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We investigate the magnetic field of a sunspot in the upper chromosphere andcompare it to the field's photospheric properties. We observed the main leadingsunspot of the active region NOAA 11124 on two days with the Tenrife InfraredPolarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT).Through inversion of Stokes spectra of the He I triplet at 1083.0 nm, weobtained the magnetic field vector of the upper chromosphere. For comparisonwith the photosphere we applied height-depended inversions of the Si I 1082.71nm and Ca I 1083.34 nm lines. We found that the umbral magnetic field strengthin the upper chromosphere is lower by a factor of 1.30-1.65 compared to thephotosphere. The magnetic field strength of the umbra decreases from thephotosphere towards the upper chromosphere by an average rate of 0.5-0.9 Gkm$^{-1}$. The difference in the magnetic field strength between bothatmospheric layers steadily decreases from the sunspot center to the outerboundary of the sunspot, with the field (in particular its horizontalcomponent) being stronger in the chromopshere outside the spot, suggestive of amagnetic canopy. The sunspot displays a twist that on average is similar in thetwo layers. However, the differential twist between photosphere andchromosphere increases rapidly towards the outer penumbral boundary. Themagnetic field vector is more horizontal with respect to the solar surface byroughly 5-20$^\circ$ in the photosphere compared to the upper chromosphere.Above a lightbridge, the chromospheric magnetic field is equally strong as thatin the umbra, whereas the lightbridge's field is weaker than its surroundingsin the photosphere by roughly 1 kG. This suggests a cusp-like magnetic fieldstructure above the lightbridge.

Three-dimensional magnetic structure of a sunspot: comparison of the photosphere and upper chromosphere