  The inverse Evershed flow (IEF) is a mass motion towards sunspots at
chromospheric heights. We combined high-resolution observations of NOAA 12418
from the Dunn Solar Telescope and vector magnetic field measurements from the
Helioseismic and Magnetic Imager (HMI) to determine the driver of the IEF. We
derived chromospheric line-of-sight (LOS) velocities from spectra of H$\alpha$
and Ca II IR. The HMI data were used in a non-force-free magnetic field
extrapolation to track closed field lines near the sunspot in the active
region. We determined their length and height, located their inner and outer
foot points, and derived flow velocities along them. The magnetic field lines
related to the IEF reach on average a height of 3 Mm over a length of 13 Mm.
The inner (outer) foot points are located at 1.2 (1.9) sunspot radii. The
average field strength difference $\Delta B$ between inner and outer foot
points is +400 G. The temperature difference $\Delta T$ is anti-correlated with
$\Delta B$ with an average value of -100 K. The pressure difference $\Delta p$
is dominated by $\Delta B$ and is primarily positive with a driving force
towards the inner foot points of 1.7 kPa on average. The velocities predicted
from $\Delta p$ reproduce the LOS velocities of 2-10 km s$^{-1}$ with a
square-root dependence. We find that the IEF is driven along magnetic field
lines connecting network elements with the outer penumbra by a gas pressure
difference that results from a difference in field strength as predicted by the
classical siphon flow scenario.
