Penumbrae are known to be area of mainly horizontal magnetic field
surrounding umbrae of relatively large and mature sunspots. In this paper, we
observationally studied the formation of penumbrae in NOAA10978, where several
penumbral formations were observed in G-band images of SOT/Hinode. Thanks to
the continuous observation by Hinode, we could morphologically follow the
evolution of sunspots and found that there are several paths to the penumbral
formation: (1) Active accumulation of magnetic flux, (2) Rapid emergence of
magnetic field, and (3) Appearance of twisted or rotating magnetic tubes. In
all of these cases, magnetic fields are expected to sustain high inclination at
the edges of flux tube concentration longer than the characteristic growth time
of downward magnetic pumping.Comment: accepted for publication in PAS

Kitai, Reizaburo

Otsuji, Ken'ichi

Watanabe, Hiroko

Publications of the Astronomical Society of Japan

English

arXiv

Reizaburo Kitai

Hiroko Watanabe

Ken'ichi Otsuji

Crossref

Morphological study of penumbral formation

Penumbrae are known to be areas of mainly horizontal magnetic field surrounding umbrae of relatively large and mature sunspots. In this paper, we observationally studied the formation of penumbrae in NOAA 10978, where several penumbral formations were observed in G-band images of the Solar Optical Telescope on board Hinode. Thanks to the continuous observation by Hinode, we could morphologically follow the evolution of sunspots and found that there are several paths to the penumbral formation: (1) active accumulation of magnetic flux, (2) rapid emergence of magnetic field, and (3) appearance of twisted or rotating magnetic tubes. In all of these cases, magnetic fields are expected to sustain high inclination at the edges of flux tube concentration longer than the characteristic growth time of downward magnetic pumping

