Magnetic properties on the boundary of an evolving pore.

Context. Analyses of magnetic properties on umbrae boundaries have led to the Jurcak criterion, which states that umbra-penumbra boundaries in stable sunspots are equally defined by a constant value of the vertical magnetic field, Bver_crit, and by 0.5 continuum intensity of the quiet Sun, Iqs. Umbrae with vertical magnetic fields stronger than Bver_crit are stable, whereas umbrae with vertical magnetic fields weaker than Bver_crit are unstable and prone to vanishing. Aims. To investigate the existence of a Bver_crit on a pore boundary and its role in the evolution of the magnetic structure. Methods. We analysed SDO/HMI vector field maps corrected for scattered light with a temporal cadence of 12 minutes during a 28-hour period. An intensity threshold (Ic = 0.55 Iqs) is used to define the pore boundary and temporal evolutions of the magnetic properties are studied there. Results. We observe well-defined stages in the pore evolution: (1)during the initial formation phase, total magnetic field strength (B) and vertical magnetic field (Bver) increase to their maximum values of 1920 G and 1670 G, respectively; (2)then the pore reaches a stable phase; (3)in a second formation phase, the pore undergoes a rapid growth in size, along with a decrease in B and Bver on its boundary. In the newly formed area of the pore, Bver remains below 1665 G and B below 1921 G; (4)ultimately, pore decay starts. We find overall that pore areas with Bver<1665 G, or equivalently B<1921 G, disintegrate faster than regions that fulfil this criteria. Conclusions. We find that the most stable regions of the pore, similarly to the case of umbral boundaries, are defined by a critical value of the vertical component of the magnetic field that is comparable to that found in stable sunspots. In this case study, the same pore areas can be equivalently well-defined by a critical value of the total magnetic field strength.