Evolution of magnetic field inclination in a forming penumbra

Transcription

1 S3-1 Publ. Astron. Soc. Japan (2014) 66 (SP1), S3 (1 8) doi: /pasj/psu080 Advance Access Publication Date: 2014 October 28 Evolution of magnetic field inclination in a forming penumbra Jan JURČÁK, 1, Nazaret BELLO GONZÁLEZ, 2 Rolf SCHLICHENMAIER, 2 and Reza REZAEI 2 1 Astronomical Institute of the Academy of Sciences Fričova 298, Ondřejov, Czech Republic 2 Kiepenheuer-Institut für Sonnenphysik Schöneckstr. 6, Freiburg, Germany * jurcak@asu.cas.cz Received 2014 February 17; Accepted 2014 August 18 Abstract As a sunspot penumbra forms, the magnetic field vector at the outer boundary of the protospot undergoes a transformation. We study the changes of the magnetic field vector at this boundary as a penumbral segment forms. We analyze a set of spectropolarimetric maps covering 2 hr during the formation of a sunspot in NOAA The data were recorded with the GFPI instrument attached to the German VTT. We observe a stationary umbra/quiet Sun boundary, where the magnetic field becomes more horizontal with time. The magnetic field inclination increases by 5, reaching a maximum value of about 59. The maximum inclination coincides with the onset of filament formation. In time, the penumbra filaments become longer and the penumbral bright grains protrude into the umbra, where the magnetic field is stronger and more vertical. Consequently, we observe a decrease in the magnetic field inclination at the boundary as the penumbra grows. In summary, in order to initiate the formation of the penumbra, the magnetic field at the umbral (protospot) boundary becomes more inclined. As the penumbra grows, the umbra/penumbra boundary migrates inwards, and at this boundary the magnetic field turns more vertical again, while it remains inclined in the outer penumbra. Key words: Sun: evolution Sun: magnetic fields Sun: photosphere sunspots 1 Introduction Sunspots and pores are dark structures observed in the solar photosphere. They are a manifestation of strong concentrations of magnetic fields that inhibit the heat transfer from deeper layers such that the photosphere becomes colder and therefore darker than its surroundings. Sunspots consist of a dark umbra surrounded by a penumbra. The processes that lead to the formation of a penumbra are still not well understood. It was shown by Rucklidge, Schmidt, and Weiss (1995) that pores and sunspots are two stable solutions of magnetic flux concentrations, where the transition from pore to sunspot is given by the critical inclination of 45 on the flux tube boundary. Despite the simplicity of the model used, this critical inclination value was recently confirmed by the MHD simulations of Rempel et al. (2009). Rempel (2012) also found that the boundary conditions for the magnetic field inclination at the top of his simulation box are crucial for the presence of a penumbra. The formation of a penumbra is favored when the horizontal component of the magnetic field strength is enhanced at the upper boundary. This numerical experiment may cast new C The Author Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. For Permissions, please journals.permissions@oup.com

2 Publications of the Astronomical Society of Japan, (2014), Vol. 66, No. SP1 S3-2 light on how the formation of a penumbra is triggered: a horizontal magnetic field in the chromosphere may initiate the penumbra formation in the photosphere. And indeed, signatures of inclined magnetic fields in the chromosphere prior to the formation of the penumbra were observed by Shimizu, Ichimoto, and Suematsu (2012) and Romano et al. (2013): they found annular zones around pores in chromospheric lines (Ca II H and Ca II nm). These were interpreted as representing strongly horizontal magnetic fields (magnetic canopy), which could initiate the formation of the penumbra. Such an annular zone was not observed by Schlichenmaier et al. (2010b) in the chromospheric Ca II K line, but they used a filter with a much larger bandwidth of 1 nm (Schlichenmaier et al. 2010b; Bello González et al. 2012). The magnetic field inclination on a pore boundary increases with its total magnetic flux. Zwaan (1987) and Leka and Skumanich (1998) observed that a magnetic flux of the order of Mx is needed to form penumbrae. Such a value is in agreement with theoretical models (Rucklidge et al. 1995). Leka and Skumanich (1998) described the evolution of a penumbra segment around a pore. They suggested three scenarios of penumbra formation: a penumbra forms from (a) the vertical magnetic field of the dark pore regions that becomes more horizontal, (b) regions with more horizontal fields surrounding the spot that still have undisturbed quiet Sun intensities, and (c) magnetic fields that emerge from deeper layers. The latest scenario is in the best agreement with their observations. However, this is not in agreement with observations showing that penumbral filaments form first on the side away from the active emergence area as is seen in Schlichenmaier et al. (2010b), and as