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

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.

12 pages, 15 figures, accepted for publication in A&A

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Cited by in corpus (8)

• Height dependence of the penumbral fine-scale structure in the inner solar atmosphere
• Magnetic Properties and Flow Angle of the Inverse Evershed Flow at Its Downflow Points
• Chromospheric magnetic field: A comparison of He I 10830 A observations with nonlinear force-free field extrapolation
• The role of the chromospheric magnetic canopy in the formation of a sunspot penumbra
• The dark side of penumbral microjets: Observations in Hα
• Magnetic field fluctuations in the shocked umbral chromosphere
• Simultaneous spectropolarimetric observations in the H$\alpha$ and Ca II 8662 Å lines of an active region
• Decay of a photospheric transient filament at the boundary of a pore and the chromospheric response