Solar coronal plumes and the fast solar wind Bhola N. Dwivedi • Klaus Wilhelm 5 1 0 2 n a J 8 Abstract The spectral profiles of the coronal Neviii 1 Introduction ] R line at 77 nm have different shapes in quiet-Sun re- S gions and coronal holes (CHs). A single Gaussian fit In a recentreview (Wilhelm et al.2011), many aspects . of the line profile provides an adequate approximation of the solar coronalplume phenomenon have been pre- h p in quiet-Sun areas, whereas a strong shoulder on the sented. In most cases the authors of the review, mem- - long-wavelength side is a systematic feature in CHs. bers of a study team of the International Space Sci- o Although this has been noticed since 1999, no physi- ence Institute (ISSI), Bern, arrived at conclusive re- r t cal reason for the peculiar shape could be given. In sults. However, some open points remained, of which s a an attempt to identify the cause of this peculiarity, we we consider three. They have been formulated with [ address three problems that could not be conclusively the abbreviations IPR (inter-plume region), SW (solar 1 resolvedinareviewarticlebyastudyteamoftheInter- wind), and FIP (first-ionization potential): v national Space Science Institute (ISSI; Wilhelm et al. 0 2011): (1)Thephysicalprocessesoperatingatthebase - Although models of plumes and their formation are 0 andinsideofplumesaswellastheirinteractionwiththe available, an exact description of the physical pro- 7 solarwind(SW).(2)Thepossiblecontributionofplume cesses operating at the base and inside of plumes 1 0 plasmatothefastSWstreams. (3)Thesignatureofthe as well as their interaction with the SW is still out- . first-ionization potential (FIP) effect between plumes standing. 6 0 and inter-plume regions (IPRs). Before the spectro- - Isthereanycontributionofplumeplasmatothefast 5 scopic peculiarities in IPRs and plumes in polar coro- SW streams at all? 1 nal holes (PCHs) can be further investigated with the - What produces the clear FIP effect signature be- : v instrument Solar Ultraviolet Measurements of Emitted tween plumes and IPRs? i Radiation(SUMER)aboardtheSolarandHeliospheric X In this paper, we propose–based on observational Observatory(SOHO),itismandatorytosummarizethe r data–tentative solutions in these problem areas. a results of the review to place the spectroscopic obser- Earlier review papers on plumes have been pub- vations into context. Finally, a plume model is pro- lished as well, and we refer the reader to them for a posed that satisfactorily explains the plasma flows up general introduction to this solar phenomenon (e.g., and down the plume field lines and leads to the shape van de Hulst 1950; Saito 1965; Newkirk and Harvey of the neon line in PCHs. 1968; DeForest et al. 1997). Nevertheless, it is neces- Keywords Sun: corona – solar wind – UV radiation sary to list some of the basic properties of plumes and IPRastheyareknownatpresentfromobservationsde- scribedbyWilhelm et al.(2011)andreferencestherein: BholaN.Dwivedi Polarplumes delineate magnetic fieldlines ofthe mini- DepartmentofPhysics,IndianInstituteofTechnology(Banaras mumcoronainPCHs andexpandsuper-radiallyinthe HinduUniversity),Varanasi-221005, India [email protected] low β-regime of the corona as can be seen from Fig. 1. KlausWilhelm Plumes observed in WL (white light) and VUV (vac- Max-Planck-Institut fu¨r Sonnensystemforschung (MPS), 37077 uum ultraviolet) result from plasma density enhance- G¨ottingen, Germany ments in CHs. The electron density ratio between [email protected] plumes and IPRs is between three and seven in the 2 Fig. 1 Thesolarcoronaduringthetotaleclipseon1August2008observedfromMongolia. Thecoronaatsolarminimum conditionshaswidePCHswithreducedradiation,openmagneticfieldlinesandmanyplumestructures. Atlowerlatitudes closed field-line regions dominate the corona and extend into coronal streamers (from Pasachoff et al. 2009, composite eclipse image by M. Druckmu¨ller, P. Aniol and V. Ruˇsin). An image in 19.5 nm of the solar disk taken from the Extreme ultravioletImagingTelescope(EIT)(Delaboudini`ere et al.1995)onSOHOatthetimeoftheeclipsehasbeeninsertedinto theshadow of the Moon (Wilhelm et al. 2011). low corona and decreases at greater heights. The elec- wherepistheplasmapressureandp isthemagnetic mag tron temperature in plumes is T ≤ 1 MK. In IPRs it pressurewith n the particle density, k the Boltzmann e B is higher by ≈0.2 MK with a tendency of even higher constant, T the plasma temperature, B the magnetic values at greater heights. A plume shows some evo- flux density, and µ the vacuum permeability. 