Spectropolarimetric inversions of the He I 10830 A multiplet in an Active Region filament

0.14 MB·English

by  C. Kuckein

ProceedingsofNSOWorkshop#25.ToappearinMem.S.A.It.2010
Spectropolarimetric inversions of the He I 10830
multiplet in an Active Region filament
C. Kuckein1,2,R. Centeno3,andV. Mart´ınezPillet1,2
0
1
0 1 Instituto de Astrof´ısica de Canarias, C/ V´ıa La´ctea s/n, E-38205 La Laguna, Tenerife,
2 Spaine-mail:[email protected]
n 2 DepartamentodeAstrof´ısica,UniversidaddeLaLaguna,E-38206LaLaguna,Tenerife,
a Spain
J 3 HighAltitudeObservatory(NCAR),Boulder,CO80301,USA

4
1
Abstract. FullStokesspectropolarimetricdata(inthe10830Åregion)ofanactiveregion

] filament were obtained in July 2005 using the Tenerife Infrared Polarimeter instrument.
R
The polarization profiles in the filament show Zeeman-like signatures. Milne-Eddington
S inversionswereperformedtoinferthechromosphericmagneticfield,inclination,azimuth,
. velocity andDoppler widthfromtheHeI 10830Åmultiplet.Fieldstrengthsof theorder
h
of600-800Gwerefoundinthefilament.Strongtransversefieldsatchromosphericlevels
p
weredetectednearthepolarityinversionline.Toourknowledge,thesearethehighestfield
-
o strengthsreliablymeasuredinthesestructures.Ourfindingssuggestthepossiblepresence
r ofafluxrope.
t
s
a Keywords.Sun:filaments–Sun:photosphere–Sun:chromosphere–Sun:magneticfields
[ –Techniques:polarimetric

