the decay of the penumbra. The magnetic tube that forms the penumbra may tend to shrink. The change of the penumbral magnetic field from horizontal to vertical can result in the disappearance of the sunspot penumbra.
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