0 lution during its lifetime. Footpoints of beam plumes The plasma of PCHs is optically thin for VUV lie near magnetic flux concentrations interacting with lines. Nevertheless, it was possible to identify two dif- small magnetic dipoles. The reconnectionactivity gen- ferent plasma regimes, plumes and IPR, by studying erates heat near the base of a plume and leads to density-andtemperature-sensitivelineratios(Wilhelm jets that probably provide some of the plume plasma. 2006). From EIT observations, Gabriel et al. (2003) The SW outflow velocity is higher in IPRs than in have found that plumes must occur with two differ- plumes. Plumes and IPRs have a distinctly different ent morphologies, beam plumes and curtain or net- abundance composition, in the sense that the ratio of work plumes, because the latter appear to be aligned low-FIP/high-FIP elements is much larger in plumes along network lanes. Network plumes, and probably than in IPRs. Rosettes in the chromospheric network beamplumes,arecomposedofindividualmicro-plumes could be of importance for the plume formation. (Gabriel et al. 2009). This aspect was further stud- The following definition has been used for β: ied by de Patoul et al. (2013) who found typical beam plumeswithalocalizedcross-sectionandthosewithan p nk T = B =β , (1) elongatedcross-sectionasexpectedfornetworkplumes. p (B2/2µ ) mag 0 3 Their tomography results show that intermediate con- 153.3nmlineattheintersectionsofchromosphericnet- figurations also exist. worklanes. However,theseareasareratherdarkinthe Fig. 1 clearly shows that the field lines of a PCH Neviii radiance maps and have a typical flow compo- open into interplanetary space. The observations of nent of Ne7+ ions of 10 kms−1 along the line of sight. the spacecraft Ulysses demonstrate that on these field The ion outflow speed can then be obtained with the lines the fast solar wind escapes from the Sun with magnetic field model of Banaszkiewicz et al. (1998) as asymptotic speeds of approximately 800 kms−1 (cf., 14 kms−1. No outflow is observed in BPs at the base e.g., Woch et al. 1997; McComas et al. 2000). of polar plumes (Wilhelm et al. 2000). Funnel-shaped magnetic flux tubes from the pho- tosphere to the corona are typical features in PCHs 2 The neon emission line near 77 nm (cf., e.g., Gabriel 1976; Tu et al. 2005; Ito et al. 2010). ThesefunnelsareseenassourceregionsofthefastSW, Thetransition2s2S −2p2P intheNe7+ ionleads but they can also contain coronal plumes. One funnel 1/2 3/2 to a prominent solar emission line in the Neviii spec- analysedbyTu et al.(2005)intheirFigs.1(F)and4at trum near 77 nm. The spectroscopic observations of x=50′′andy =175′′doesnotshowanyoutflowspeed. this line are of major importance in this study and Ithadearlierbeenidentifiedasaplume(Wilhelm et al. therefore some backgroundinformation might be help- 2000). Two plumes seen by Hassler et al. (1999) also ful. Thefirstwavelengthdeterminationλ =77.042nm appear to be stationary. 