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EUV SUNSPOT PLUMES OBSERVED WITH SOHO P. Maltby, N. Brynildsen, P. Brekke, S. V. H. Haugan, O. Kjeldseth-Moe, Ø. Wikstøl Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, 0315 Oslo, Norway and 8 9 T. Rimmele 9 National Solar Observatory, Sacramento Peak, Sunspot, NM 88349, USA 1 n a J 5 ABSTRACT 1 1 Bright EUV sunspot plumes have been observed in five out of nine sunspot regions v withtheCoronalDiagnosticSpectrometer –CDSonSOHO.Intheotherfourregionsthe 4 brightest line emissions may appear inside the sunspot but are mainly concentrated in 4 1 small regions outside the sunspot areas. These results are in contrast to those obtained 1 during the Solar Maximum Mission, but are compatible with the Skylab mission results. 0 8 The present observations show that sunspot plumes are formed in the upper part of the 9 transition region, occur both in magnetic unipolar– and bipolar regions, and may extend / h from the umbra into the penumbra. p - o Subject headings: Sun: corona — sunspots — Sun: transition region — Sun: UV radia- r t s tion a : v i X r a 1 1. Introduction Table 1: Observed active regions Based on monochromatic EUV images obtained during the Skylab mission Foukal et al. (1974) intro- NOAA Date θ Mag. duced the notation “sunspot plumes”, defined as ar- (degrees) Class easabovesunspotumbraethatare“thebrightestfea- 7973 1996 June 26 16 A tures in an active region by an order of magnitude”. 7981 1996 August 2 16 B This led to the idea that sunspot plumes are regions 7986 1996 August 29 17 A withinlargemagneticloops,extendingtoaltitudesof 7999 1996 November 28 36 B severalthousandkilometersabovethephotosphere,in 8011 1997 January 16 13 B whichthe temperature is one to twoordersofmagni- 8073 1997 August 16 15 A tude lower than in the corona of the surrounding ac- 8076 1997 August 30 20 B tiveregion(Noyesetal. 1985). Incontrast,Brueckner 8083 1997 September 9 35 B and Bartoe (1974) observed enhanced line emission 8085 1997 September 15 44 B both over plages and sunspots and Cheng, Doschek, &Feldman(1976)foundthatemissionlinesformedat temperaturesbelow2.4×105Kshowedthe sameline Theangleθ =heliocentricangle. Magneticfieldclassification: emission over the sunspot as over the quiet network. A=unipolar,B=bipolar. Evidence against the importance of sunspot plumes came from the UVSP instrument on the Solar Max- imum Mission - SMM. Kingston et al. (1982) found contains information from 60 adjacent locations for that the emission measures over the penumbrae were ten emission lines. Table 2 gives the line list and the higherthanoverthe umbrae. TheSMM observations corresponding ionization temperatures. For each ob- ledGurman(1993)toconcludethattheumbraltran- serving sequence several rasters, up to thirteen, were sitionregionwasgenerallyindistinguishablefromthe obtained. quiet transition region. There appears to be at least two possible expla- Table 2: Selected spectral lines nations for the discrepancy between the observations obtained with the S055 Skylab and the UVSP SMM ID λ (˚A) log T (K) instruments. One possibility is that sunspot plumes He I 522.2 4.3 are formed in the upper part of the transition re- He I 584.3 4.3 gionandthereforeareeasierobservedwithS055than O III 599.5 5.0 with UVSP. Another possibility is that one type of O IV 554.5 5.2 sunspots have plumes whereas others do not. The O V 629.7 5.4 presentobservationsshowthat sunspotplumes occur Ne VI 562.8 5.6 both in magnetic unipolar and bipolar active regions Mg VIII 315.0 5.9 andaremostapparentinemissionlinesformedinthe Mg IX 368.0 6.0 upper part of the transition region. Fe XIV 334.1 6.3 Fe XVI 360.7 6.4 2. Observations and Data Reduction Observationsof nine sunspot regions (see Table 1) The data acquisition and detector characteristics