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Title Morphological study of penumbral formationAuthor(s)Kitai, Reizaburo; Watanabe, Hiroko; Otsuji, Ken'ichiCitationPublications of the Astronomical Society of Japan (2014),66(SP1)Issue Date2014-10-28URL http://hdl.handle.net/2433/200194RightThis is a pre-copyedited, author-produced PDF of an articleaccepted for publication in 'Publ Astron Soc Jpn' followingpeer review. The version of record [Reizaburo Kitai, HirokoWatanabe, and Ken'ichi OtsujiMorphological study of penumbral formationPubl Astron Soc Jpn (December 2014) 66 (SP1), S11,doi:10.1093/pasj/psu084 first published online October 28,2014] is available online at:http://pasj.oxfordjournals.org/content/66/SP1/S11.; The full-text file will be made open to the public on 28 October 2015 inaccordance with publisher's 'Terms and Conditions for Self-Archiving'.Type Journal ArticleTextversionauthorKyoto UniversityPubl. Astron. Soc. Japan (2014) 00 (0), 1–5doi: 10.1093/pasj/psu084Advance Access Publication Date: 2014 01Morphological study of penumbral formationReizaburo KITAI,1,∗ Hiroko WATANABE,2 and Ken’ichi OTSUJI31Kwasan and Hida Observatories, Kyoto University, Yamashina, Kyoto, Kyoto 607-8471, Japan2Unit of Synergetic Studies for Space, Kyoto University, Yamashina, Kyoto, Kyoto 607-8471, Japan3National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan*E-mail: kitai@kwasan.kyoto-u.ac.jpReceived 2014 February 14; Accepted 2014 March 24AbstractPenumbrae are known to be areas of mainly horizontal magnetic field surroundingumbrae of relatively large andmature sunspots. In this paper, we observationally studiedthe formation of penumbrae in NOAA10978, where several penumbral formations wereobserved in G-band images of the Solar Optical Telescope on board Hinode. Thanks tothe continuous observation by Hinode, we could morphologically follow the evolution ofsunspots and found that there are several paths to the penumbral formation: (1) activeaccumulation of magnetic flux, (2) rapid emergence of magnetic field, and (3) appear-ance of twisted or rotating magnetic tubes. In all of these cases, magnetic fields areexpected to sustain high inclination at the edges of flux tube concentration longer thanthe characteristic growth time of downward magnetic pumping.Key words: convection—Sun: magnetic fields—Sun: photosphere—sunspots1 IntroductionWell-developed sunspots are fledged with surroundingpenumbrae. The magnetic field in the penumbral area isknown to be nearly horizontal (Stix 2002). The formationmechanism of penumbrae has been studied bymany authorstheoretically or observationally (Borrero & Ichimoto 2011;Rempel 2011, and references cited therein).Rucklidge, Schmidt, andWeiss (1995) studied the energyflows in a model sunspot including the lateral energy fluxfrom the surrounding inclined magnetic field, and showedthat sunspot MHD structure has two equilibrium states(pores and sunspots) and that the structure will changeabruptly or transit from pore state to sunspot state whenthe inclination of the surrounding magnetic field gets largerthan a threshold value. Succeeding numerical simulationsby Hurlburt and Rucklidge (2000) and Tildesley andWeiss (2004) led to the idea of a downward magneticpumping mechanism around sunspots (Weiss et al. 2004).Cheung et al. (2010) and then Rempel (2012) closelystudied the necessary chromospheric boundary conditionsfor extended penumbral formation.Observationally, Shimizu, Ichimoto, and Suematsu(2012) detected a case of penumbral formation where theappearance of a chromospheric dark annular zone, whichmay correspond to a horizontal magnetic field, precedes theformation of photospheric penumbra. The work inspired usto observationally follow the evolution of penumbral struc-ture in many varieties of sunspots, continuously in time,with the Solar Optical Telescope (SOT) on board Hinode(Suematsu et al. 2008; Tsuneta et al. 2008).In this paper, we observationally studied the formationof penumbrae in NOAA 10978, where several penum-bral formations were observed in G-band images ofSOT/Hinode. We will briefly report that there are severalpaths where the surrounding magnetic fields of sunspots areinclined and eventually lead to the formation of penumbrae.C© The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan.All rights reserved. For Permissions, please email: journals.permissions@oup.com Publications of the Astronomical Society of Japan Advance Access published October 28, 2014 by guest on October 30, 2014http://pasj.oxfordjournals.org/Downloaded from 2 Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 02 Morphological study of penumbralformation in NOAA 10978NOAA 10978 was a very large sunspot group, covered awide area on the solar surface, and showed several activeemerging flux regions (EFRs) which developed to formsunspots with penumbrae. Figure 1 shows white light andmagnetic maps of the region on 2007 December 12. Fluxemergence in the region was very active from December 11through 13. From visual inspection of the G-band movie,we selected three sub-regions as examples of different pathsof penumbral formation.Fig. 1. NOAA 10978 on 2007 December 12. Top: White lightimage. Bottom: Map of line-of-sight magnetic field. Courtesy ofSolarMonitor.org.2.1 Active accumulation of magnetic fluxIn figure 2, we show a case of penumbral formation dueto rapid accumulation of magnetic flux. The accumulationcontinued for around 40 hr in this case until the formationof fully developed penumbra. Let us simply assume thatFig. 2. Rapid accumulation of magnetic flux. The polarity over the area ispositive. Black arrows indicate the penumbral area, while white arrowsshow the umbra/photosphere boundary without penumbral structure.The FOV is about 75′ ′ × 66′ ′. by guest on October 30, 2014http://pasj.oxfordjournals.org/Downloaded from Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0 3the magnetic field of a sunspot takes a canopy structuredue to the force balance between the inside magnetic pres-sure and the outside gas pressure. When the magnetic fluxincreases, the inclination of the magnetic field at the outerperiphery of the sunspot increases accordingly. Thus, as themagnetic flux is accumulated to the spot, the field will beinclined more and becomes liable to be horizontal by down-ward magnetic pumping. This case is typical of penumbralformation.It is interesting to note that the magnetic polarity ispositive all over this particular area. While the northernperiphery of the region, indicated by black arrows infigure 2, forms a penumbral structure, no penumbra isformed at the central part of the umbra, indicated by whitearrows. The latter part is in the midst of a positive polarityarea, and so the magnetic field will be nearly vertical even atthe apparent boundary between the umbra and