  Context. The Evershed effect, a nearly horizontal outflow of material seen in
the penumbrae of sunspots in the photospheric layers, is a common
characteristic of well-developed penumbrae, but is still not well understood.
Even less is known about photospheric horizontal inflows in the penumbra, also
known as counter Evershed flows. Aims. Here we present a rare feature observed
in the penumbra of the main sunspot of AR NOAA 10930. This spot displays the
normal Evershed outflow in most of the penumbra, but harbors a fast
photospheric inflow of material over a large sector of the disk-center
penumbra. We investigate the driving forces of both, the normal and the counter
Evershed flows. Methods. We invert the spectropolarimetric data from Hinode
SOT/SP using the spatially coupled version of the SPINOR inversion code, which
allows us to derive height-dependent maps of the relevant physical parameters
in the sunspot. These maps show considerable fine structure. Similarities and
differences between the normal Evershed outflow and the counter Evershed flow
are investigated. Results. In both the normal and the counter Evershed flows,
the material flows from regions with field strengths of the order of 1.5-2 kG
to regions with stronger fields. The sources and sinks of both penumbral flows
display opposite field polarities, with the sinks (tails of filaments)
harboring local enhancements in temperature, which are nonetheless colder than
their sources (heads of filaments). Conclusions. The anti-correlation of the
gradients in the temperature and magnetic pressure between the endpoints of the
filaments from the two distinct penumbral regions is compatible with both the
convective driver and the siphon flow scenarios. A geometrical scale of the
parameters is necessary to determine which is the dominant force driving the
flows.
