EDP Sciences
Free Access
Issue
A&A
Volume 597, January 2017
Article Number A60
Number of page(s) 4
Section The Sun
DOI https://doi.org/10.1051/0004-6361/201628547
Published online 23 December 2016

© ESO, 2016

1. Introduction

Penumbra formation is not yet fully understood. Rucklidge et al. (1995) showed that the efficiency of energy transport across the magnetopause increases dramatically when the magnetic field inclination exceeds the critical value of 45° (γcrit). Despite the simplicity of the model used, the γcrit of 45° for the penumbral formation was confirmed by recent MHD simulations of Rempel et al. (2009). Jurčák et al. (2014a) confirmed observationally the importance of magnetic field inclination in triggering penumbra formation.

The magnetic field inclination on a protospot boundary is related to the total magnetic flux as the magnetic field inclination increases with the increasing flux. According to theoretical prediction of Rucklidge et al. (1995), the necessary magnetic flux for a penumbra formation is between 1−7 × 1020 Mx. These values are in agreement with observed values of 5 × 1020 Mx (Zwaan 1987), 1 × 1020 Mx (rudimentary penumbra, Leka & Skumanich 1998), and 4 × 1020 Mx (protospot with forming penumbral segments, Rezaei et al. 2012).

Observations of orphan penumbrae (Zirin & Wang 1991; Kuckein et al. 2012; Lim et al. 2013; Jurčák et al. 2014b; Zuccarello et al. 2014) show that they are comparable in all aspects to regular sunspot penumbrae. These observations prove that sufficient magnetic flux is not a necessary condition for a penumbra formation. A favourable configuration of magnetic field strength and inclination can result in the penumbra formation.

Once the penumbra formation is triggered, the penumbra extends mostly at the expense of granular regions. Jurčák et al. (2015) show that the inner penumbral end extends also to the umbral area where the umbra-penumbra (UP) boundary settles at a distinct canonical value of the vertical component of the magnetic field, of 1.8 kG. This confirms the results of Jurčák (2011) who found constant values of Bver along UP boundaries of stable sunspots, where the actual value of seems to be weakly dependent on the sunspot size. Larger samples of sunspots must be investigated to clarify whether or not such a dependence is real.

thumbnail Fig. 1

G-band images showing selected stages of pore and penumbra evolution in AR 10960. The orange contours indicate pore areas with . The arrows point towards the disc centre (north is up and west is right as marked in f)). A video showing the temporal evolution of the penumbra at the pore boundary is available online.

Open with DEXTER

In this paper, we present the Hinode observations of a small pore near the polarity inversion line of AR 10960. We follow the formation and development of a penumbra forming at the expense of the magnetic flux of the pore while colonising it. Six spectro-polarimetric scans of the region are used to infer the maps of the magnetic field vector. Details of the studied data are described in Sect. 2 along with the analysis methods. We describe and discuss the results in Sect. 3, and summarise them in Sect. 4.

2. Observations and data analysis

Our analysis is based on data observed with the Solar Optical Telescope (SOT, Tsuneta et al. 2008) aboard the Hinode satellite (Kosugi et al. 2007). We made use of spectro-polarimetric (SP) data in the Fe i lines at 630.15 and 630.25 nm, and images taken through a G-band filter centred at 430.5 nm with a bandpass of 0.8 nm.

We study the evolution of a pore in AR 10960 between 19:00 UT on 6 June 2007 and 8:00 UT on 8 June 2007. The pore was located 6° S and between 14° E and 6° W during the analysed time period. The cadence of G-band images vary between 60 s and 100 s (see Fig. 2). At all times, the spatial sampling is 0.̋11 resulting in a diffraction-limited spatial resolution of 0.̋22. During the analysed time period, six Hinode/SP scans of the region were acquired. The SP observations were taken in a fast mode with a pixel sampling of 0.̋32 and a noise level of 10-3Ic. These data were calibrated using the available routines in the Hinode SolarSoft package.

thumbnail Fig. 2

Evolution of the pore area with (black + symbols). Blue symbols connected by the dashed line show the total positive magnetic flux (Φ) in the pore. Red Λ symbols connected by the dash-doted line show Φ in the penumbra. The horizontal blue lines at the top of the plot indicate the times of G-band observations, where the light blue correspond to the imaging cadence of 60 s. The vertical red lines correspond to the times of SP scans and the vertical black lines labelled a)f) show the times of images shown in Figs. 1, 3, and 4.

