Magnetic structures of an emerging flux region in the solar photosphere and chromosphere

Abstract

Aims. We investigate the vector magnetic field and Doppler velocity in the photosphere and upper chromosphere of a young emerging flux region of the sun close to disk center. Methods. Spectropolarimetric scans of a young active region made using the second generation Tenerife Infrared Polarimeter (TIP II) on the German Vacuum Tower Telescope (VTT) are analyzed. The scanned area contained multiple sunspots and an emerging flux region. An inversion based on the Milne-Eddington approximation was performed on the full Stokes vector of the chromospheric He I 10 830 Å and the photospheric Si I 10 827.1 Å lines. This provided the magnetic vector and line-of-sight velocity at each spatial point in both atmospheric layers. Results. A clear difference is seen between the complex magnetic structure of the emerging flux region (EFR) in the photosphere and the much simpler structure in the upper chromosphere. The upper chromospheric structure is consistent with a set of emerging loops following elongated dark structures seen in the He I 10 830 Å triplet, similar to arch filament systems (AFS), while in the photosphere we infer the presence of U-loops within the emergence zone. Nonetheless, in general the upper chromospheric field has a similar linear relationship between inclination angle and field strength as the photospheric field: the field is weak (≈300 G) and horizontal in the emergence zone, but strong (up to 850 G) and more vertical near its edges. The field strength decreases from the photosphere to the upper chromosphere by approximately 0.1-0.2 G km −1 (or even less) within the emergence zone and by 0.3-0.6 G km −1 in sunspots located at its edge. We reconstructed the magnetic field in 3D based on the chromospheric vector field under the assumption that the He I 10 830 Å triplet forms along the magnetic field loops. The reconstructed loops are quite flat with supersonic downflows at both footpoints. Arguments and evidence for an enhanced formation height of He I 10 830 Å in arch-filaments seen in this line are provided, which support the validity of the reconstructed loops. Conclusions. The main chromospheric properties of EFRs previously deduced for a single region NOAA 9451 are shown to be valid for another region as well, suggesting that the main original results may have a wider application. The main exception is that only the first region displayed a current sheet in the chromosphere. We propose a scenario in which the relatively complex photospheric structure evolves into the simpler chromospheric one.

Properties of solar plage from a spatially coupled inversion of Hinode SP data

“…the hydrostatic atmospheres returned by SPINOR. The B value of 350 G in the filament is comparable (within a factor of 2) to the field strengths found in AR filaments by Xu et al (2010), Kuckein et al (2012), Sasso et al (2011) in the chromosphere sampled by the He I 10 830 Å triplet. The azimuthal orientation of the magnetic field within the filament is almost invariant across the whole filament.…”

Granular-Scale Elementary Flux Emergence Episodes in a Solar Active Region

“…Moving bipolar features (MDFs; Bernasconi et al, 2002) are detected in the central part of the EFR and move at velocities similar to that of the MMFs, though they are likely to drift towards sunspots and not away from them. This intricate pattern of magnetic features is commonly observed in the central part of EFR at the photospheric level (Xu, Lagg, and Solanki, 2010) and it also is present in output of realistic numerical simulations (Cheung et al, 2008).…”

Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope

“…Solanki et al (2003) and Lagg et al (2007) observed an AFS in the infrared helium line at 1083 nm and determined its magnetic structure. The chromospheric magnetic field of another AFS is investigated by Xu et al (2010). In the zone of flux emergence they find a magnetic field strength of 300 G, and the magnetic field is almost horizontal.…”