was already described by Zwaan (1992). Also, Rezaei, Bello González, and Schlichenmaier (2012) find transient filaments on the side of the emerging flux that disappear on a dynamical time scale. They surmise that the emergent flux hinders the magnetic field staying horizontal. Only on the side opposite to the emergence site can the filaments stretch out, become more horizontal, and form a stable penumbra. On the other hand, the emerging flux can be responsible for the formation of so-called orphan penumbrae (Lim et al. 2013; Zuccarello et al. 2014) that have similar characteristics to sunspot penumbrae (Jurčák et al. 2014), i.e., filamentary structure, inclined magnetic fields, and Evershed flow. In this paper, we are extending the analysis of a unique and comprehensive data set capturing the formation of a sunspot in the active region NOAA Schlichenmaier et al. (2010b) described the morphological aspects of the formation process using G-band and Ca II K filtergrams. They found that the penumbra forms in segments. The first segments form next to the light bridge on the spot side opposite to the region with emerging flux, and only later do the forming segments surround the whole umbra. During the formation, the total umbral area stays constant. As discussed in Schlichenmaier et al. (2010a) and Rezaei, BelloGonzález,andSchlichenmaier (2012), they found evidence that the additional flux needed to form the penumbra is supplied by small magnetic flux patches that join the spot from the emergence site. It appears as if the magnetic flux is transported through the light bridges to the opposite spot side where the magnetic field encounters conditions to make it increase its inclination and to form stable penumbral segments. Here we analyze the magnetic field properties on the boundary between the sunspot and the surrounding granulation as it transforms into the umbra/penumbra boundary. The dataset used for the analysis was extensively studied by Schlichenmaier et al. (2010a, 2010b), Rezaei, Bello González, and Schlichenmaier (2012), and Bello González, Kneer, and Schlichenmaier (2012) we briefly describe it in section 2. The changing magnetic field properties on the boundary studied are described in section 3, along with the reliability of the retrieved results. The conclusions are summarized in section 4. 2 Observations and data analysis Our analysis is based on spectropolarimetric data taken with the GFPI (GREGOR Fabry-Pérot Interferometer) at the German Vacuum Tower Telescope (Puschmann et al. 2006; Bello González & Kneer 2008). The instrument was used to scan four Stokes parameters at 31 wavelength points in the Fe I nm line with a spectral sampling of 1.48 pm, a resulting S/N ratio of 125, and a spatial resolution of The data processing is described in Rezaei, Bello González, and Schlichenmaier (2012). We focus on the data taken on 2009 July 4, between 8:32 UT and 10:31 UT. During this period, the Kiepenheuer Adaptive Optics System (KAOS: von der Lühe et al. 2003; Berkefeld et al.2010) successfully compensated for the image motions and distortions for most of the time. There are 109 GFPI scans taken at a cadence of 56 s, but occasional longer gaps exist. The active region was located 6 E and 25 N from disk center. The intensity maps reconstructed from the continuum of the Stokes I profile are used to spatially align the consecutive GFPI scans (cross-correlation in the Fourier domain). To determine the magnetic field properties from the observed Stokes profiles, we use the inversion code VFISV (Very Fast Inversion of the Stokes Vector: Borrero et al. 2011). The code works under the assumption of a Milne Eddington atmosphere, i.e., the plasma parameters

3 S3-3 Publications of the Astronomical Society of Japan, (2014), Vol. 66, No. SP1 are constant with height and the source function is linearly dependent on optical depth. For our study, the VFISV code has been adapted to work with the GFPI data (instead of its default use on SDO-HMI data) by including the proper spectral PSF. The magnetic filling factor is set to unity for all inverted pixels by default, i.e., we do not take into account a stray-light component. Multiple starting models of atmosphere are used at each pixel and the best solution is selected to create the final maps of magnetic field strength, B, inclination, and azimuth. We resolve the 180 ambiguity by assuming a radial orientation of the magnetic field in the sunspot. We convert the values of magnetic field inclination and azimuth from the line-of-sight (LOS) frame into the local reference frame (LRF) by applying routines from the AZAM code (Lites et al. 1995). The inclination γ is hereafter used as a symbol for magnetic field inclination in the LRF. Complementary to the GFPI scans, we use co-temporal speckle reconstructed G-band images (filter centered at 403 nm with bandwidth of 1 nm), which are aligned to the continuum GFPI images. The G-band images have a spatial resolution better than 0. 3 (Schlichenmaier et al. 2010b). 