ON THE DYNAMICS OF BRIGHT FEATURES IN ... - IOPscience

THE ASTROPHYSICAL JOURNAL, 447 : L133–L134, 1995 July 10 1995. The American Astronomical Society. All rights reserved. Printed in U.S.A.

ON THE DYNAMICS OF BRIGHT FEATURES IN SUNSPOT UMBRAE ´ A. BONET, 2 MANUEL VA ´ZQUEZ, 2 MICHAL SOBOTKA, 1 JOSE

AND

ARNOLD HANSLMEIER 3

Received 1995 March 6; accepted 1995 May 2

ABSTRACT Time series of white-light pictures of the sunspot NOAA 7522, obtained at the Swedish Vacuum Solar Telescope (La Palma), were analyzed to study the proper motion of bright features in sunspots. For the first time, its relation with the dark nuclei present in the umbra is investigated. The bright features are visible in the penumbra as bright grains moving into the umbra. A few of them cross the penumbra/umbra boundary, becoming peripheral umbral dots, which move farther into the umbra until they meet dark nuclei, braking their motion and disappearing. In some cases the encounter with a dark nucleus produces a brightening of the central umbral dots placed on the opposite side of the nucleus. A similar phenomenon is observed in the grains of a faint light bridge, when bright penumbral grains collide with one of the edges of the bridge. Subject headings: sunspots — Sun: photosphere and showing small motions or none at all (Ewell 1992), and possibly having a different physical origin (Weiss 1990). Let us try to study the role played by the dark nuclei in the dynamics of the PUDs flowing into the umbra, and how they are related to the various umbral fine structures.

1. INTRODUCTION

Sunspots display a complicated pattern of fine structures, both in the penumbra (bright and dark filaments) and in the umbra (umbral dots, light bridges, and dark nuclei, embedded in a diffuse background). These structures are probably linked to different strengths and inclinations of the magnetic field, and to the onset of some type of altered convection. A global view of the dynamics of these structures is needed to understand the physical processes controlling the evolution of a sunspot. In a previous work (Sobotka, Bonet, & Va´zquez 1993) we showed that the brightness of umbral dots depends on the brightness of the adjacent diffuse background and that the intensity minimum of the umbra is a crucial parameter to describe the sunspot’s thermal structure. Figure 1 of the cited paper describes in more detail the nomenclature used throughout this Letter. The dark nuclei correspond to the voids described by Livingston (1991). The absence of umbral dots in dark nuclei, where high values of the magnetic field strength are expected, is due to the inhibition of convection in these regions, or to its onset at layers deeper than the visible surface (Moreno Insertis & Spruit 1989). Muller (1973) and To ¨njes & Wo ¨hl (1982) reported evidence that bright penumbral grains flow into the umbra. Ewell (1992), Wang & Zirin (1992), and Molowny-Horas (1994) observed that some umbral dots, perhaps associated with the bright penumbral grains, flow into the umbra too. Under the hypothesis of penumbral convection produced in a comblike magnetic field structure, the individual flux tubes gain thermal energy and move inward and upward, so that their intersection with the visible surface migrates radially inward, giving rise to the proper motions of bright penumbral grains. A similar explanation could be adopted for moving umbral bright features, presumably associated with magnetic fields whose inclination to the vertical is greater than that in the surrounding umbra (Thomas & Weiss 1992). These moving features are called peripheral umbral dots (PUDs), to be distinguished from central umbral dots (CUDs) located between dark nuclei

