  Context. Fully fledged penumbrae have been widely studied both
observationally and theoretically. Yet the relatively fast process of penumbra
formation has not been studied closely with high spatial resolution. Aims. We
investigate the stages previous to and during the formation of penumbral
filaments in a developing sunspot. Methods. We analysed Milne-Eddington
inversions from spectro-polarimetric data of the leading sunspot of NOAA 11024
during the development of its penumbra. We focused on selected areas of this
protospot in which segments of penumbra develop. Results. We find that few
types of distinctive flow patterns develop at the protospot limb and centre
sides previous to penumbra formation. The flow in the centre side is often
characterised by a persistent (>20 min) inflow-outflow pattern extending
radially over 4 arcsec at the direct periphery of the protospot umbra. This
inflow-outflow system often correlates with elongated granules, as seen in
continuum intensity maps, and is also coupled with magnetic bipolar patches at
its edges, as seen in magnetograms. The field is close to horizontal between
the bipolar patches, which is indicative of its possible loop configuration.
All of these aspects are analogous to observations of magnetic flux emergence.
In the protospot limb side, however, we observed a mostly regular pattern
associated with small granules located near the protospot intensity boundary.
Locally, an inflow develops adjacent to an existing penumbral segment, and this
inflow is correlated with a single bright penumbral filament that is brighter
than filaments containing the Evershed flow. All investigated areas at the
centre and limb side eventually develop penumbral filaments with an actual
Evershed flow that starts at the umbral boundary and grows outwards radially as
the penumbral filaments become longer in time
