Fine-Scale Structures of the Evershed Effect Observed by the Solar Optical Telescope aboard Hinode

PASJ: Publ. Astron. Soc. Japan 59, S593–S599, 2007 November 30 c 2007. Astronomical Society of Japan.
Fine-Scale Structures of the Evershed Effect Observed by the Solar Optical Telescope aboard Hinode Kiyoshi I CHIMOTO,1 Richard A. S HINE,2 Bruce L ITES,4 Masahito K UBO,4 Toshifumi S HIMIZU,3 Yoshinori S UEMATSU,1 Saku T SUNETA,1 Yukio K ATSUKAWA,1 Theodore D. TARBELL,2 Alan M. T ITLE,2 Shin’ichi NAGATA,6 Takaaki YOKOYAMA,5 and Masumi S HIMOJO1 1 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Toyko 181-8588 2 Lockheed Martin Solar and Astrophysics Laboratory, 3251 Hanover Street, Palo Alto, CA 94304, USA 3 Japan Aerospace Exploration Agency, Institute of Space and Astronoutical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 4 High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, USA 5 Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 6 Hida Observatory, Kyoto University, Takayama, Gifu 506-1314 (Received 2007 July 14; accepted 2007 August 17) Abstract The small-scale structure of the Evershed effect is being studied using data obtained by the Spectropolarimeter and the Broadband Filter Imager of the Solar Optical Telescope aboard Hinode. We find that the Evershed flow starts at the leading edge of inwardly migrating bright penumbral grains, and turns to nearly a horizontal flow preferentially in the dark lanes of the penumbra. A number of small elongated regions that have an upward motion of  1 km s1 are found in the deep photosphere distributed over the penumbra. They are cospatial with bright grains and have relatively horizontal magnetic fields. A number of patches having a strong downward motion associated with the opposite magnetic polarity from the sunspot are also found in the mid and outer penumbra. They could be identified as foot points of the Evershed flow channels, though the identification of individual pairs is not straightforward. Our results provide strong support for some recent findings from ground-based high-resolution observations, and are in general agreement with the well-known picture of the uncombed structure of the penumbra, in which the penumbrae consist of rising flux tubes carrying nearly horizontal Evershed flows embedded in more vertical background magnetic fields. Key words: Evershed flow — space vehicles — Sun: atmospheric motion — Sun: magnetic field — sunspots 1. Introduction The Evershed effect (Evershed 1909), a horizontal outflow of material in the photosphere, is one of the most distinctive properties of sunspot penumbrae. It is evident from recent high-resolution observations that the flow is closely related to the fine-scale structures of the penumbra (for reviews, see Solanki 2003; Thomas & Weiss 2004). A generally accepted picture of the geometry of the penumbral magnetic fields is the so-called uncombed structure, in which the inclination of the magnetic field vector fluctuates in azimuthal direction with the spatial scale of the penumbral filaments, and the material flow preferentially takes place in radial filaments, which have a nearly horizontal magnetic field (Degenhardt & Wiehr 1991; Schmidt et al. 1992; Title et al. 1993). The flow vector is parallel to the magnetic fields (Bellot Rubio et al. 2004; Borrero et al. 2005). Most observations show that the magnetic fields in bright filaments of penumbrae are more vertical than in dark filaments (e.g., Langhans et al. 2005). The relationship between the flow and intensity of the penumbral filament has been, however, more or less contradictory. Many authors (e.g., Beckers & Schröter 1969; Title et al. 1993; Shine et al. 1994; Rimmele 1995; Rouppe van der Voort 2002) have presented evidence that the Evershed flow is concentrated in dark filaments, while some studies claimed that there is no correlation (Wiehr & Stellmacher 1989; Lites et al. 1990). Rimmele (1995) showed that the correlation becomes better when one compares the intensity and velocity originating from the same height, and also gave a hint that the correlation is different between inner and outer penumbrae. Recently, Schlichenmaier et al. (2005) and Bellot Rubio et al. (2006) presented evidence that the Evershed flow takes place preferentially in bright filaments in the inner penumbra, but in dark filaments in the outer penumbra. The correlation between the flow and the brightness (and eventually the gas pressure) may provide clues about the acceleration mechanism of the Evershed flow. Another important aspect that should be addressed to understand the Evershed flow is mass continuity. There are a number of reports on the vertical components of the flow in penumbrae. Up flows in penumbrae were observed by Johannesson (1993), Rimmele (1995), Schlichenmaier and Schmidt (1999, 2000), Tritschler et al. (2004), Bellot Rubio et al. (2005), Rimmele and Marino (2006), and Sanchez Almeida et al. (2007), while down flows were observed in and around the outer edge of penumbrae by, e.g., Westendorp Plaza et al. (1997), Schmidt and Schlichenmaier (2000), Schlichenmaier et al. (2004), Bellot Rubio et al. (2004), S594 K. Ichimoto et al. Sanchez Cuberes et al. (2005). Most material carried by the Evershed flow is supposed to plunge into the photosphere at the downflow patches (Westendorp Plaza et al. 2001), while some fractions of the material may continue to flow across the penumbral outer edge along the elevated magnetic field to form a canopy (Solanki et al. 1994). The coexistence of up and down flows in penumbrae and their mass flux balance was infered from a sunspot observation near to the disk center under the MISMA hypothesis, though individual flow regions were not spatialy resolved (Sánchez Almeida 2005). Definite identifications of the source and the destination of the Evershed flow and quantitative evaluations of the mass conservation are still open issues. To account for the filamentary structure of penumbrae, a number of models have been proposed, e.g., the embedded or rising flux tube model (Solanki & Montavon 1993; Schlichenmaier et al. 1998) in which the Evershed flow channels are explained as rising flux tubes embedded in more vertical background magnetic fields in the penumbra, the gappy penumbral model (Spruit & Scharmer 2006; Scharmer & Spruit 2006) in which the penumbral bright filaments are regarded as manifestations of the protrusion of non-magnetized, convecting hot gas into the background oblique magnetic fields of the penumbra, and the downward pumping model (Thomas et al. 2002; Weiss et al. 2004) in which the penumbral fine structure is created by the localized submergence of penumbral magnetic fields forced by photospheric convection around the outer edge of penumbrae. There is still no consensus about the physical nature of the penumbrae, and the origin of the penumbral fine structure remains a mystery (see Bellot Rubio 2007 for review). Since (...truncated)