  Context. Analyses of magnetic properties on umbrae boundaries 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 critical value of
the vertical magnetic field 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 min during a
26.5-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 stages in the pore evolution: (1)during an initial
formation 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, the
pore undergoes a rapid growth in size, along with a decrease in B and Bver on
its boundary. In the newly formed area, Bver remains mostly below 1731 G and B
remains everywhere below 1921 G; (4)ultimately, pore decay starts. We find
overall that pore areas with Bver<1731 G, or equivalently B<1921 G,
disintegrate faster than regions that fulfil this criteria. Conclusions. The
most 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 in
stable sunspots. In addition, in this case study, the same pore areas can be
similarly well-defined by a critical value of the total magnetic field
strength.