the normalphotosphere, without forming a penumbral area.2.2 Rapid emergence of magnetic fieldFigure 3 shows the emergence of an EFR at the southernpart of the region shown in figure 2. The EFR devel-oped very rapidly in 10 hr. In the middle part of theEFR, penumbral structures were seen face to face in bothpolarities. Recent observations and numerical simulationsshow that the emerging magnetic flux stays horizontal fora while beneath the solar photosphere before emergence(Otsuji et al. 2011; Magara 2012; Toriumi & Yokoyama2013). So we can expect that the magnetic field will benearly horizontal in the middle part of the EFR, and man-ifested as photospheric dark lanes often observed in themiddle part of EFRs (Otsuji et al. 2007). Furthermore,the EFR in question emerges at the bottom of a relativelyolder pre-existing bi-polar region and so the expansionof newly emerged magnetic flux to the higher layers willbe suppressed. As the magnetic field is horizontal in themiddle part of the EFR, the downward pumping mecha-nism is highly likely to work in this situation, leading to theformation of penumbral structure.2.3 Appearance of twisted or rotatingmagnetic tubeFigure 4 shows a case of penumbral formation where atwisted or rotating magnetic tube appears in the photo-spheric surface. The apparent counter-clockwise rotationof the umbra continued for 5–6 hr. The situation might beexplained by actual counter-clockwise field rotation or bythe vertical emergence of a twisted field. In either case, thefield will be inclined at the umbra/photosphere boundary.Fig. 3. Rapidly emerging magnetic flux region. Penumbral areas areseen in the middle part of both polarities. The FOV is about 95′ ′ × 67′ ′.This can be seen from the non-radial pattern of the penum-bral structure. So in this case, the magnetic field at theperiphery of umbra is expected to be highly inclined and isliable to be affected by the magnetic pumping mechanismto form penumbral structure.3 Discussion and conclusionThanks to the continuous observation by Hinode, we couldmorphologically follow the evolution of sunspots and foundthat there are a few different paths to penumbral forma-tion: (1) active accumulation of magnetic flux, (2) rapidemergence of new magnetic flux, and (3) appearance oftwisted or rotating magnetic tubes. In all of these cases,magnetic fields are expected to sustain high inclination atthe edges of flux tube concentration longer than the char-acteristic growth time of downward magnetic pumping by guest on October 30, 2014http://pasj.oxfordjournals.org/Downloaded from 4 Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0Fig. 4. Penumbral formation around a rotating or twistedmagnetic tube.The umbra rotates counter-clockwise as is indicated by an arrow. TheFOV is about 40′ ′ × 45′ ′.(3–4 hr), which is estimated as 20 times the thermal dif-fusion time of photospheric convection (Tildesley & Weiss2004).As our report is only based on the morphological study,our findings must be confirmed by polarimetric observationof the three-dimensional magnetic field around penumbra-forming sunspots. Such an observation not only in the for-mation phases but also in the decaying phases of penumbraeis quite indispensable to clarify the detailed mechanism ofpenumbral formation.Recently, such studies have been published by severalauthors. Penumbral formation in the preceeding spot ofNOAA11024 was studied by Schlichenmaier et al. (2010a,2010b) and Rezaei, Bello Gonza´lez, and Schlichenmaier(2012) with spectro-polarimetric observation done at theGerman Vacuum Tower Telescope. The spot grew in areaand in total magnetic flux through the merging of smallflux patches. After a few hours of flux merging, a penum-bral area was formed in the spot at the opposite side ofthe flux merging. They argue that the total magnetic fluxattained a threshold for penumbral formation and the lightbridge of the inclined magnetic field helped the penumbralformation. As they observed the relatively vertical magneticfield at the merging side and the inclined field at the oppo-site side, they suggested that these magnetic environmentscould control the actual penumbral formation. We thinkthat the penumbral formation in NOAA11024 is a case ofactive accumulation of magnetic flux and not a case of rapidflux emergence, and agree that the magnetic environment isa key factor in the actual penumbral formation, as we alsonotice the one-sided penumbra in figure 3.Louis et al. (2013) analyzed the penumbral formation inNOAA11283 with Hinode spectro-polarimetric observa-tion. They found the coalescence of a pore with a decayingsunspot supplied enough magnetic flux and triggered theformation of penumbra at the location of the merger, notat the opposite side.We think that the penumbral formationin NOAA11283 is basically a case of rapid flux accumu-lation. As the pore was a foot point of an active EFR, wesuspect that rapid emergence may play a role in forming thepenumbra at the merger side.Romano et al. (2013) undertook spectro-polarimetricobservation of NOAA11490 and detected the status of thesunspot atmosphere just before the penumbral formation.They found a magnetic canopy structure at the chromo-spheric heights and uncombed photospheric magnetic struc-ture in the spot atmosphere. We think that the penumbralformation there is also a case of active accumulation ofmag-netic flux forming a magnetic canopy structure of inclinedmagnetic field at the periphery.Although these studies contribute much to our under-standing of penumbral formation in the case of rapid fluxaccumulation, detailed behavior in the other cases of rapidflux emergence and rotating spots have not been fullyobserved yet. Further spectro-polarimetric observation ofvarious cases will be necessary. by guest on October 30, 2014http://pasj.oxfordjournals.org/Downloaded from Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0 5AcknowledgmentsThis work is partly supported by a Grant-in-Aid from the JapaneseMEXT (No. 23540264). Hinode is a Japanese mission developedand launched by ISAS/JAXA, with NAOJ as domestic partner andNASA and STFC (UK) as international partners. It is operated bythese agencies in cooperation with ESA and NSC (Norway).ReferencesBorrero, J. 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A. 2010b, A&A, 512, L1Shimizu, T., Ichimoto, K., & Suematsu, Y. 2012, ApJ, 747, L18Stix, M. 2002, The Sun, 2nd ed. (Berlin: Springer)Suematsu, Y., et al. 2008, Sol. Phys., 249, 197Tildesley, M. J., & Weiss, N. O. 2004, MNRAS, 350, 657Toriumi, S., & Yokoyama, T. 2013, A&A, 553, A55Tsuneta, S., et al. 2008, Sol. Phys., 249, 167Weiss, N. O., Thomas, J. H., Brummell, N. H., & Tobias, S. M.2004, ApJ, 600, 1073 by guest on October 30, 2014http://pasj.oxfordjournals.org/Downloaded from 

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