Abstract
Since the hint of the existence of an invariable value of the vertical magnetic field (Bver) on umbra-penumbra (UP) boundaries in sunspots by Jurčák (2011), many investigations have focused on studying this property. UP boundaries of stable sunspots are defined by a critical Bver.
On the other hand, when an umbra or pore do not hold a sufficiently strong Bver, i.e. the inclined field is weaker than the critical value, penumbral magneto-convection overtakes. Here we introduce the study of an evolving pore that does not develop a penumbra.
This study suggests the existence of a critical value of Bver that hinders magneto-convection in pores as well. In addition, in this case study, the stable regions of the pore can be similarly well defined by a critical value of the total magnetic field strength.