3 Results In figure 1, the left and right panels display the G-band images of the sunspot close to the beginning and the end of our time series. The middle panel shows co-spatial and co-temporal GFPI narrow band images at continuum wavelength. In figure 2, we show three snapshots of maps of B (middle column) and γ (right column), along with co-spatial G-band intensity maps (left column). The orange, red, and blue contours show the intensity boundaries at 50%, 55%, and 60% of the mean QS (quiet Sun) intensity, respectively. These boundaries are based on GFPI continuum intensity maps that are spatially and temporally smoothed and are hereafter referenced as, e.g., 50% boundary. The maps of B and γ show the typical behavior within sunspots as the strongest and most vertical field is located in the darkest parts of the umbra (see the review by Solanki 2003). The penumbral filaments protruding deeper into the umbra on the left- and right-hand sides in the G-band images have a weaker and more inclined magnetic field (cf. Solanki 2003 and references therein). We note that the field inclination, γ, is only meaningful when the strength, B, has a significant value. Hence, the inclinations in the upper portion of the maps outside the spot are not determined reliably. 3.1 Effects of degrading seeing and increasing dust in the atmosphere on the inversion results As time proceeds the spatial resolution of the magnetic maps degrades. Also, the magnetic field strength values in the dark core of the umbra decrease with time. On average, the magnetic field decreases by 100 G in the umbral region during the two hours. The decrease of B is comparable with the results of Rezaei, Bello Gonza lez, and Schlichenmaier (2012), who obtained the same trend and amplitude of magnetic field decrease in all umbral cores of this spot (cf. figure 8 in Rezaei et al. 2012). We ascribe this to the degrading seeing conditions and a significant increase of the dust concentration in the atmosphere, which results in increasing stray-light contamination. In figure 3, we show the decreasing intensity RMS (black line) along with the increasing dust concentration (red line). We argue that increasing stray light reduces the Fig. 1. On left and right we show the speckle reconstructed G-band images at 8:53 UT and 10:29 UT. In the middle, we show intensity maps reconstructed from the blue continuum of the Stokes I profile scanned by the GFPI instrument. These maps are co-temporal and co-spatial with boxes marked in the G-band images. The arrow in the left panel points toward the disk center.

4 Publications of the Astronomical Society of Japan, (2014), Vol. 66, No. SP1 S3-4 Fig. 2. From left to right: G-band intensity, magnetic field strength, and magnetic field inclination maps. The observation times are shown in the upper-left corner of the G-band images. The orange, red, and blue contours mark the positions of the intensity boundaries at 50%, 55%, and 60% of the mean QS intensity, respectively. (Color online) Fig. 3. Temporal evolution of intensity RMS ( symbols), where the solid black line corresponds to the linear fit of the measured values. The solid red line corresponds to the dust density measured for the nearby STELLA instrument. (Color online) Rezaei, Bello González, and Schlichenmaier (2012), who assumed a temporally constant stray-light contamination of 12%. They obtained higher B values, but comparable γ values. We note that the inversion code encounters problems at certain locations, as can be seen, e.g., in the middle B map at [14,0 ], where the inferred field strength is unrealistically high. Such problems are caused occasionally by destretching the narrow band images before assembling the Stokes profiles. Approximately 20% of the boundary segments are influenced by such regions. We exclude from further analysis boundary segments where the standard deviations of either γ or B are larger than 3 or 150 G, respectively (typical values are 1. 5 and 55 G, respectively). obtained magnetic field strength as the amplitudes of the Stokes Q, U,andV profiles decrease, but the magnetic field inclination is not affected as it is proportional to the amplitude ratio of linearly and circularly polarized light. This argument is supported by comparison with results from 3.2 Temporal evolution of B and γ To study the temporal evolution of the magnetic field strength and inclination, we select segments of the intensity boundaries shown in figure 2. The intensity boundaries are well defined and changing gradually only at the

