  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.
