  12 stable single sunspots were investigated in two aspects. The outer
boundaries of the sunspots umbra were determined by an independent mathematical
method and the vertical component of the magnetic field at these boundaries was
found. We analyzed data from the SDO station using the segments of the
continuum and magnetic field. It is shown that in a wide range of sunspot
fields, the field average along the contour changes weakly, while the vertical
component of the magnetic field itself, in a particular spot, changes
significantly along the defined contour which reflects the fibrous structure of
the penumbral field.