5 S3-5 Publications of the Astronomical Society of Japan, (2014), Vol. 66, No. SP1 Fig. 4. G-band images showing the evolution of penumbra on the upper boundary of the spot. The orange and red lines show the positions of the segments of the umbra boundary used for detailed analysis. The lines correspond to the position of the 55% boundary. (Color online) upper umbral boundary, i.e., for x-coordinates between 6 and 9. Only in this region do the boundaries stay close to each other for the whole observational sequence. Elsewhere, there are abrupt changes of the boundary shapes and positions due to the evolution of sunspot fine structure, and plots analogous to those presented for the selected segments are thus not interpretable. In figure 4, we show the G-band images illustrating the evolution of penumbra during the two-hour period. For the detailed analysis, we selected two segments of the boundaries with length 1 and separated by The positions of these segments on the 55% boundary are marked by orange and red lines in figure 4. At 8:30 UT, there are no penumbral filaments at the boundary segment marked by the red line in figure 4, while there are hints of short filaments at the boundary marked by the orange line. Hereafter, we call these boundary segments red boundary and orange boundary, respectively. Initially, the orange boundary is located closer to the umbral core. At 9:00 UT we observe short filaments that start to influence the red boundary. Around this time, both red and orange boundaries have approximately the same distance from the umbral core as the orange boundary moved outwards due to the evolution of penumbral filaments. At 9:10 UT, short penumbral filaments are observed at both red and orange boundaries. In time, the penumbral filaments are getting longer and a well-developed penumbra segment can be seen after 10:00 UT. The prolongation of the penumbral filaments results in the migration of both boundary segments towards the umbral core. For each scan, we compute the mean values of magnetic field strength and inclination along the red and orange boundaries marked in figure 4 that corresponds to segments of the 55% boundary. Analogously, mean values of B and γ are computed at segments corresponding to the 50% and 60% boundaries. We also compute the mean positions of all boundary segments with respect to the umbral core Red boundary In figure 5, we show the resulting evolution of B and γ at the red boundary marked in figure 4. In the initial 30 min of the analyzed sequence, we observe an increase of the magnetic field inclination by approximately 5 (red + symbols and lines), which is not accompanied by any motion of the boundary (the dashed black lines). The most inclined magnetic field at the red boundary is observed around 9:00 UT, which is co-temporal with the appearance of the first penumbral filaments at this boundary, as can be seen in figure 4. Around 9:20 UT, the magnetic field inclination starts to decrease again and this is accompanied by the migration of the red boundary towards the umbral core. Towards the inner umbra, the magnetic field is stronger and more vertical and hence the migrating boundary shows decreasing inclination, γ. In this phase (9:30 UT 10:00 UT), the magnetic field slightly increases at the boundary (solid black lines in figure 5), even though the degraded seeing (and increasing dust) causes a general decrease of the inferred magnetic field strength (dash-dotted black lines in figure 5). Similarly to the behavior of γ, the strengthening of the magnetic fields stops at 10:00 UT.

6 Publications of the Astronomical Society of Japan, (2014), Vol. 66, No. SP1 S3-6 Fig. 5. Temporal evolution of magnetic field inclination (red + symbols) and strength (blue + symbols) at the boundary segment marked by the red line in figure 4. The values of γ and B smoothed over 10 minutes are shown by red and blue lines. The dashed black lines in the γ plots show the positions of the boundaries with time, where the axis on the right-hand side shows the span of the motions. The black dash-dotted lines in the B plots represent linear fits of the obtained B values, except for the period between 9:30 UT and 10:00 UT when the trend is fitted by the solid black line. From left to right the plots correspond to the 50%, 55%, and 60% boundaries, respectively. (Color online) Fig. 6. Analogous to figure 5, but corresponding to the boundary segment marked by the orange line in figure 4. (Color online) Orange boundary We observe a similar evolution on the orange boundary located approximately 1. 5 from the red boundary. In figure 6, we show the corresponding changes of B, γ,and location with time. We observe an increase of the magnetic field inclination from the beginning until 9:30 UT. This is the same behavior as for the red boundary, even though in this case the inclination increase is favored by the fact that the orange boundary migrates outwards, i.e., towards the weaker and more horizontal magnetic field. Starting at 9:30 UT, the penumbral filaments grow in length and a penumbral segment forms. At this time, the inclination starts to decrease and the boundary migrates inwards. This is analogous to what is happening on the red boundary. In contrast to the red boundary, an increase of B is not detected. 3.3 Reliability of B and γ variations To confirm the reliability of the temporally smoothed variations of magnetic field strength and inclination (solid red and blue lines in figures 5 and 6), we estimate the uncertainties of these parameters. We consider the standard deviation of temporal differences between consecutive B values as a representative uncertainty of B for a given boundary segment. The same approach is used to estimate the uncertainty of γ. The uncertainties of magnetic field strength and inclination on all six studied boundary segments are around 70 G and 2. 5, respectively. The increase and decrease of magnetic field inclination at the red boundary segment have amplitudes of 5 and 10, respectively. Also, on the orange boundary, the amplitudes of γ variations are around 5. These changes are larger

