  We studied the variations of line-of-sight photospheric plasma flows during
the formation phase of the penumbra around a pore in Active Region NOAA 11490.
We used a high spatial, spectral, and temporal resolution data set acquired by
the Interferometric BIdimensional Spectrometer (IBIS) operating at the NSO/Dunn
Solar Telescope as well as data taken by the Helioseismic and Magnetic Imager
onboard the Solar Dynamics Observatory satellite (SDO/HMI). Before the penumbra
formed we observed a redshift of the spectral line in the inner part of the
annular zone surrounding the pore as well as a blueshift of material associated
with opposite magnetic polarity further away from the pore. We found that the
onset of the classical Evershed flow occurs in a very short time scale -- 1-3
hours -- while the penumbra is forming. During the same time interval we found
changes in the magnetic field inclination in the penumbra, with the vertical
field actually changing sign near the penumbral edge, while the total magnetic
field showed a significant increase, about 400 G. To explain these and other
observations related to the formation of the penumbra and the onset of the
Evershed flow we propose a scenario in which the penumbra is formed by magnetic
flux dragged down from the canopy surrounding the initial pore. The Evershed
flow starts when the sinking magnetic field dips below the solar surface and
magnetoconvection sets in.
