  The amplitudes of the Evershed flow are measured using pairs of carefully
selected FeI and FeII spectral lines located close in wavelength and registered
simultaneously. A sunspot belonging to the NOAA 11582 group was scanned using
the spectrograph of the German Vacuum Tower Telescope (Observatorio del Teide,
Tenerife). Velocities were extracted from intensity profiles using the
lambda-meter technique. The formation heights of the observed spectral lines
were calculated using semi-empirical models of a bright and dark penumbral
filament taking into account the sunspot location at the limb. Our objective is
to compare azimuthally averaged amplitudes of the Evershed flow extracted from
neutral and ion lines. We find measurable differences in the radial component
of the flow. All five pairs of lines show the same tendency, with a few hundred
m/s larger amplitude of the flow measured from FeI lines compared to FeII
lines. This tendency is preserved at all photospheric heights and radial
distances in the penumbra. We discuss the possible origin of this effect.
