  We studied the physical parameters of the penumbra in a large and
fully-developed sunspot, one of the largest over the last two solar cycles, by
using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and
chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional
Spectrometer. Inverting measurements with the NICOLE code, we obtained the
three-dimensional structure of the magnetic field in the penumbra from the
bottom of the photosphere up to the middle chromosphere. We analyzed the
azimuthal and vertical gradient of the magnetic field strength and inclination.
Our results provide new insights on the properties of the penumbral magnetic
fields in the chromosphere at atmospheric heights unexplored in previous
studies. We found signatures of the small-scale spine and intra-spine structure
of both the magnetic field strength and inclination at all investigated
atmospheric heights. In particular, we report typical peak-to-peak variations
of the field strength and inclination of $\approx 300$ G and $\approx
20^{\circ}$, respectively, in the photosphere, and of $\approx 200$ G and
$\approx 10^{\circ}$ in the chromosphere. Besides, we estimated the vertical
gradient of the magnetic field strength in the studied penumbra: we find a
value of $\approx 0.3$ G km$^{-1}$ between the photosphere and the middle
chromosphere. Interestingly, the photospheric magnetic field gradient changes
sign from negative in the inner to positive in the outer penumbra.
