  The emergence of the magnetic field through the photosphere has multiple
manifestations and sunspots are the most prominent examples of this. One of the
most relevant sunspot properties, to study both its structure and evolution, is
the sunspot area: either total, umbra or penumbra area. Recently Schlichenmaier
et al. (2010) studied the evolution of the active region (AR) NOAA 11024
concluding that during the penumbra formation the umbra area remains constant
and that the increase of the total sunspot area is caused exclusively by the
penumbra growth. In this presentation the Schlichenmaier's conclusion is
firstly tested, investigating the evolution of four different ARs. Hundreds of
Intensitygram images from the Helioseismic and Magnetic Imager (HMI) images are
used, obtained by the Solar Dynamics Observatory, in order to describe the area
evolution of the above ARs and estimate the increase and decrease rates for
umbra and penumbra areas, separately. A simple magnetohydrodynamic model is
then tentatively used in a first approximation to explain the observed results.