0 with a standard uncertainty of 0.003 nm was per- Outflow speeds observed in PCHs by many re- formed inthe laboratoryby Fawcett et al. (1961). The searchers have been compiled by Wilhelm et al. (2011) large Doppler width of the line emitted from high- andareincludedhereinFig.2togetherwiththeescape velocities at heliocentric distances, R: temperature plasmas limits the accuracy of such mea- surements. Solar observations, therefore, provide the 2G M best values of the rest wavelength in vacuum λ0 = VF(R)=r NR ⊙ , (2) (77.0428±0.0003) nm (Dammasch et al. 1999). The Neviii line is formed at an electron temperature of where G is the gravitational constant and M the N ⊙ 620000 K (cf., Wilhelm et al. 2002). The contribu- mass of the Sun. The speeds measured in IPRs and tion function has a long tail towards higher temper- PCHs (without distinction between IPRs and plumes, atures typical for lithium-like ions. The line is thus which have a small filling factor) increase with in- ideally suited for studies of the upper transition re- creasing heliocentric distance without too much scat- gion and its interface with the low corona. Mea- ter and attain escape velocities near R = 3 R , ⊙ surements of Doppler shifts of this line in quiet-Sun whereas the speeds published for plumes vary consid- (QS) regions initially provided inconsistent results for erably and nowhere reach the escape velocity. We ar- the average shift. The problems were, however, re- gue that there are probably two reasons for the high lated to the inaccurate knowledge of the vacuum rest variability: (1) Plumes evolve during their lifetime and wavelength (Doschek et al. 1976; Hassler et al. 1991; maydisplaydifferentcharacteristicsatdifferentstages. Brekke et al. 1997; Chae et al. 1997). Later studies (2) Jets with high outflow speeds are often observed showed an average blue shift of ≈ 1 km s−1 in QS re- nearthe footpointsofplumes(e.g.,Raouafi et al.2008; gions (Peter and Judge 1999; Dammasch et al. 1999), de Patoul et al. 2013; Raouafi & Stenborg 2014). andamorepronouncedaverageblueshiftof≈6kms−1 This is in line with a suggestion by van de Hulst in PCHs. (1950) that plumes are rather static with occasional plasma injections along the field lines. This has been confirmed by the findings of Sheeley et al. (1997) that 3 Observed outflow speeds in polar coronal the direction of time is easy to see with the Large An- holes gle Spectroscopic Coronagraph(LASCO) on SOHO by tracing lateral inhomogeneities in coronal streamers, but difficult to identify over PCHs. Before an attempt can be undertaken to answer the question: Are there plume signatures in the fast solar wind? the observed outflow speeds in plumes and IPR 4 First-Ionization Potential effects in the solar have to be considered as well as the elemental compo- atmosphere and the solar wind sition of the solar photosphere and the polar corona. Strong outflows were observed by Hassler et al. Inthe corona,theabundancesofelementswithrespect (1999) in a PCH above bright areas as seen in the Siii tothephotospherevaryandtheFIPeffectplaysadom- 4 Fig. 2 Flow speeds observed in polar coronal holes (PCHs). Plume and inter-plume-region (IPR) measurements are plottedseparately. TheobservationsintheOvilineemittedbyO5+ ionsrefertoaPCH.Alsoshownistheescapevelocity as function of theheliocentric distance (cf., Wilhelm et al. 2011). inant rˆole. Elements with a FIP value of I < 10 eV identified against the IPRs by their high electron den- X are defined as low-FIP elements and those with FIP sities obtained from the density-sensitive Siviii (144.6, of I > 10 eV as high-FIP elements, separated by the 144.0) nm ratio and the lower electron temperatures X photon energy hν =10 eV of the Hi Lyα line. evident in the line ratio of two ionization stages of The photosphere is generally assumed to represent silicon. Both the low-FIP elements magnesium and theelementalcompositionoftheouterlayersoftheSun. sodium are enriched relative to the high-FIP element This could be confirmed by Sheminova and Solanki neon in plumes with respect to IPRs. (1999) who showed that only a very minor part of the Widing and Feldman(2001)havefoundinactivere- element segregation observed in the outer solar atmo- gions (AR) that the confinement time of a plasma is sphere seems to take place in photospheric and sub- a decisive parameter for abundance variations. A FIP photosphericlayers. Widing and Feldman(1992)found bias of nearly ten was reached after ≈ 6 d. If these FIPeffectsinstrongplumes,whereasnosignificantFIP findings can be applied to plumes, we would expect effect was observed in an IPR of a CH (cf., Feldman confinement times of a day or so–not too different 1998;Landi2008). Accordingto Doschek et al.