Open with DEXTER

In order to retrieve the physical parameters, the SP data were inverted with the SIR code (Stokes Inversion based on Response function, Ruiz Cobo & del Toro Iniesta 1992). We used a simple atmosphere model supposing all free parameters of the inversion, except for temperature, to be constant with height. We took into account the spectral point spread function of the Hinode SP in the inversion process. We assumed the magnetic filling factor to be unity and assumed no stray light. The macro-turbulence was set to zero while micro-turbulence was a free parameter of the inversion. We applied the code AMBIG (Leka et al. 2009) to solve the 180° ambiguity of the LOS azimuth. The disambiguated LOS vector magnetic field was then transformed to the local reference frame (LRF) with the help of routines from the AZAM code (Lites et al. 1995).

The apparent motions of photospheric structures were determined from the G-band images using local correlation tracking (LCT, November & Simon 1988) with a Gaussian tracking window of FWHM 1′′. We first aligned the images and removed the p-mode oscillations by applying a kω filter with a cut-off of 5 km s-1.

thumbnail Fig. 3

Arrows indicate the direction and amplitude of the apparent motions in the forming penumbra region at times around the snapshots shown in Figs. 1c, f.

Open with DEXTER

thumbnail Fig. 4

From left to right: maps of continuum intensity, Bver, and magnetic field inclination at times of the snapshots shown in Figs. 1c, e, f. The blue and black contours indicate the areas of the studied pore and penumbra, respectively. Streamlines of the horizontal component of the magnetic field (red lines) are plotted over the magnetic field inclination maps.

Open with DEXTER

3. Results

In Fig. 1, we show the pore and penumbra evolution in AR 10960 at selected stages. A video attached to Fig. 1 is available online. At the beginning of the studied period (a), the pore with a small light bridge and no penumbra is observed. Approximately two hours later (b), a penumbra segment west from the pore can be observed; the penumbral filaments, which are not directly adjacent to the pore, have south-north (S-N, cf. Fig. 1f) orientation (heads of the filaments are located on the southern end). The animation on the evolution shows that this penumbral segment in not coupled to the investigated pore. Later (c), new penumbral filaments appear with their heads – penumbral grains – connected to the pore boundary. Initially, these filaments have a NS orientation (c), and at later stages (d, e, f), they show an EW orientation.

The evolution of the pore area (pixels with ) shown in Fig. 1 is shown in Fig. 2. The pore area increases until 4:00 UT on 7 June (Fig. 1c). Afterwards, the pore area decreases. The evolution of the pore size is closely related to the evolution of penumbral filaments that are directly adjacent to the pore. These filaments appear around 2:00 UT on 7 June (see the animation), and we immediately observe proper motions of penumbral bright grains into the pore (as observed in stable and forming penumbrae, Wang & Zirin 1992; Sobotka et al. 1999; Márquez et al. 2006; Jurčák et al. 2015). In Fig. 3, we show the proper motions in the close surrounding of the studied pore at the beginning of the penumbra formation (left) and towards the end of the pore lifetime (right). A characteristic property of a penumbra is clearly depicted: the apparent inward (outward) motion of the inner (outer) penumbra.

In Fig. 4, we show maps of the magnetic field configuration in the area of the forming penumbra for the three SP scans that captured the evolving pore-penumbra system. The first SP scan of the region where the penumbra is attached to the pore is shown in the upper row (c). The penumbral filaments are directed away from the polarity inversion line and the field lines (shown in the inclination map) do not connect the pore-penumbra system with the pores of opposite polarity located north and west of it. Figure 4, middle row (e), shows the magnetic field configuration around 17:10 UT. At that time, a transient second penumbral segment develops oriented along the polarity inversion line, partially connected by the field lines to the west pore of opposite polarity around (12′′, 12′′). Yet, three hours later (bottom row of Fig. 4f), the penumbral filaments are no longer oriented westwards and the magnetic field creating the penumbra is not longer connected with the west pore of opposite polarity.

At all times, the pore-penumbra system under study belongs to the same polarity in this bipolar region. The positive magnetic flux in the penumbra increases in time (Fig. 2), while the magnetic flux in the pore diminishes. This indicates that the penumbra grows at the expense of the pore magnetic flux as signatures of emerging flux are not seen during the relevant time span. The maximum flux found in the penumbra reaches 84% of the maximum flux in the pore, that is, even if some magnetic flux of the pore was cancelled out by the nearby regions of opposite polarity, the majority is transformed into the penumbral magnetic flux. Therefore, the fundamental cause of the pore disappearance is its transformation into the penumbra.

