Formation Process of a Light Bridge Revealed with the Hinode Solar Optical Telescope

PASJ: Publ. Astron. Soc. Japan 59, S577–S584, 2007 November 30 c 2007. Astronomical Society of Japan.
Formation Process of a Light Bridge Revealed with the Hinode Solar Optical Telescope Yukio K ATSUKAWA,1 Takaaki YOKOYAMA,2 Thomas E. B ERGER,3 Kiyoshi I CHIMOTO,1 Masahito K UBO,4 Bruce L ITES,4 Shin’ichi NAGATA,5 Toshifumi S HIMIZU,6 Richard A. S HINE,3 Yoshinori S UEMATSU,1 Theodore D. TARBELL,3 Alan M. T ITLE,3 and Saku T SUNETA1 1 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 2 Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 3 Lockheed Martin Solar and Astrophysics Lab, B/252, 3251 Hanover St., Palo Alto, CA 94304, USA 4 High Altitude Observatory, National Center for Atmospheric Research, P.O.Box 3000, Boulder, CO 80307, USA 5 Hida Observatory, Kyoto University, Takayama, Gifu 506-1314 6 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 (Received 2007 July 16; accepted 2007 September 15) Abstract The Solar Optical Telescope (SOT) on-board Hinode successfully and continuously observed the formation process of a light bridge in a matured sunspot of the NOAA active region 10923 for several days with high spatial resolution. During its formation, many umbral dots were observed to be emerging from the leading edges of penumbral filaments, and rapidly intruding into the umbra. The precursor of the light bridge formation was also identified as a relatively slow inward motion of the umbral dots, which emerged not near the penumbra, but inside the umbra. The spectro-polarimeter on SOT provided physical conditions in the photosphere around the umbral dots and the light bridges. We found that the light bridges and the umbral dots had significantly weaker magnetic fields associated with upflows relative to the core of the umbra, which implies that there was hot gas with weak field strength penetrating from the subphotosphere to near the visible surface inside those structures. There needs to be a mechanism to drive the inward motion of the hot gas along the light bridges. We suggest that the emergence and the inward motion are triggered by a buoyant penumbral flux tube as well as subphotospheric flow crossing the sunspot. Key words: Sun: magnetic fields — Sun: photosphere — Sun: sunspot 1. Introduction A sunspot provides us with a unique site to understand the interaction between very strong magnetic fields and convective flows driven by subsurface heat transfer. Although convective flows are especially important in the breakup and disappearance of a sunspot during its lifetime, the process is still poorly understood. One of the well-known signatures of sunspot breakup is the formation of a light bridge (LB), which is a lane of relatively bright material dividing an umbra into two parts (Bray & Loughhead 1964). The formation is a result of the reestablishment of a granular surface as a precursor for the decay of a spot (Vázquez 1973). LBs have been classified based on the morphological arrangement and brightness. A strong LB, which separates umbral cores, is further distinguished as either penumbral or photospheric according to the fine structures observed within it (Sobotka et al. 1993, 1994). A faint (or umbral) LB, which is a faint narrow lane within the umbra, most likely consists of a chain of umbral dots (Muller 1979). The classification is somewhat phenomenological, but implies that there should be some sort  Movie for figure 1 is available in the electronic version hhttp://pasj.asj.or. jp/v59/s3/59s302/i. of relationship among LBs, umbral dots, and penumbrae. It is obvious that gas in a LB must have a temperature higher than a surrounding umbra because of its brightness. It is important to know the magnetic and velocity structures of a LB to understand how hot gas is continuously provided to the LB; otherwise, the gas inside the LB is cooled down and the LB disappears. Spectrometric observations of photospheric Zeeman-sensitive lines indicate that LBs typically have a weakened magnetic field strength relative to the nearby umbra with field lines inclined from the local vertical (Lites et al. 1991; Rüedi et al. 1995; Leka 1997). Furthermore, it is found that the field strengths and inclinations increase and decrease with the height, respectively, by a detailed analysis of the Stokes spectra (Jurčák et al. 2006), which suggest a canopy-like magnetic structure above the LB. There are no systematic findings considering the vertical velocities in LBs (Leka 1997), but a positive correlation between the brightness and upflow velocities is reported by Rimmele (1997), which is interpreted as evidence of the hot gas originating from subphotospheric convection. These observational results can be explained theoretically in terms of a cluster model, where an umbra consists of a tight bundle of isolated flux tubes separated by field-free columns of hot gas (Parker 1979; Spruit & Scharmer 2006). But it is still unknown what triggers the S578 Y. Katsukawa et al. [Vol. 59, Fig. 1. Sunspot umbra in the NOAA active region 10923 observed with the blue continuum (BC) channel on SOT from 2006 November 13 0UT to 2006 November 17 12UT. Original images covered the entire field-of-view of SOT with 2  2 summing, but the images shown here are only a part of the original images, and cover an area of 3500  3500 (25  25 Mm2 ) centered at the sunspot umbra. The arrow in the image at 2006 November 13 11:57 indicates an example of an extreme penumbral intrusion observed before the formation of LBs. A movie of the umbra provides the dynamical evolution of the umbra and LBs more clearly. development of a LB, and what is the role of penumbrae and umbral dots in the development. LBs are also important from the viewpoint of chromospheric and coronal activities. Observations in H˛ show that surges are ejected from a LB in some situations (Roy 1973; Asai et al. 2001; Bharti et al. 2007). Berger and Berdyugina (2003) found a constant brightness enhancement over a LB in 1600 Å ultraviolet images from the Transition Region and Coronal Explorer (TRACE), and suggested a steady chromospheric heat source over a LB. Katsukawa (2007) found that the formation of a LB is spatially and temporally coincident with the heating of coronal loops seen in TRACE 171 Å images. These observations suggest that LBs serve a role not only to dissolve a sunspot, but to release or dissipate magnetic energies stored in a sunspot. The Solar Optical Telescope (SOT: Tsuneta et al. 2007; Suematsu et al. 2007; T. D. Tarbell et al. 2007, in preparation; Ichimoto et al. 2007) on the new Japanese spacecraft Hinode (Kosugi et al. 2007) enables us to observe the dynamics and evolution in the photosphere not only with high spatial resolution (0:002) under a seeing-free condition, but with uninterrupted coverage longer than one day owing to the sun-s (...truncated)