were obtained with the Normal Incidence Spectrom- that are relevant for this study were described by eter (NIS) of the Coronal Diagnostic Spectrometer – Harrisonetal. (1995). Briefly,the CDSdataarecor- CDS (Harrison et al. 1995), as part of a joint ob- rected for geometrical distortions, the CCD readout serving program on the Solar and Heliospheric Ob- bias is removed, the non-wavelength-dependent cali- servatory – SOHO. A large fraction of the observing bration parameters peculiar to the detector are ap- time was used to raster an area of 120′′× 120′′, mov- plied, including the exposure time, the amplification ′′ ingthenarrow2.0 spectrometerslitperpendicularto of the microchannel plate, and a flat-field correction. ′′ the slit direction in steps of 2.0 . The exposure time The final step in calibrating is to convert the photon was20s,eachrasterwasrecordedduring25minand eventsintoabsoluteunits. Theline parameters,peak 2 intensity, wavelength shift and line width are deter- ure3inFoukaletal. (1974). Figure1showsthatthe mined by a least squares fit to the observations, see brightest emission above the umbra in NOAA 7986 Brynildsen et al. (1997). The data material consists is observed in a compatible temperature range since of line profiles that are well represented by a single the brightestemissionis observedin O IV554˚A,O V Gaussian shape. Small regions with complicated line 629 ˚A, and Ne VI 562 ˚A. Next consider the spatial profilesandregionswithrapidtimeevolutionareout- extentofthe brightestemission. AccordingtoFoukal side the scope of this paper. et al. (1974) the plume’s “minimum half-width is The CDS images were coaligned with white light definitely smaller than the diameter of the umbra”. imagesusingmagnetogramsobservedwiththeMichel- However, their Figure 4 shows that the spatial ex- son Doppler Imager (Scherrer et al. 1995) from tent changes from one emission line to another. In SOHO.Todeterminethelocationofregionswithpeak Ne VII 465 ˚A, where the plume is most apparent, the ′′ line intensity I > I , where I is a preselected value FWHM of the plume exceeds 30 which is consid- p p of I, we introduce the notations: erably larger than the umbral diameter (≈ 10′′). We findthatthesizeofthebrightfeaturesinthesunspots F (I >I ) = fraction of umbra covered with I >I , U p p ofNOAA7986andMcMathregion12543arecompat- F (I >I )=fractionofsunspotcoveredwithI >I , S p p ible. Based on these comparisons we conclude that f (I > I ) = fraction of area with I > I located U p p the sunspot in NOAA 7986 (Fig. 1) shows a sunspot above the umbra, plume. f (I > I ) = fraction of area with I > I located S p p Asunspot plume is locatedabovethe sunspotand above the sunspot. is the brightest feature within the active region. It The brightest features, such as the sunspot plumes, is important to note that both the extent and the will be located by I > Ip = 5I, where I is the aver- locationofenhancedlineemissiondependonthepre- agepeaklineintensityvaluewithintherasteredarea, selected intensity level I . We show in Figure 3 the p 120′′× 120′′. fraction, f (I > I ), of the area with peak line in- S p tensity I > I which is located above the sunspot p 3. Results as function of the intensity ratio, I /I. For several p active regions we find that as I /I increases towards p In Figures 1 and 2 (Pl.00 and 00) the brightest thebrightestfeaturesanincreasingfractionoftheline features with peak line intensity I >5I are encircled emission in O IV 554 ˚A, O V 629 ˚A, and Ne VI 562 ˚A by yellow contours, whereas medium bright features is located inside the sunspot. This means that in a with I > 2.5I are encircled by green contours. Be- searchfor sunspotplumes one cannotsimply look for low we give our reasons for identifying the brightest enhanced line emission, but must select the