Evolution of magnetic field inclination in a forming penumbra

S3-1 Publ. Astron. Soc. Japan (2014) 66 (SP1), S3 (1–8) doi: 10.1093/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 REZAEI2 1 2 Astronomical Institute of the Academy of Sciences Fričova 298, 25165 Ondřejov, Czech Republic Kiepenheuer-Institut für Sonnenphysik Schöneckstr. 6, 79104 Freiburg, Germany *E-mail: 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 11024. 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 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. For Permissions, please email: Publications of the Astronomical Society of Japan, (2014), Vol. 66, No. SP1 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 854.2 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 1020 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 11024. 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 S3-2 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, Bello González, and Schlichenmaier (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/penu (...truncated)