Upper Chromospheric Magnetic Field of a Sunspot Penumbra: Observations of Fine Structure

Abstract

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.

8 pages, 7 figures, accepted for publication in A&A

References in corpus (7)

• Observations of Subarcsecond Bright Dots in the Transition Region above Sunspots with the Interface Region Imaging Spectrograph
• The influence of coronal EUV irradiance on the emission in the He I 10830 A and D3 multiplets
• Evidence of magnetic field wrapping around penumbral filaments
• Deep probing of the photospheric sunspot penumbra: no evidence for magnetic field-free gaps
• The properties of penumbral microjets inclination
• Magnetic fields of opposite polarity in sunspot penumbrae
• Lateral downflows in sunspot penumbral filaments and their temporal evolution

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• Spectropolarimetric analysis of an active region filament. II. Evidence of the limitations of a single component model
• Three-dimensional magnetic field structure of a flux emerging region in the solar atmosphere
• Temporal Evolution of the Inverse Evershed Flow
• Detection of emission in the Si i 1082.7 nm line core in sunspot umbrae
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• Determining the Magnetic Field in the Atmosphere of a Solar Active Region Observed by the CLASP2.1 Sounding Rocket Experiment
• Simultaneous spectropolarimetric observations in the H$\alpha$ and Ca II 8662 Å lines of an active region
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