brightest feature in NOAA 7986, observed on 29 August 1996 regions by choosing a criterion, such as, I >I =5I. p (see Fig. 1), with a sunspot plume. In contrast, Fig- If it is required that the sunspot plume must be ure 2 shows that nearly all the brightest features in positioned directly above the umbra, only two of the NOAA 7999, observed on 28 November 1996, are lo- nine sunspots contain a sunspot plume. Taking into cated outside the sunspot. The size and the location account the measured size of sunspot plumes a more of the brightest emission features with peak line in- reasonable requirement is that a plume is located in tensityI >5I aregiveninTable3fortheentiresetof an area above the sunspot that includes the umbra observations. We limit the list to the three emission or parts thereof and may extend into the penum- lines where the sunspot plumes are most apparent. bra. Withthisrequirementthefollowingfivesunspots Anexcellentillustrationofasunspotplumeispre- show plumes in O V 629 ˚A and Ne VI 562 ˚A: NOAA sented in Figure 2 of Foukal et al. (1974), based on 7973, 7986, 8011, 8073, and 8085. In these sunspots observations of McMath region 12543. To compare the plumes are centered in the penumbra, the umbra the brightestemissionfeatureinNOAA7986(Fig.1) (see Fig. 1), the umbra, the rim of the umbra and with that of a sunspot plume let us first consider the the penumbra, respectively. It is possible that also variation with the line formation temperature. The NOAA8076shouldberegardedascontainingsunspot line emissionin the sunspotplume inMcMathregion plumessincetwoofthebrightestlineemissionregions 12543exceedsthatabovetheadjacentplageregionin in O V 629 ˚A and Ne VI 562 ˚A are located within the O IV 554 ˚A, O VI 1032 ˚A, and Ne VII 465 ˚A, see Fig- largest,leadingsunspotandthethirdbrightlineemis- 3 sion region covers most of a following sunspot, but in magnetic unipolar and bipolar regions, and may extends outside the sunspot. The other three active extend outside the umbra and into the penumbra. regions show bright emission both inside and outside the sunspot in Ne VI 562 ˚A. Wewouldliketothankallthemembersofthelarge international CDS team for their extreme dedication Table 3: Size and location of the brightest emission indevelopingandoperatingthisexcellentinstrument, regions in O IV 554 ˚A, O V 629 ˚A and Ne VI 562 ˚A theMichelsonDopplerImagerteamforpermissionto use their data for coalignment purposes and the Re- searchCouncilof Norwayfor financialsupport. Data NOAA ID size fU fS FU FS (arcsec)2 I>5I from Mees Solar Observatory, University of Hawaii, 7973 OIV 7 0.00 0.00 0.00 0.00 are produced with the support of NASA grant NAG OV 31 0.11 0.67 0.14 0.04 5-4941 and NASA contract NAS8-40801. SOHO is NeVI 99 0.03 0.48 0.14 0.08 7981 OIV 384 0.03 0.42 0.03 0.10 a mission of international cooperation between ESA OV 493 0.03 0.63 0.04 0.19 and NASA. NeVI 248 0.00 0.59 0.00 0.09 7986 OIV 105 0.26 1.00 0.80 0.21 OV 218 0.16 0.91 1.00 0.39 REFERENCES NeVI 265 0.13 0.86 1.00 0.46 7999 OIV 156 0.00 0.00 0.00 0.00 Brueckner, G. E. and Bartoe, J.-D. F. 1974, Solar OV 129 0.00 0.00 0.00 0.00 NeVI 122 0.25 0.25 0.05 0.02 Phys., 38, 133 8011 OIV 0 0.00 0.00 0.00 0.00 OV 14 0.75 0.75 0.04 0.04 Brynildsen, N. et al. 1997, Solar Phys., in press NeVI 24 0.29 0.29 0.03 0.03 8073 OIV 3 0.00 1.00 0.00 0.01 OV 37 0.36 0.91 0.17 0.07 Cheng, C. C., Doschek, G, and Feldman, U. 1976, NeVI 78 0.22 0.61 0.21 0.10 ApJ, 210, 836 8076 OIV 54 0.06 0.12 0.03 0.01 OV 221 0.20 0.62 0.45 0.14 NeVI 238 0.16 0.29 0.38 0.07 Foukal,P.V.,Huber,M.C.E.,Noyes,R.W.,Reeves, 8083 OIV 377 0.06 0.26 0.08 0.08 E. M., Schmahl, E. J., Timothy, J. G., Vernazza, OV 262 0.08 0.26 0.07 0.06 NeVI 102 0.07 0.40 0.02 0.03 J. E., and Withbroe, G. L. 1974, ApJ193, L143 8085 OIV 102 0.10 1.00 0.07 0.09 OV 282 