2. OBSERVATIONS

Time series of white-light images (l 5 5257 H 29 Å) of the sunspot NOAA 7522 (heliocentric position m 5 0.90) were obtained at the Swedish Vacuum Solar Telescope (Observatorio del Roque de los Muchachos, La Palma) using an 8 bit CCD camera (1360 3 1036 pixels) with a dynamic range of 0 –255 counts. The maximum intensity of the undisturbed photosphere was set to approximately 235 counts (exposure time 14 ms) by means of a neutral filter and two crossed polarizers. The dark-current level amounted to 13 counts, with a rms value of H0.1. The optical setup produced a spatial scale of 00. 062 pixel 21 . From this material, one of the series taken on 1993 June 13 appeared to be particularly suitable for our study, owing to the high image quality throughout the whole duration of the series (51 minutes). The images, taken with an average time spacing of 32 s using a frame selection system, were corrected for instrumental profile, and de-stretched to minimize seeing distortion. The level of scattered light was estimated to be less than 1% of the mean intensity of the undisturbed photosphere (cf. Sobotka et al. 1993; Bonet, Sobotka & Va´zquez 1995). The proper motions of umbral fine structures were determined by applying the method of local correlation tracking described by November & Simon (1988). The spatial resolution of the tracking was 00. 8. As a result we obtained the map of the proper motions, averaged over the whole 51 minute time series (Fig. 1 [Pl. L20]). 3. RESULTS

The sunspot shows an asymmetric distribution of brightness, with most of the dark nuclei placed toward the right (Fig. 1). They form a slowly evolving network which determines the internal structure of the umbra. We expect a stronger (and less inclined) magnetic field in these structures than in the rest of the umbra. We observe moving dots, located mostly in the peripheral

1 Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondrˇejov, Czech Republic. 2 Instituto de Astrofı ´sica de Canarias, 38200 La Laguna, Spain. 3 Institut fu ¨r Astronomie, Universita¨tsplatz 5, 8010 Graz, Austria.

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parts of the umbra (PUDs). Their motion is related to or is a direct continuation of that of bright grains across the whole penumbra. Most of the bright penumbral grains cease their inward motion at the penumbra/umbra border. No bright grains penetrate if a dark nucleus is present there, but on the ‘‘left’’ side of the umbra (in the absence of dark nuclei) many penumbral grains continue to move as PUDs. After surmounting this obstacle, the movement of the PUDs into the umbra is controlled by the dark nuclei, which deflects them from the original trajectory or stops their motion, decreasing the brightness of PUDs, until they eventually disappear. The arrows in Figure 1 represent the average proper motions of the umbral fine structures. Tracking individual structures, we find that bright penumbral grains—moving in the inner penumbra with velocities of about 1 km s 21 — penetrate the penumbra/umbra border at about 800 m s 21 . PUDs have velocities of up to 600 m s 21 . When they encounter a dark nucleus, they slow down to 180 m s 21 , which is the noise level in our measurements. The velocities of CUDs were found to be below this level. In addition, the dark patches in the outer penumbra show strong outward motions (MolownyHoras 1994; Shine et al 1994). In three cases, the ‘‘collision’’ of a PUD with a dark nucleus is accompanied by a brightening of an already existing CUD on the opposite side of the dark nucleus (Fig. 2 [Pl. L21). If this brightening were induced by some sort of perturbation (a magnetoacoustic wave) propagating across the dark nucleus, the velocity of propagation could be estimated at about 10 km s 21 . CUDs, however, also show individual intensity fluctuations which are not related to the above-mentioned effect. On the other hand, smaller and brighter dark nuclei are deformed, pushed away, and, eventually, broken in pieces by moving PUDs. The brightness and velocity of PUDs always decrease during this process. A faint light bridge in the right-hand part of the umbra [the

