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

## TL;DR

This study compares the magnetic field structure of a sunspot in the photosphere and upper chromosphere, revealing differences in strength, twist, and orientation, and suggesting a magnetic canopy and cusp-like structures.

## Contribution

It provides the first detailed comparison of magnetic fields in a sunspot between the photosphere and upper chromosphere using spectropolarimetric inversions.

## Key findings

- Umbral magnetic field strength decreases by a factor of 1.30-1.65 from photosphere to chromosphere.
- Magnetic field becomes more horizontal in the chromosphere, especially outside the sunspot.
- Lightbridge shows a cusp-like magnetic structure with comparable field strength in chromosphere and umbra.

## Abstract

We investigate the magnetic field of a sunspot in the upper chromosphere and compare it to the field's photospheric properties. We observed the main leading sunspot of the active region NOAA 11124 on two days with the Tenrife Infrared Polarimeter-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, we obtained the magnetic field vector of the upper chromosphere. For comparison with the photosphere we applied height-depended inversions of the Si I 1082.71 nm and Ca I 1083.34 nm lines. We found that the umbral magnetic field strength in the upper chromosphere is lower by a factor of 1.30-1.65 compared to the photosphere. The magnetic field strength of the umbra decreases from the photosphere towards the upper chromosphere by an average rate of 0.5-0.9 G km$^{-1}$. The difference in the magnetic field strength between both atmospheric layers steadily decreases from the sunspot center to the outer boundary of the sunspot, with the field (in particular its horizontal component) being stronger in the chromopshere outside the spot, suggestive of a magnetic canopy. The sunspot displays a twist that on average is similar in the two layers. However, the differential twist between photosphere and chromosphere increases rapidly towards the outer penumbral boundary. The magnetic field vector is more horizontal with respect to the solar surface by roughly 5-20$^\circ$ in the photosphere compared to the upper chromosphere. Above a lightbridge, the chromospheric magnetic field is equally strong as that in the umbra, whereas the lightbridge's field is weaker than its surroundings in the photosphere by roughly 1 kG. This suggests a cusp-like magnetic field structure above the lightbridge.