evolution of physical characteristics of umbral dots and penumbral ...

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Mar 23, 2009 - covering 3 hr of umbra and penumbra evolution. The retrieved maps of plasma parameters show the spatial distribution of temperature ...
The Astrophysical Journal, 694:1080–1084, 2009 April 1  C 2009.

doi:10.1088/0004-637X/694/2/1080

The American Astronomical Society. All rights reserved. Printed in the U.S.A.

EVOLUTION OF PHYSICAL CHARACTERISTICS OF UMBRAL DOTS AND PENUMBRAL GRAINS ´ 1,2 M. Sobotka1 and J. Jurˇcak 1

Astronomical Institute, Academy of Sciences of the Czech Republic (v.v.i.), 251 65 Ondˇrejov, Czech Republic; [email protected] 2 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan; [email protected] Received 2008 November 5; accepted 2008 December 31; published 2009 March 23

ABSTRACT A time series of full-Stokes spectropolarimetric observations of the sunspot NOAA 10944, acquired with HINODE/ SOT in 2007 February, is analyzed. The data were inverted using the code SIR into a series of 34 maps covering 3 hr of umbra and penumbra evolution. The retrieved maps of plasma parameters show the spatial distribution of temperature, line-of-sight velocity, magnetic field strength, and inclination in two different ranges of optical depths corresponding to the low and high photosphere. In these maps, the evolution of central and peripheral umbral dots (CUDs and PUDs) and penumbral grains (PGs) was traced. While CUDs do not show any excess of line-of-sight velocity and magnetic field inclination with respect to the surrounding umbra, upflows of 400 m s−1 and a more horizontal magnetic field are detected in the low photospheric layers of PUDs. PGs have even stronger upflows and magnetic field inclination in the low photosphere than PUDs. The absolute values of these parameters decrease when PGs evolve into PUDs. It seems that PGs and PUDs are of a similar physical nature. Both classes of features appear in regions with a weaker and more horizontal magnetic field and their formation height reaches the low photosphere. On the other hand, CUDs appear in regions with a stronger and more vertical magnetic field and they are formed too deep to detect upflows and changes in magnetic field inclination. Key words: Sun: photosphere – sunspots – techniques: spectroscopic

any important LOS velocity signature. In the low layers of bright penumbral filaments in the inner penumbra, Jurˇca´ k et al. (2007) revealed a weaker and nearly horizontal magnetic field along with an increased LOS velocity reaching 4 km s−1 . In the upper photospheric layers, the physical characteristics of PUDs, CUDs, and bright penumbral filaments do not differ significantly from those in their surroundings, indicating that these features are formed deep in the photosphere and below it (Riethm¨uller et al. 2008; Jurˇca´ k et al. 2007). The physical nature of UDs is connected with heat transport in a strong magnetic field of the umbra either in the form of oscillatory convection (see the review by Thomas & Weiss 2004) or by intrusions of nonmagnetic plasma into a cluster-type sunspot (Parker 1979). According to recent theoretical MHD simulations (Sch¨ussler & V¨ogler 2006), UDs are nonstationary narrow plumes of rising hot plasma with strongly reduced magnetic field. It is possible that elongated plumes are formed in the inclined magnetic field of the inner penumbra, giving rise to PGs attached to bright penumbral filaments (Rempel et al. 2009). In this paper, we study a time sequence of two-dimensional spectropolarimetric scans observed with HINODE/SOT and show some examples of the temporal evolution of temperature, magnetic field vector, and LOS velocity in CUDs, PUDs, and PGs including cases where PGs evolve directly into PUDs.

1. INTRODUCTION Sunspots show a variety of fine structures in both the umbra and the penumbra. Umbrae are populated with small bright dotlike features, named umbral dots (UDs) by Danielson (1964). Bright penumbral filaments host small-scale brightenings called penumbral grains (PGs; Muller 1973). Many PGs are located at the inner, umbral ends of penumbral filaments. Usually, UDs are divided into two groups: peripheral umbral dots (PUDs) and central umbral dots (CUDs), according to their position inside the umbra (Grossmann-Doerth et al. 1986). PUDs are often brighter than CUDs, because they appear in the brighter peripheral parts of the umbra and the brightness of UDs is generally related to the brightness of their umbral surroundings (Sobotka & Hanslmeier 2005). While the majority of CUDs do not show systematic horizontal motions, PUDs drift toward the center of the umbra (Ewell 1992). A similar drift is observed for PGs (Muller 1973). Some PGs separate from penumbral filaments at the edge of the penumbra and move into the umbra as PUDs (Sobotka et al. 1995). Detailed studies of the physical characteristics of UDs such as temperature stratification, magnetic field vector, and line-ofsight (LOS) velocity are necessary to understand the nature of UDs and the underlying MHD physics. Socas-Navarro et al. (2004) found from ground-based spectropolarimetric observations upflows of 200 m s−1 in UDs together with a weaker and more inclined magnetic field compared to the surrounding umbra. Hartkorn & Rimmele (2003) detected in narrowband filtergrams upflows of 100–300 m s−1 connected with PUDs and weak downflows in CUDs. Substantial progress has been achieved using the seeing-free spectropolarimetric data from HINODE/SOT: Riethm¨uller et al. (2008) found from observations of 30 PUDs and 21 CUDs that, at the continuum formation level, the magnetic field is 510 G (PUDs) and 480 G (CUDs) weaker than in the surrounding umbra and is more inclined in PUDs than in CUDs. They observed significant upflows of 800 m s−1 in PUDs, while CUDs did not show

