Umbral Fine Structures in Sunspots Observed with Hinode Solar Optical Telescope

PASJ: Publ. Astron. Soc. Japan 59, S585–S591, 2007 November 11 c 2007. Astronomical Society of Japan.
Umbral Fine Structures in Sunspots Observed with Hinode Solar Optical Telescope Reizaburo K ITAI, Hiroko WATANABE, Tahei NAKAMURA, Ken-ichi OTSUJI, Takuma M ATSUMOTO, Satoru U E N O, Shin’ichi NAGATA, and Kazunari S HIBATA, Kwasan Observatory, Graduate School of Science, Kyoto University, 17 Ohmine-cho Kita Kazan, Yamashina-ku, Kyoto 607-8471 Hida Observatory, Graduate School of Science, Kyoto University, Kurabashira, Kamitakara-cho, Takayama, Gifu 506-1314 Richard M ULLER, Midi-Pyrènèes Observatory, 14, avenue Edouard Belin, 31400 Toulouse, France Kiyoshi I CHIMOTO, Saku T SUNETA, Yoshinori S UEMATSU, and Yukio K ATSUKAWA, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 Toshifumi S HIMIZU, Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 Theodore D. TARBELL, Richard A. S HINE, and Alan M. T ITLE, Lockheed Martin Solar and Astrophysics Laboratory, 3251 Hanover Street, Palo Alto, CA 94304, U.S.A. and Bruce L ITES High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, U.S.A. (Received 2007 May 31; accepted 2007 August 26) Abstract A high resolution imaging observation of a sunspot umbra was made with the Hinode Solar Optical Telescope. Filtergrams at wavelengths of the blue and green continua were taken during three consecutive days. The umbra consisted of a dark core region, several diffuse components, and numerous umbral dots. We derived basic properties of umbral dots (UDs), especially their temperatures, lifetimes, proper motions, spatial distribution, and morphological evolution. The brightness of UDs is confirmed to depend on the brightness of their surrounding background. Several UDs show fission and fusion. Thanks to the stable condition of the space observation, we could for the first time follow the temporal behavior of these events. The derived properties of the internal structure of the umbra are discussed from the viewpoint of magnetoconvection in a strong magnetic field. Key words: Sun: magnetoconvection — Sun: sunspot — Sun: umbral dots 1. Introduction The umbral fine structure in sunspots has been studied by many authors. Recent reviews are given in Thomas and Weiss (2004) and in references cited therein. The study of umbral fine features is very essential for our understanding of the magnetoconvection in a strong magnetic field atmosphere of celestial bodies. Because the spatial size of the umbral fine structures, such as umbral dots (UDs), is very fine, it was very hard to obtain their basic characteristics. It was especially very difficult to follow the temporal evolution of the fine features from ground-based telescopes, due to the influences of variable atmospheric seeing conditions. Solar Optical Telescope (SOT) on board Hinode, successfully launched on 2006 September 23, was designed to observe the solar fine structure with a 50 cm mirror from space (Ichimoto et al. 2007; Kosugi et al. 2007; Shimizu et al. 2007; Suematsu et al. 2007; Tsuneta et al. 2007). The resolving power in the flight condition was confirmed to have nearly the theoretical one of 0:00 2. With Hinode/SOT, we observed the temporal evolution of the umbral fine structures during the period of 2007 March 2–4. The spatial distribution of the umbral structure as well as its temporal evolution, lifetimes, proper motions, and temperatures were studied during a three-day period. Besides the basic characteristics stated above, we could follow the temporal evolution of fission and fusion events of the umbral dots. In the following section we describe the details of our observation, and the analysis procedures in section 2; we give our results in section 3, and finally discuss and summarize our results in section 4. 2. Observation and Reduction We observed a roundish sunspot in an active region, NOAA 10944, from 2007 March 2 through March 4. The region was fairly inactive during the three-day period, and disintegrated on March 5. The region observed in H˛ with the Domeless Solar Telescope (DST) at Hida Observatory is shown in figure 1. The main sunspot remained as the ˛ type for three days. Among the data taken with the Hinode/SOT, we report on the results obtained from a time-series imaging observation by the Broadband Filter Imager (BFI), shown in table 1. The green continuum images were taken through a filter (
= 5550 Å, ∆
' 5 Å), while the blue continuum images were through a different filter (
= 4504 Å, ∆
' 5 Å). Both continuum images were taken in a cadence of 1 frame/30 s. The pixel S586 R. Kitai et al. [Vol. 59, Fig. 1. H˛ image of NOAA10944 on 2007 March 2 taken by DST at Hida Observatory. Table 1. Observation. Date Time 2007 March 2 2007 March 3 2007 March 4 00:14–03:15 UT 00:10–03:30 UT 00:15–03:05 UT Filter green continuum green continuum green and blue continua resolution of the images was 0:00054. The field of view (FOV) of the continuum images was 55:00 8  55:00 8. To follow the temporal evolution correctly without the projection effect, we transformed all of the images as if they are seen from the top. The daily evolution of the umbral region in the green continuum is shown in figure 2. We applied a median filter ( window : 100  100 ) to all of the images to identify slowly varying features, such as the dark core area and diffuse components. The effect of median filter processing for structure identification is shown in figure 3. All of the images were co-aligned among them by finding image displacements, which gave the maximum correlation between consecutive frames. The proper motions of UDs were derived by tracking the identified features along the time series of the images. Identification of the features was done visually on a PC screen. The lifetimes of UDs were determined by measuring the time spans, during which the UDs showed a 1.2-times larger brightness than their surrounding background. The temperatures of the umbral features were estimated from their color values, i.e., the intensity ratio I (blue)/I (green). The relation between the intensity ratio and the temperature was calculated assuming blackbody radiation. The temperature distribution over the region is shown in figure 4. The Fig. 2. Daily evolution of the sunspot in the green continuum. No. S3] Umbral Fine Structures in Sunspots S587 Fig. 4. Temperature distribution on 2007 March 4. temperatures of normal granules surrounding the spot are ' 6000 K, while those of intergranular lanes are ' 5000 K. These temperature values are consistent with those thus-far known. 3. Internal Structure of Umbral Region As shown in figure 2, the brightness distribution of the umbral area is not uniform. The umbra observed by us consists of a dark core region, diffuse components, and bright umbral dots, as was observed in previous ground-based work (...truncated)