0.23 0.93 0.46 0.22 Gurman, J. B. 1993, ApJ, 412, 865 NeVI 333 0.12 0.76 0.29 0.21 Harrison, R. A. et al. 1995, Solar Phys., 162, 233 In contrast to the SMM observations of Gurman Kingston,A.E.,Doyle,J.G.,Dufton,P.L.,andGur- (1993),thepresentobservationsconfirmtheexistence man, J. B. 1982, Solar Phys., 81, 47 of sunspot plumes. The SMM observations were ob- tainedclosertoasunspotmaximumthanthe present Noyes, R. W., Raymond, J. C., Doyle, J. G., and observations. We cannot exclude a selection effect Kingston, A. E. 1985,ApJ, 297, 805 since the sunspots listed by Gurman (1993) are all Scherrer, P. H. et al. 1995, Solar Phys., 162, 129 magneticbipolar,whereasunipolarsunspotswereob- served both by Foukal et al. (1974) and by us (see Table1). However,thiscannotbetheentireexplana- tion since we also observe sunspot plumes in bipolar sunspots. It appearsthatthe CDS instrumentis well suited to measure emission lines formed in the upper part of the transition region, where the sunspot plumes are most apparent. The higher sensitivity of theCDSinstrumentthantheUVSPSMMinstrument tolineemissioninthisregionofthe solaratmosphere may be one of the reasons for the difference in results between the two instruments. The present ob- servationssuggestthat sunspotplumes are formedin This 2-column preprint was prepared with the AAS LATEX the upper part of the transition region, occur both macrosv4.0. 4 Fig. 1.— Images of peak line intensities in NOAA 7986 observed on August 29, 1996. Regions with enhanced intensityareshownasdarkorangeregions. Areaswithpeaklineintensity,I,largerthan2.5and5timestheaverage intensity, I, are encircled by green and yellow contours. The images are ordered after increasing line formation temperature,startingintheupperlefthandcorner. Thecontoursoftheumbraandpenumbraarefromwhitelight observations at the Mees Solar Observatory, Haleakala, Hawaii. The scales in arc sec are in a reference system where the origin coincides with the centre of the solar disk. Fig. 2.— Images of peak line intensities in EUV emission lines observed on 28 November 1996. The same color code etc. as in Figure 1. White light image of NOAA 7999 observed the previous day with the Vacuum Tower Telescope at the National Solar Observatory, U.S.A. is shown (top). Only minor changes in the sunspot contour were observed from one day to the next. Fig. 3.— The fraction,f (I >I ), of the areawith peak line intensityI >I that is locatedabovethe sunspot as S p p a function of the intensity ratio I /I. Note the tendency for some sunspots to show the brightest emission above p sunspots in O IV 554 ˚A, O V 629 ˚A, and Ne VI 562 ˚A. 5 N _ 2.5 I _ 5.0 I W OO IIIIII 559999 ÅÅ OO IIVV 555544 ÅÅ −−226600 −−226600 cc ee ss c c −−331100 −−331100 rr aa −−336600 −−336600 −−114400 −−9900 −−4400 −−114400 −−9900 −−4400 OO VV 662299 ÅÅ NNee VVII 556622 ÅÅ −−226600 −−226600 cc ee ss c c −−331100 −−331100 rr aa −−336600 −−336600 −−114400 −−9900 −−4400 −−114400 −−9900 −−4400 MMgg IIXX 336688 ÅÅ FFee XXVVII 336600 ÅÅ −−226600 −−226600 cc ee ss c c −−331100 −−331100 rr aa −−336600 −−336600 −−114400 −−9900 −−4400 −−114400 −−9900 −−4400 aarrcc sseecc aarrcc sseecc N W OO IIIIII 559999 ÅÅ OO IIVV 555544 ÅÅ OO VV 662299 ÅÅ −−7700 −−7700 −−7700 cc ee ss −−112200 −−112200 −−112200 cc rr aa −−117700 −−117700 −−117700 553300 558800 663300 553300 558800 663300 553300 558800 663300 NNee VVII 556622 ÅÅ MMgg IIXX 336688 ÅÅ FFee XXVVII 336600 ÅÅ −−7700 −−7700 −−7700 cc ee ss −−112200 −−112200 −−112200 cc rr aa −−117700 −−117700 −−117700 553300 558800 663300 553300 558800 663300 553300 558800 663300 aarrcc sseecc aarrcc sseecc aarrcc sseecc NOAA 7986 O III O IV O V Ne VI Mg IX Fe XVI NOAA 8085 NOAA 8076 NOAA 7973 NOAA 8083 ) p I > I ( S f NOAA 8073 NOAA 7981 NOAA 8011 NOAA 7999 1.0 2.0 _ 4.0 1.0 2.0 _ 4.0 I /I I /I p p