2. OBSERVATIONS AND DATA REDUCTION A medium-size regular sunspot NOAA 10944 (Figure 1) with a well developed penumbra was located in the central zone of the solar disk (heliocentric position μ = 0.97) on 2007 February 27. The sunspot was observed with the spectropolarimeter (Lites et al. 2001) of the 0.5 m Solar Optical Telescope (SOT; Tsuneta et al. 2008) on board HINODE during the period 11:52–17:54 UT. After about 3 hr of observations, the umbra drifted out from the scanning field of the instrument due to the drift of the correlation tracker. For further analysis we took 34 repeated 1080

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EVOLUTION OF UMBRAL DOTS AND PENUMBRAL GRAINS

Figure 1. Sunspot NOAA 10944 observed in the G band on 2007 February 27, 12:11 UT, with HINODE/SOT. The 8 × 16 white rectangle corresponds to the studied field of view of the spectropolarimeter.

scans of full Stokes spectra in the Fe i lines 6301.5 and 6302.5 Å, observed from 11:52 to 14:57 UT. The spectra were sampled with a step of 21 mÅ per pixel. The spectropolarimeter was working in its normal map mode, so that both the sampling along the slit and the slit-scan sampling were 0. 16, that is, the spatial resolution of 0. 32 is near the diffraction limit of 50 cm SOT at 6302 Å. The time separation between the repeated scans was 333 s. The spectra were corrected for dark current, flat field, and instrumental polarization with standard routines available in the Hinode Solar Software package. For inversion purposes, the spectra were normalized to the continuum intensity of the quiet photosphere. Also, the zero of the line-of-sight velocity was defined as a temporal and spatial average of velocities in the quiet photosphere. In addition to the spectropolarimetric observations, a series of 185 Broadband Filter Imager frames was taken from 11:52 to 14:58 UT in the G-band channel (4305 ± 4 Å) with the cadence of 1 frame per 60 s and spatial sampling of 0. 0545 per pixel. The frames were processed using the Solar Software routines and then deconvolved for the modulation transfer function of the telescope with diffraction-limited spatial resolution of 0. 22 at 4305 Å. The restored G-band images in the series were spatially correlated to remove the drift of the correlation tracker. The obtained alignment shifts were used to co-align the positions of the spectropolarimetric scans. The resulting field of view of the sequence of spectropolarimetric scans is 8 × 16 (see Figure 1).

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cases when the line of sight is not parallel with magnetic field lines. However, tentative changes of the hydrostatic equilibrium parameters in the code did not show any substantial or systematic errors of the calculated plasma parameters. Since the spatial resolution (0. 32) was comparable with the size of the studied features, inversions were made for a single-component model atmosphere in order to reduce the number of free parameters. However, the studied features (CUDs in particular) are very close to the spatial resolution limit and thus we performed some tests with a polarized stray-light component. For this purpose we used averaged Stokes profiles from an area surrounding the studied feature. We increased the stray-light factor up to 30%, but the results were practically identical to those without any stray light. This result confirms the finding of Riethm¨uller et al. (2008), i.e., the instrumental scattered light is negligible. Thus, only the results obtained without the stray light component are discussed in this paper. As mentioned in Section 1, the studied phenomena are most probably restricted to the lowest photospheric layers. To detect rapid changes of plasma parameters with height, we enabled five nodes for temperature T, LOS velocity vLOS , magnetic field strength B, and its inclination γ in the final step of the inversion process. Additionally, we used two nodes for magnetic field azimuth and one node for microturbulent velocity—altogether, 23 free parameters. As a result, we obtained a sequence of 34 three-dimensional maps of T, vLOS , B, and γ . Each map was averaged in the vertical direction in the ranges −0.5 < log τ < −0.2 (low photosphere) and −2.0< log τ < − 1.4 (high photosphere), where τ is optical depth at 5000 Å. An example of the vertically averaged maps for the low-photosphere range is shown in Figure 2. Longlived CUDs, PUDs, and PGs were tracked in the sequence of maps to study the temporal evolution of T, vLOS , B, and γ . The resulting values of magnetic field inclination are evaluated with respect to the line of sight. Since we are interested in the differences between the studied features and the surrounding areas and the observations were taken close to the disc center, transformation to the local reference frame is not necessary. To obtain these physical characteristics also for the umbral or penumbral surroundings of the features, we selected 3–4 locations at a distance of 0. 64 from the center of each feature and averaged the values measured at these locations. Complementary information about the brightness IGband (normalized to the mean intensity of√ the undisturbed photosphere), effective diameter deff = 0. 0545 4A/π (where A is the area in pixels), and horizontal velocity vHOR of tracked CUDs, PUDs, and PGs was obtained from the series of G-band images. The features were identified in images that were segmented using a combination of the multilevel tracking algorithm (Bovelet & Wiehr 2001; T. Roudier 2003, private communication) and the curvature determination algorithm (Hamedivafa 2008). The minimum observed IGband in the umbra was 0.08. This fact confirms the very low stray light level in the telescope and also in the Broadband Filter Imager.