(1998), from plume and BP lifetimes of days (e.g., Wang 1998; the Si/Ne abundance ratio in IPRs in CHs is close to DeForest et al. 2001a; Wilhelm et al. 2011). the photospheric value at temperatures near 106 K. The different elemental compositions of plumes and The abundance ratio of magnesium (a low-FIP ele- IPRsthussuggestthatplumesincontrasttoIPRspro- ment) to neon (a high-FIP element) in plumes is en- vide some kind of containment for the solar plasma for hanced relative to IPRs by factors of 1.5 and 3.5 a period of days, in which the FIP effect can operate. (Wilhelm and Bodmer 1998; Young et al. 1999). In Fig.25ofWilhelm et al.(2011), plume andIPRobser- vations arecompiledto characterizethe changesinele- mental abundances over a PCH. Plumes can clearly be 5 5 Are there plume signatures in the fast solar wavelength wing if seen in PCHs on the solar disk1. wind? Attempts to explain this shoulder by Sii line blends seeninthefirstorderofdiffraction(whereastheNeviii If the IPRs are indeed the source regions of the fast line is recorded in the second order with the SUMER SW, no composition changes would be expected in the detectorA)werenotsuccessful(Dammasch et al.1999; high-speed streams in accordance with observations of Wilhelm et al. 2000). The conclusion was that two Geiss et al.(1995). Heber et al.(2013)alsoreachedthe spectral components with a Doppler separation of conclusion that the SW originating in regions of open 34 km s−1 were present nearly symmetrically with re- magneticfield,wouldprobablynotcontainmatterwith spect to the rest wavelength, but the nature of these any significant mass fractionation. components remained unclear. Thieme et al. (1990) identified with the help of The main purpose of this section and the next is to plasma and magnetic field data obtained by the two clarifythesituationandprovideaphysicalexplanation Helios solar probes 41 fast SW streams between 0.3 ua for the Neviii profile in PCHs. and 1 ua often with a strong anticorrelation between The observations re-analysed here have been pre- the variations in the gas pressure and the magnetic sentedbyHassler et al.(1999);Dammasch et al.(1999); pressurewerefoundwhilethetotalpressurewasnearly Wilhelm et al. (2000). We consider the Neviii line constant. Ulysses observations (Reisenfeld et al. 1999) profile in Fig. 5 of the latter paper and apply multi- ofthehigh-latitudeSWhaveshownthatontimescales Gaussian fits on them. This line is very weak in PCHs of less than one day, the polar SW is dominated by (Fig.7ofWilhelm et al.1998)and,inparticular,inre- pressurebalancestructures(PBSs). Fluctuationsofthe gionswithhighoutflowspeeds(Fig.3ofWilhelm et al. plasmaβ withinPBSsappeartobestronglycorrelated 2000), presumably IPRs. We, therefore, assume that with fluctuations in the helium abundance. The au- most of the radiation analysed in Fig. 3a stems from thorssuggestaninterpretationofthe highβ portionof bright plumes and not from IPRs. PBSs as the SW extensions of polar plumes. However, TheresultsareshownseparatelyinFig.3aandbfor the abundance of helium (a high-FIP element) should the PCH and QS regions. They confirm that the QS notbeenhancedinplumes,iftheneonobservationsare profile is of a near Gaussian shape. The PCH profile, taken into account. however,is builtup ofthreecomponents inthe 2ndor- Direct observation of plumes with SOHO instru- der spectrum: (1) A blue-shifted component (Doppler ments have been made up to 15 R , “where they fade shift: 19kms−1)witharelativecontributionof≈45% ⊙ intothebackgroundnoise”accordingtoDeForest et al. to the total line radiance; (2) one with a redshift of (1997). Very strong plumes could be followed to 15kms−1 andacontributionof35%;(3)acomponent 30 R , but beyond that distance there is no clear in- with a blueshift of ≈ 14 km s−1 and a 15 % contribu- ⊙ dication for the presence of plume plasma in the SW tion. All Doppler shifts refer, of course, to line-of-sight (see, e.g., Poletto et al. 1996; DeForest et al. 2001b; components. The outflow speeds along the magnetic