  Comparison of photographic observations and vector-magnetograph measurements
demonstrate, that the outer boundary of the sunspot penumbra --even in complex
sunspot groups-- closely follows the 0.075T isogauss line of the total value of
the magnetic field, corresponding approximately to the equipartition value in
the photosphere. Radio observations also show this feature. The thick penumbra
model with interchange convection (Jahn and Schmidt, 1994) gives the best
explanation of the penumbral structure.