The process of pore disappearance is not straightforward. At some stages there are almost no regions with (Fig. 1d), while at later times these regions revive again (Fig. 1e). During the whole sequence, we observe proper motions of penumbral bright grains into the pore (Fig. 3). As shown in Jurčák et al. (2015), such motions “displace” the UP boundary towards the umbral core in the case of a forming sunspot penumbra, until the Bver reaches the value. Analogously, the area of the studied pore decreases at the expense of the evolving penumbra. Since the strongest vertical component of the magnetic field reaches only 1.4 kG in the studied pore, the stable boundary between the pore and the forming penumbra cannot establish and the penumbral magneto-convective mode takes over in the pore area.

The pore disappearance at the expense of the evolving penumbra can be also understood as a mechanism for an orphan penumbra formation. The predominantly vertical magnetic field of the pore becomes horizontal and we observe an orphan penumbra for a certain time before its disappearance. These horizontal fields creating the orphan penumbra submerge in time,

possibly due to downward pumping as described by Tobias et al. (1998). This is an alternative to the formation of an orphan penumbra by a flux emergence that is blocked by an overlaying magnetic field (Zuccarello et al. 2014) or by the relation of an orphan penumbra to an active region filament (Kuckein et al. 2012; Buehler et al. 2016).

4. Conclusions

We describe the evolution of a penumbra at the boundary of a small pore. The penumbra eventually colonises the pore area leading to its extinction. We link these observations with previous studies that found constant values of the vertical component of magnetic field strength on UP boundaries of stable sunspots (Jurčák 2011) and how this canonical value establishes a demarcation line in the forming UP boundary (Jurčák et al. 2015).

The studied pore has the vertical component of the magnetic field around 1.4 kG at its maximum (Fig. 4) which is smaller than the of about 1.8 kG (Jurčák 2011; Jurčák et al. 2015). Therefore, the protrusion of penumbral magneto-convection into the pore area is not hindered, a stable pore-penumbra boundary does not establish, and the pore is eventually colonised by the penumbra. As the penumbral bright grains protrude into the pore, the magnetic flux of the pore is transformed into penumbral magnetic flux, and the predominantly vertical field becomes horizontal. This scenario describes a mechanism by which an orphan penumbra forms.

Acknowledgments

The support from GA CR 14-04338S and RVO:67985815 is gratefully acknowledged. N.B.G. acknowledges financial support by the Senatsausschuss of the Leibniz-Gemeinschaft, Ref.-No. SAW-2012-KIS-5. R.R. acknowledges financial support by the DFG grant RE 3282/1-1 and by the Spanish Ministry of Economy and Competitiveness through project AYA2014-60476-P. Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in cooperation with ESA and NSC (Norway).

References

Online material

Movie of Fig. 1 (Access here)

All Figures

thumbnail Fig. 1

G-band images showing selected stages of pore and penumbra evolution in AR 10960. The orange contours indicate pore areas with . The arrows point towards the disc centre (north is up and west is right as marked in f)). A video showing the temporal evolution of the penumbra at the pore boundary is available online.

Open with DEXTER
In the text
thumbnail Fig. 2

Evolution of the pore area with (black + symbols). Blue symbols connected by the dashed line show the total positive magnetic flux (Φ) in the pore. Red Λ symbols connected by the dash-doted line show Φ in the penumbra. The horizontal blue lines at the top of the plot indicate the times of G-band observations, where the light blue correspond to the imaging cadence of 60 s. The vertical red lines correspond to the times of SP scans and the vertical black lines labelled a)f) show the times of images shown in Figs. 1, 3, and 4.

Open with DEXTER
In the text
thumbnail Fig. 3

Arrows indicate the direction and amplitude of the apparent motions in the forming penumbra region at times around the snapshots shown in Figs. 1c, f.

Open with DEXTER
In the text
thumbnail Fig. 4

From left to right: maps of continuum intensity, Bver, and magnetic field inclination at times of the snapshots shown in Figs. 1c, e, f. The blue and black contours indicate the areas of the studied pore and penumbra, respectively. Streamlines of the horizontal component of the magnetic field (red lines) are plotted over the magnetic field inclination maps.

Open with DEXTER
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.