Magnetic properties of the umbral boundary during sunspot decay. A comparative study of multiple datasets
Abstract
Context: 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.
Aims: 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.
Methods: We analyse the sunspot embedded in the active region NOAA 12797 during 6~days in 30 SP/Hinode scans, and 704 HMI/SDO for both regular maps and maps corrected for scattered light, HMI_dcon. We study the correlation of the magnetic properties and continuum intensity among datasets within the spot, and investigate the differences on the umbra-penumbra boundary. We examine the decaying process in detail using the full temporal resolution of the HMI_dcon maps.
Results: We find a good one-to-one correspondence between the magnetic properties in SP and HMI_dcon maps, however, the continuum intensity of the spots in SP maps is found 0.04I_QS brighter than in HMI_dcon maps. The considerable influence of scattered light in HMI maps makes it the least ideal dataset to study the boundary of spots without a penumbra. The properties on the umbra-penumbra boundary evolve slowly during the sunspot's 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 a linear decay both 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 hints of a 3.5 h lag between the dissipation of the vertical fields on the umbral region and the photometric decay of the umbral ! area.
Conclusions: The differences found in the continuum intensity and in the vertical component of the magnetic field, B_ver, between the analysed datasets explain the discrepancies among the B_ver values found on the boundaries of umbrae and pores in previous studies.