Magnetic properties of the umbral boundary during sunspot decay

\nContext. In recent years, the magnetic properties of the umbra-penumbra boundary of sunspots and the boundary of pores at various evolutionary stages have been characterised using datasets from different instruments.\nAims. We aim to study the intrinsic differences between the intensity and vector magnetic field properties derived from Hinode/SP and SDO/HMI observations of a decaying sunspot.\nMethods. We analysed the sunspot embedded in active region NOAA 12797 during six days in 30 SP/Hinode scans and 704 HMI/SDO for both regular maps and maps corrected for scattered light, HMIdcon. We studied the correlation of the magnetic properties and continuum intensity in the datasets within the spot, and we investigated the differences at the umbra-penumbra boundary. We examined the decaying process in detail using the full temporal resolution of the HMIdcon maps.\nResults. We find a good one-to-one correspondence between the magnetic properties in the SP and HMIdcon maps, but the continuum intensity of the spots in the SP maps is found to be 0.04 IQS brighter than in the HMIdcon maps. The considerable influence of scattered light in the HMI maps makes it the least ideal dataset for studying the boundary of spots without a penumbra. The properties at the umbra-penumbra boundary evolve slowly during the sunspot decay stage, while the penumbra still provides some stability. In contrast, they respond more abruptly to areal changes in the naked-spot stage. During the sunspot decay, we find linear decay in the area and in the magnetic flux. Moreover, the umbra shows two characteristic decaying processes: a slow decay during the first three days, and a sudden fast decay during the final dissipation of the penumbra. We find indications of a 3.5 h lag between the dissipation of the vertical fields in the umbral region and the photometric decay of the umbral area.\nConclusions. The differences found in the continuum intensity and in the vertical component of the magnetic field, Bver, between the analysed datasets explain the discrepancies among the Bver values found at the boundaries of umbrae in previous studies.\n