Abstract
Active regions are the most prominent manifestations of solar magnetic fields; their generation and dissipation are fundamental problems in solar physics. Light bridges are commonly present during sunspot decay, but a comprehensive picture of their role in the removal of photospheric magnetic field is still missing. We study the three dimensional configuration of a sunspot and in particular its light bridge during one of the last stages of its decay. We present the magnetic and thermodynamical stratification inferred from full Stokes inversions of the photospheric Si I 10827 \AA\ and Ca I 10839 \AA\ lines obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope at Observatorio del Teide, Tenerife, Spain. The analysis is complemented by a study of continuum images covering the disk passage of the active region, which are provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. The sunspot shows a light bridge with penumbral continuum intensity that separates the central umbra from a smaller umbra. We find that in this region the magnetic field lines form a canopy with lower magnetic field strength in the inner part. The photospheric light bridge is dominated by gas pressure (high-$\beta$), as opposed to the surrounding umbra where the magnetic pressure is higher. A convective flow is observed in the light bridge. This flow is able to bend the magnetic field lines and to produce field reversals. The field lines close above the light bridge and become as vertical and strong as in the surrounding umbra. We conclude that it develops because of two highly magnetized regions which come closer during the sunspot evolution.
Highlights
Light bridges are commonly present during sunspot decay, but a comprehensive picture of their role in the removal of the photospheric magnetic field is still lacking
The determination of the magnetic topology of sunspots has been an important topic in solar physics during the past decades
The topology of the magnetic field in the light bridge can be pictured as a region with a low horizontal magnetic field that is covered by a canopy of stronger and more vertical field, similar to the surrounding umbral field (Jurcák et al 2006; Lagg et al 2014; Schlichenmaier et al 2016)
Summary
The determination of the magnetic topology of sunspots has been an important topic in solar physics during the past decades. Most sunspots exhibit a large-scale outflow, called moat flow (Meyer et al 1974), beyond their visible outline, whose physical origin differs from that of the Evershed flow (Löhner-Böttcher & Schlichenmaier 2013) It was first detected spectroscopically (Sheeley 1972) and by tracking moving magnetic features (MMFs, Harvey & Harvey 1973). MMFs with the same polarity as the umbra have been associated with sunspot decay (e.g., Martínez Pillet 2002) because they extract net flux from the spot (Zhang et al 1992).
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