7 S3-7 Publications of the Astronomical Society of Japan, (2014), Vol. 66, No. SP1 than the uncertainty of the magnetic field inclination. As γ is not influenced by degraded seeing and increasing stray-light contamination, we argue that the evolution of the magnetic field inclination obtained is reliable. On average, the magnetic field strength decreases by 140 G on the red and orange boundary segments during the two hours. This is also above the uncertainty of B. However, the weakening of the magnetic field is probably caused by degraded seeing and increasing stray-light contamination, and it is not a reliable variation of B. Between 9:30 UT and 10:00 UT, the magnetic field strengthens on the red boundary segments by 100 G. This increase is above the uncertainty level of 70 G and it is against the general trend. We surmise that it is a real change of B. 3.4 Critical value of the magnetic field inclination Our measurements of the magnetic field inclination at the umbra/penumbra boundary can be compared to the critical inclination which is predicted by theory for a forming penumbra: Rucklidge, Schmidt, and Weiss (1995) and Rempel et al. (2009) found that the onset of penumbra formation happens after the magnetic field vector reaches 45 on the boundary of the spot. Depending on the intensity threshold of the boundary, we find values for the inclination, γ, between 57 (on the 50% boundary) and 61 (on the 60% boundary). Even at later stages, when the penumbra filaments are well developed, we obtain magnetic field inclinations between 48 and 53 (cf. figure 5) at the umbra/penumbra boundary, i.e., our measured values are larger than the model values. The model values correspond to the background (umbral) component of the magnetic field. Our values are inferred from a Milne Eddington inversion and represent mean values. If the emerging profiles are a mixture of background and highly inclined filamentary magnetic field (see, e.g., Jurčák et al. 2007), our inferred values are larger than those of the background component. This can explain the difference in theoretical and observed values of critical magnetic field inclination. 4 Conclusions In theoretical terms a sunspot consists of two stratifications in which distinct modes of magnetoconvection are present. In the umbra, the magnetic field is dominantly vertical and the largely suppressed convection takes place in smallscale cells manifested by umbral dots. In the penumbra the convective motions are stronger, but they are affected by magnetic forces. Hence, the penumbra is characterized by radiatively driven magnetoconvection in an inclined magnetic field. These considerations apply for a stationary sunspot. Note that the existence of so-called orphan penumbrae (e.g., Jurčák et al. 2014) indicates that the penumbral mode of magnetoconvection can also exist without an accompanying umbra. When does a penumbra form around protospots? How does the transition take place? Does the magnetic field in the outer part of the protospot become more inclined to form a penumbra, i.e., is there a transition between the two modes? Or does the penumbra form out of newly emerging flux, such that the corresponding magnetic field is in the penumbral magnetoconvective mode from the beginning, as seems to be the case in orphan penumbrae? In this paper we address this question by analyzing the data set of the spot forming on 2009 July 4. In this case, some inclined magnetic field of kg strength exists outside the visible protospot boundary (Rezaei et al. 2012). As it is not visible in the continuum, it is presumably present as a strongly inclined field in the higher layers of the photosphere (Shimizu et al. 2012; Romano et al. 2013). We surmise that the presence of this horizontal magnetic field results in the penumbra formation. As shown in the previous section, we find evidence that the field at the boundary becomes more inclined just at the onset of penumbra formation. Subsequently, the boundary migrates inwards and the magnetic field on the boundary becomes more vertical. From this we conclude that some of the more vertical flux of the protospot transforms from the umbral type of magnetoconvection into the penumbral type of magnetoconvection. Acknowledgments We are grateful to Juan Manuel Borrero for providing us with the inversion results obtained by his code VFISV. The support from GA CR P209/12/0287, GA CR S, and RVO: is gratefully acknowledged. R. R. acknowledges financial support by the DFG grant RE 3282/1-1. 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