3. INVERSION METHOD AND DATA ANALYSIS We applied the inversion code SIR (Stokes inversion based on response functions; Ruiz Cobo & del Toro Iniesta 1992) to the observed spectra. This code works under the assumption of local thermodynamical equilibrium and hydrostatic equilibrium. We are aware that the assumption of hydrostatic equilibrium is not justified in the upper parts of UDs due to the magnetic field gradient and dynamic action of moving plasma, as well as in

4. RESULTS In total, seven features were tracked: three CUDs (Nos. 2a, 2b, and 4), one PUD (No. 1), one PG (No. 5), and two PGs that converted into PUDs (Nos. 3 and 6). The positions of the tracked features are shown in Figure 2. At position 2, two CUDs, 2a and 2b, were observed at different time periods. Graphs of the temporal evolution of T, vLOS , B, and γ in the low and high

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Figure 2. Maps of temperature, LOS velocity, magnetic field strength, and inclination in the low photosphere (−0.5 < log τ < −0.2), observed at 13:34 UT. The gray scale ranges between the minimum (black) and maximum (white) of the displayed quantities are T : 3850–5760 K; vLOS : −2.21 to 0.29 km s−1 ; B : 40 − 3090 G; γ : 10◦ − 75◦ . Crosses depict the trajectories of tracked features.

Figure 3. Plots of temporal evolution of temperature, LOS velocity, magnetic field strength, and inclination for CUD No. 4 (solid lines) and its umbral surroundings (dashed lines). Black lines correspond to the low photosphere, gray lines to the high photosphere. Negative values of LOS velocity represent upflows.

photosphere for features 4, 1, 3, 5, and their surroundings are plotted in Figures 3–6. Three CUDs (2a, 2b, 4), located near the left edge of the field, did not show any systematic horizontal motion. Their evolution was very simple: an increase of T and a reduction of B with respect of the surrounding umbra in the first half of life, then a decrease of T and no observed B reduction. These changes were detected only in the low photosphere. In vLOS and γ , CUDs did not differ significantly from the surroundings at all photospheric heights (see Figure 3). The long-lived PUD 1 (Figure 4) detached from an end of a penumbral filament 11 minutes after its 83 minute long tracking

Figure 4. Same as Figure 3 but for PUD No. 1. The vertical dotted line marks the time of detachment of the UD from a penumbral filament.

began and moved into the umbra with a constant vHOR = 360 m s−1 . In the low photosphere, its T was approximately constant for 40 minutes and then decreased, while T in the surroundings decreased steadily as PUD 1 penetrated deeper into the umbra. Its reduced B fluctuated (probably due to the inversion uncertainties) and we observed a trend to diminish the difference in B between PUD 1 and its surroundings. Decreased B was also detected in the high photosphere in the first 30 minutes of life. While in the high photosphere vLOS and γ did not differ from the values in the surroundings, in the low photosphere PUD 1 maintained a stable upflow of 600 m s−1 for 70 minutes and a constant γ = 40◦ for 55 minutes. These parameters decreased substantially before the disappearance of PUD 1.

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Table 1 Time-averaged Values of Physical Parameters in the Low (High)a Photosphere Feature CUD 2a CUD 2b CUD 4 PUD 1 PUD 3 PUD 6 PG 5 PG 3 PG 6

tb 15 17 17 83 21 17 60 17 33

IGband

deff

0.26 0.33 0.25 0.58 0.69 0.41 0.88 0.87 0.90

0. 29

0. 33 0. 29 0. 35 0. 35 0. 34 0. 42 0. 41 0. 44

c vHOR

0.02 < 0.13