  We present the properties of the inverse Evershed flow (IEF) based on the
center-to-limb variation of the plasma speed and loop geometry of chromospheric
superpenumbral fibrils in eleven sunspots that were located at a wide range of
heliocentric angles from 12 to 79 deg. The observations were acquired at the
Dunn Solar Telescope in the spectral lines of Halpha at 656nm, CaII IR at 854
nm and HeI at 1083 nm. All sunspots display opposite line-of-sight (LOS)
velocities on the limb and center side with a distinct shock signature near the
outer penumbral edge. We developed a simplified flexible sunspot model assuming
axisymmetry and prescribing the radial flow speed profile at a known loop
geometry to replicate the observed two-dimensional IEF patterns under different
viewing angles. The simulated flow maps match the observations for
chromospheric loops with 10-20 Mm length starting at 0.8-1.1 sunspot radii, an
apex height of 2-3Mm and a true constant flow speed of 2-9km/s. We find on
average a good agreement of the simulated velocities and the observations on
elliptical annuli around the sunspot. Individual IEF channels show a
significant range of variation in their properties and reach maximal LOS speeds
of up to 12km/s. Upwards or downwards directed flows do not show a change of
sign in the LOS velocities for heliocentric angles above 30 deg. Our results
are consistent with the IEF being caused by a siphon flow mechanism driving a
flow at a constant sonic speed along elevated loops with a flattened top in the
chromosphere.
