  The fine-structure of magnetic field of a sunspot penumbra in the upper
chromosphere is to be explored and compared to that in the photosphere. High
spatial resolution spectropolarimetric observations were recorded with the
1.5-meter GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The
observed spectral domain includes the upper chromospheric He I triplet at
1083.0 nm and the photospheric Si I 1082.7 nm and Ca I 1083.3 nm spectral
lines. The upper chromospheric magnetic field is obtained by inverting the He I
triplet assuming a Milne-Eddington type model atmosphere. A height dependent
inversion was applied to the Si I 1082.7 nm and Ca I 1083.3 nm lines to obtain
the photospheric magnetic field. We find that the inclination of the magnetic
field shows variations in the azimuthal direction both in the photosphere, but
also in the upper chromosphere. The chromospheric variations remarkably well
coincide with the variations in the inclination of the photospheric field and
resemble the well-known spine and inter-spine structure in the photospheric
layers of penumbrae. The typical peak-to-peak variations in the inclination of
the magnetic field in the upper chromosphere is found to be 10-15 degree, i.e.,
roughly half the variation in the photosphere. In contrast, the magnetic field
strength of the observed penumbra does not show variations on small spatial
scales in the upper chromosphere. Thanks to the high spatial resolution
observations possible with the GREGOR telescope at 1.08 microns, we find that
the prominent small-scale fluctuations in the magnetic field inclination, which
are a salient part of the property of sunspot penumbral photospheres, also
persist in the chromosphere, although at somewhat reduced amplitudes. Such a
complex magnetic configuration may facilitate penumbral chromospheric dynamic
phenomena, such as penumbral micro-jets or transient bright dots.