coordinates of its core are (150, 90) in Fig. 1], formed by bright grains, displays rapid changes in its internal structure: bright grains that form and then disappear, and inward motions on the right-hand side adjacent to the penumbra. The most conspicuous effect is the almost simultaneous brightening of two clusters of bright grains at the extremes of the faint light bridge. This is probably connected with the motions of grains in the adjacent penumbra (see Fig. 3 [Pl. L21]). The cluster on the left is motionless, while the right-hand one moves inward along the bridge. The dominant structures in sunspot umbrae are dark nuclei—regions with a stronger and more vertical magnetic field—which in some ways resemble the fragments proposed by Garcı´a de la Rosa (1987). Instead of a monolithic or multitube structure, we have a multinucleus structure. The dark nuclei—the intrinsic sunspot components—are separated by CUDs, faint and strong light bridges which, together with the diffuse background, probably represent different kinds of convection altered by the magnetic field (Sobotka, Bonet, & Va´zquez 1994). This altered convection is present from the first stages of a sunspot’s evolution (Bonet et al. 1995). The bright penumbral grains, which appear to flow inward across the whole penumbra, and PUDs physically related to them, seem also to reflect this convective mechanism. The support provided by R. Kever and G. Hosinsky during the observations is gratefully acknowledged. We thank F. Moreno Insertis and V. Martı´nez Pillet for fruitful discussions, and Mo ´nica Murphy for revising the English. The SVST is operated on the island of La Palma by the Royal Swedish Academy of Sciences in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofı´sica de Canarias. The work was partially funded by the Spanish DGICYT project 91-0530, the Czech Academy of Sciences grant 303111, and the Austrian-Spanish ‘‘Acciones Integradas’’ 13-B.

REFERENCES Bonet, J. A., Sobotka, M., & Va´zquez M. 1995, A&A, 296, 241 Ewell, M. W. 1992, Sol. Phys., 137, 215 Garcı´a de la Rosa, J. I. 1987, Sol. Phys., 112, 49 Livingston, W. 1991, Nature, 350, 45 Molowny-Horas, R. 1994, Sol. Phys., 154, 29 Moreno Insertis, F., & Spruit, H. 1989, ApJ, 342, 1158 Muller, R. 1973, Sol. Phys., 29, 55 November, L. J., & Simon, G. W. 1988, ApJ, 333, 427 Shine, R. A., Title, A. M., Tarbell, T. D., Smith, K., & Frank, Z. A. 1994, ApJ, 430, 413

Sobotka, M., Bonet, J. A., & Va´zquez, M. 1993, ApJ, 415, 832 ———. 1994, ApJ, 426, 404 Thomas, J. H., & Weiss, N. O. 1992, in Sunspots: Theory and Observations, ed. J. H. Thomas & N. O. Weiss (Dordrecht: Kluwer), 3 To ¨njes, K., & Wo ¨hl, H. 1982, Sol. Phys., 75, 63 Wang, H., & Zirin, H. 1992, Sol. Phys., 140, 41 Weiss, N. O. 1990, in Basic Plasma Processes on the Sun, ed. E. R. Priest & V. Krishan (Dordrecht: Kluwer), 139

PLATE L20

FIG. 1.—Flow map representing the proper motions (black arrows), averaged over the whole 51 minute time series, of the bright and dark structures in the umbra and adjacent penumbra. The underlying white-light image is one of the best frames in the middle of the series. The typical behavior of the bright features is described in the text. The small arrows in the central parts can be explained by seeing and/or by slow changes in the morphology of dark nuclei and clusters of central umbral dots. The apparent motions in some of the dark nuclei close to the penumbra/umbra border are a consequence of their morphological evolution during the series. The coordinates are in pixels. The distance between two ticks is 00. 62. SOBOTKA et al. (see 447, L133)

PLATE L21

FIG. 2

FIG. 3 FIG. 2.—Time evolution of the ‘‘collision’’ of a peripheral umbral dot (1) with a dark nucleus on its right-hand side. The ‘‘collision’’ triggered a rise in the brightening of a central umbral dot (2). Time (minutes : seconds) starts when the series begins. The center of this box corresponds roughly to the coordinates (85, 140) of Fig. 1. SOBOTKA et al. (see 447, L134) Fig. 3.—Time evolution of the faint light bridge, showing an increase in brightness of two clusters of bright grains (labeled 1 and 2 in the second panel) at the extremes of the bridge. This brightening could be related to the motions of bright penumbral grains, adjacent to the bridge on the right. Time (minutes : seconds) starts when the series begins. The center of this box corresponds roughly to the coordinates (150, 90) of Fig. 1. SOBOTKA et al. (see 447, L134)