Wilhelm et al. 2011). field lines are approximately a factor of 1.4 higher (see Microstreams–identifiedinUlyssesdata–havebeen Sect. 3). The weak 15 % peak will be attributed to the analysed by Neugebauer et al. (1995), who concluded outflow in the IPRs with about 14 km s−1. that these were not to be identified with plumes. The Although several first-order Sii lines blend the same result was reported by von Steiger et al. (1999), Neviiiline, they do notproducethe shoulder(as men- because no significant depletion of the Ne/Mg abun- tioned above), and can be disregarded here. The red- dance and charge-state deviation in these structures shiftedcomponent(theshadedareainFig.3a)isthere- could be detected. fore difficult to understand and is an important topic of this article. An explanation will be presented in the next section based on a specific plume model. 6 Spectroscopic peculiarities in polar coronal holes 1Allraw dataacquiredareinthepublicdomainandcanbeob- tainedeitherfromtheSOHOArchiveorfromtheSUMERImage In most of the studies performed with the SUMER Databaseatwww2.mps.mpg.de/ instrument on SOHO (Wilhelm et al. 1995), it was no- projects/soho/sumer/FILE/SumerEntryPage.html (accessed on ticed that the profiles of the Neviii line were of Gaus- 16Dec. 2014). sian shape in QS regions and in the corona above the limb, but exhibited a strong shoulder on the long- 6 Fig. 3 SpectralprofilescoveringtheNeviiilinerecordedwithdetectorAofSUMERinthesecondorderinaPCHonthe solar disk (a) and in a QS region (b). The profiles are normalized to one and havebeen approximated by three Gaussian fits shown in dotted lines for second-order contributions and as dashed-dotted line for a suspected first-order blend of 5 % at 2×77.027 nm. Therestwavelength oftheNeviiilineisindicated at λ0=77.0428 nm. Thetotalprofilesareconsistent with a mean blueshift of 6.2 kms−1 in PCHs and 0.8 kms−1 in QS areas (Dammasch et al. 1999). In panel (a) the 45 % peak is shifted to theblue by19 kms−1, the15 % peak by ≈14 kms−1, and the35 % peak by15 kms−1 tothe red side. 7 Proposed plume model is: what arethe conditionsfor a plume formationcom- pared to those for a normal funnel? The funnel activ- An outflow velocity of Vout ≈ 300 kms−1 of H0 is ity consists of small-scale reconnection events close to reached in IPRs at R ≈ 3R⊙ (Kohl et al. 1998). We the TR and in the low corona. This, in turn, creates will approximate Vk, the component parallel to the heatedplasma and wavesthat are obviouslycapable of magnetic field, by Vout, and take the transverse veloc- expanding the coronal plasma against gravity and ac- ity, V⊥, also into account in defining the total velocity celerating the fast SW to speeds of ≈800 kms−1. One answer could be that such an active funnel at some V = V2+V2 . (3) stage“burns out”. The alternative answerthat the ac- q k ⊥ tivity ina funnel has notyetreachedthe levelrequired The Ultraviolet Coronagraph Spectrometer (UVCS) to produce the SW is less likely in view of observed observations on SOHO of Hi Lyα line-width indicate BP/plume evolution sequences (cf., Wang 1998). thatV ofprotonsisoftheorderof200kms−1at3R , Letusnowconsiderhowsuchashutdowncouldhap- ⊥ ⊙ ifcharge-exchangeprocessesequalizethe hydrogenand pen. A narrow funnel interacting with advected small proton speeds. This gives V ≈ 360 kms−1 and, con- loops will–in addition to generating particle and wave sequently, most of the material below ≈ 3R is still energy–grow through reconnection. However, not all ⊙ gravitationally bound to the Sun (cf., also Fig. 2) as ofthe advectedloopswillhavetherightorientationfor long as no post-acceleration is in operation. Such an a successful interaction. These loops will accumulate acceleration depends on waves generated by reconnec- around the funnel and might eventually shield it from tion processes at or near the footpoints of the funnel loopscapableofcreatingreconnectionevents. Thiscon- (cf., e.g., Ofman 2006). figuration, if visualized in three dimensions, resembles Given the fact that plumes exist on open magnetic withthatofarosette,acharacteristicmagneticfeature