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Context. Analyses of magnetic properties on umbrae boundaries led to theJurcak criterion, which states that umbra-penumbra boundaries in stablesunspots are equally defined by a constant value of the vertical magneticfield, Bver_crit, and by 0.5 continuum intensity of the quiet Sun, Iqs. Umbraewith vertical magnetic fields stronger than Bver_crit are stable, whereasumbrae with vertical magnetic fields weaker than Bver_crit are unstable andprone to vanishing. Aims. To investigate the existence of a critical value ofthe vertical magnetic field on a pore boundary and its role in the evolution ofthe magnetic structure. Methods. We analysed SDO/HMI vector field mapscorrected for scattered light with a temporal cadence of 12 min during a26.5-hour period. An intensity threshold (Ic = 0.55 Iqs) is used to define thepore boundary and temporal evolutions of the magnetic properties are studiedthere. Results. We observe stages in the pore evolution: (1)during an initialformation phase, total magnetic field strength (B) and vertical magnetic field(Bver) increase to their maximum values of ~1920 G and ~1730 G, respectively;(2)then the pore reaches a stable phase; (3)in a second formation phase, thepore undergoes a rapid growth in size, along with a decrease in B and Bver onits boundary. In the newly formed area, Bver remains mostly below 1731 G and Bremains everywhere below 1921 G; (4)ultimately, pore decay starts. We findoverall that pore areas with Bver<1731 G, or equivalently B<1921 G,disintegrate faster than regions that fulfil this criteria. Conclusions. Themost stable regions of the pore, similarly to the case of umbral boundaries,are defined by a critical value of Bver that is comparable to that found instable sunspots. In addition, in this case study, the same pore areas can besimilarly well-defined by a critical value of the total magnetic fieldstrength.

Magnetic properties on the boundary of an evolving pore