field structures, their geometries are not too different in the chromospheric network. from the magnetic funnels described, e.g. by Tu et al. The scenario described will not lead to an abrupt (2005), for CH regions in general. The first question shutdownof an active funnel, but to a slow diminution 7 ofthereconnectionactivity,presumablywiththeeffect Sun at the upper end of the flux tube. The continu- that plasma is injected into the funnel without enough ation of the plume flux tube will thus be more or less post-accelerationto leave the gravitationalpotential of void of plasma (cf., Wilhelm et al. 2011). At the sun- theSun. Thesituationisnowcomparabletoregionson ward side only particles within the loss cone will be the Sun with closedmagnetic field regions–the plasma lost, the others will be mirrored. van de Hulst (1950) density will increase and the FIP differentiation would demonstrated that the Lorentz force will not influence commence. A coronalplume is formed, and at its base the hydrostaticequilibrium, but in our configurationit a BP might be seen during this phase. willconfinetheplumeplasmaduringtheradiativecool- At a later stage, the energy input by reconnection ing phase. During this phase, we would expect plasma will decrease even more. One could speculate that this flowsupanddowntheplumefieldlinesandsuggestthat isrelatedtothe factthatthecross-sectionofagrowing the blue- and red-shifted strong components in Fig. 3a funnel base will increase faster than its circumference. correspondtotheseflows,ifseenalongthelineofsight. The BP will fade out, but the plume will not immedi- ately collapse under the gravitational pull of the Sun as one might think, even if the thermal energy could 8 Conclusion be dumped at the base of the plume, which is now assumed to be cool. To show this, we will treat the The different elemental compositions of plumes and plasmaoftheplumeinasingle-particleapproximation, IPRs strongly suggest that plumes in contrast to IPRs justified by the low density of n ≤ 1×108 cm−3 (cf., provide some kind of containment for the solar plasma e Lie-Svendsen et al. 2002). In a low β regime of a mag- foraperiodofdays,inwhichtheFIPeffectcanoperate. netized plasma, the protons will have a magnetic mo- Thecontinuationoftheplumefluxtubemaybemore ment thatcanbe writteninthe non-relativisticcaseas or less void of plasma (cf., Wilhelm et al. 2011). At the sunward side, only particles within the loss cone [W −m U(s)] will be lost, the others will be mirrored. van de Hulst µ = p sin2α(s) , (4) p (1950)demonstratedthatthe Lorentzforcewillnotin- B(s) fluence the hydrostatic equilibrium. In our model, it where W is the total proton energy, m the proton p willconfinetheplumeplasmaduringtheradiativecool- mass, U(s) the gravitational potential with s a spatial ing phase. During this phase, we would expect that parameter along the field direction, B(s) the magnetic plasma flows up and down the plume field lines and field and the pitch angle, α, defined by thatthestrongblue-andred-shiftedcomponentsofthe V neon line correspond to these flows. k α=arccos . (5) V Themagneticmomentwillbeaconstantofthemotion, We thank the Max-Planck-Institut fu¨r Sonnen- the first adiabatic invariant, as long as Coulomb colli- systemforschung for administrative support and an sions and wave-particle interactions can be neglected. anonymous referee for constructive comments. The This concept was first formulated by Alfv´en, details of SUMER instrument and its operation are financed which are described, for instance, by Roederer (1970) by the Deutsches Zentrum fu¨r Luft- und Raum- for applications in the magnetosphere of the Earth. fahrt (DLR), the Centre National d’Etudes Spatiales If we consider a plume at one of the solar poles, the (CNES), the National Aeronautics and Space Admin- gravitationalpotential is istration (NASA), and the European Space Agency’s G M (ESA)PRODEXprogramme(Swisscontribution). The U(s)=U(R)=− N ⊙ . (6) R instrument is part of ESA’s and NASA’s SOHO mis- sion. 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