1
v
4 1. Introduction parts(seeAulanier&De´moulin2003,andref-
3 erencestherein).
4 Filaments canbe seen as longanddark struc-
Partly due to the limited availability of
2 tures in Hα observations on the solar disk
1. and are formed of denser and cooler plasma spectropolarimetric data, there are very few
studies of magnetic field strengths in AR fil-
0 thantheirsurroundings.Wecandistinguishbe-
aments. Wiehr&Stellmacher (1991) inferred
0 tween two types of filaments: active region
the longitudinal magnetic field in an AR
1 (AR) and quiescent filaments. They are lo-
: catedalongpolarityinversionlines(PILs)and prominenceusing the Stokes V spectra of the
v CaII 8542 Å. Values between 75 and 180G
i their exact height is not easily inferred from
X werefound.Furthermore,byanalyzingthefull
on-disk data. From observations and three-
Stokes vector of the HeI 10830Å of a mul-
r dimensional models it has been showed that
a ticomponent flaring active region, Sassoetal.
AR filaments are stronger and lie lower in
(2007) have inferred magnetic field strengths
the atmosphere than their quiescent counter-
around380G.
This paper will focus on the full Stokes
Sendoffprintrequeststo:C.Kuckein inversionof HeI 10830Å multipletmeasured
2 Kuckeinetal.:InversionsoftheHeI10830ÅmultipletinanARfilament
Fig.1. Left:SiIStokesVmapnormalizedtothecontinuumintensity.Right:HeI“red”coreintensitymap
reconstructedfromtheslitscanpositions.ThearrowsindicateSolarWestandNorth.
in an AR filament near the solar disk center. signal-to-noise ratio a 3px binning in the
Section 2 begins by laying out the observa- spectral domain and a 6px binning in the
tions,section3describesthespectropolarimet- spatial direction were applied to the data. For
ric inversionsand results. The last section as- amoredetaileddescriptionoftheobservations
sessesthediscussion. seeKuckeinetal.(2009).
2. Observations 2.1.Description
The analyzed data were acquired on the 5th The HeI “red” component at ∼ 10830.3Å
of July, 2005, with the Tenerife Infrared slit-reconstructed image in the right panel
Polarimeter (TIP-II, Colladosetal. 2007) at of Figure 1 shows a dense concentration of
the German Vacuum Tower Telescope (VTT, plasma above the PIL. Highly twisted struc-
Tenerife, Spain). The active region NOAA turescanbeidentifiedin thelowerpartofthe
10781 was localized near the disk center at map. On the contrary, in the upper part these
N13-W29aroundµ = 0.92.The filamentwas dark structures seem to align with the PIL.
lying above a compact active region neutral Thesefindingscouldsuggestthepresenceofa
line. fluxrope.Onequestionthatneedstobeasked,
Various spatial scans, using in addi- however,is whether we are seeing the dipsor
tion the adaptive optics system KAOS thetopsofthatfluxrope.Anexhaustivestudy
(vonderLueheetal. 2003), were taken along ofthevectormagneticfieldmustbecarriedout
the AR with the slit parallel to the polarity toobtainthisinformation.Thisisfuturework
inversionline, coveringan areaof35′′ ×30′′. thatwillbepresentedinournextpaper.
Moreover, one time series was acquired with The Stokes profiles of HeI 10830Å ob-
the slit fixed at the PIL. The spectral range tained in this campaign (see Fig. 1 of
covered the chromospheric HeI 10830 Å Kuckeinetal.2009)indicateastrongpredom-
multiplet,thephotosphericSiI10827.1Åline inanceofZeeman-likesignaturesintheARfil-
and at least one telluric line with a spectral ament, characteristically with three-lobe pro-
sampling of ∼ 11.1mÅpx−1. The data reduc- files in Stokes Q and U. Observationsof qui-
tion, as well as the polarimetric calibration escent filaments using the same HeI multi-
(Collados 1999, 2003), were performed for plet have shown that the polarization is dom-
all the data-sets. In order to improve the inated by atomic level polarization and its
Kuckeinetal.:InversionsoftheHeI10830ÅmultipletinanARfilament 3
modification through the Hanle effect (see atmosphere configuration in order to obtain
TrujilloBuenoetal.2002). thebestfit.
Figure 2 presents the magnetic field (B),
LOS velocity (v), inclination (θ) and azimuth
3. Spectropolarimetricinversions
(φ) with respect to the line-of-sight and the
Three methods were used to invert the Doppler width obtained from the ME inver-
Stokesprofiles:magnetographanalysis,Milne- sions. A HeI, centeredat the red core,slit re-
Eddington (ME) inversions and principal constructed image is also shown to identify
component analysis (PCA), the latter tak- the filament. On average, the magnetic field
ing into account the physics of atomic strengthinferredfromtheMEinversionsinthe
level polarization and the Hanle effect (see filamentisaround600−800G.Moreover,high
Lo´pezAriste&Casini 2002, for a review of inclination values between 80◦ and 100◦ with
this code). The magnetographanalysis, based respect to the line-of-sight (LOS) are found
on the weak-field approximation, gives us a atthePILindicatingpredominantlytransverse
first approach of the vector magnetic field. fields.
Furthermore,theseresultswerecomparedwith The azimuth origin is referenced to the
more accurately ME and PCA-based inver- Earth’sNorth-Southdirection.Fromthelower
sions. Kuckeinetal. (2009) concluded that rightmapofFig.2itappearsthattheazimuth
the magnetic field strengths inferred with the is aligned with the dark structures of the fil-
magnetographanalysisareunder-estimatedby ament. Nevertheless, these results mustbe in-
around100−150G,whiletheinclinationsand terpretedwith cautionbecausethe 180degree
azimuthsareinverygoodagreement. ambiguityisnotyetresolved.Thisisanimpor-
The time series were inverted using the tantissueforfuturework.
ME and the PCA-based inversion codes. AscanbeseenfromFigure2,theDoppler
Kuckeinetal.(2009)presented,forbothinver- widthisaround210−220mÅinthefilament,
sioncodes,thebestfitsfortheStokesprofiles smaller than its surroundings and it seems to
oftwoselectedpointsofthedata-setandfound shapethesurfaceformofthefilament.
that both techniques yielded almost the same TheLOSvelocitieswerenotcalibratedfor
magnetic field strengths, inclinations and az- the orbital motions which contribute to the
imuths. The Stokesprofileshave Zeeman-like wavelength shifts. Instead, we calculated the
shapes and atomic polarization is almost ab- meanvelocityfromdifferentareasofthemap
sentatthefilament. outsidethefilamentandsubtracteditfromthe
This paper will focus on the results velocityofeverypixel.Consequently,thismap
achieved with the ME inversion code for the givesassomehintwhethertheplasmaismov-
entiremapoftheARfilament. ing upward (negative v) or downward (posi-
tivev).Ifweonlytakeintoaccounttheveloc-
ities which correspond to Doppler widths be-
3.1.Milne-Eddingtoninversions
low220mÅ,thefilamentisrisingwithrespect
to its surroundingsat mean speedsas largeas
The HeI Stokes profiles were inverted using
2.2kms−1.
MELANIE (Socas-Navarro 2001). This in-
version code computes the Zeeman-induced
Stokes spectra, in the incomplete Paschen-
4. Discussion
Back effect regime (Socas-Navarroetal.
2004), that emerge from a model atmosphere We have presented Milne-Eddington inver-
describedbytheMilne-Eddingtonapproxima- sions using the MELANIE code for an ac-
tion. A set of eleven parametersof the model tive region filament lying near the solar disk
atmospherearemodifiedbytheinversioncode center. The Stokes profiles in the filament are
to obtain the best fits to the observed Stokes completely dominated by the Zeeman effect.
profiles. We performed several inversions for It is somewhat surprising that no atomic po-
the same point changing the initial model larization was found.Casinietal. (2009) sug-
4 Kuckeinetal.:InversionsoftheHeI10830ÅmultipletinanARfilament
Fig.2.Fromtoptobottomandlefttoright:HeIredcoreintensity,magneticfieldstrengthinGaussunits,
LOSvelocity(seedefinitioninsection3.1),LOSinclinationθindegrees,DopplerwidthinmÅandLOS
azimuthφindegrees.
gest that the presence of an unresolved mag- (2007), who argue that the radiation field of
neticfieldwitharandomcomponentcouldex- the optical thick filament itself decreases the
plainthelackofatomicpolarizationsignatures anisotropyradiationmakingitnegligible.
intheStokesprofiles.Anotherexplanationwas
At the formation heightof the HeI strong
proposedbyTrujilloBueno&AsensioRamos transverse magnetic fields are found of 600−
Kuckeinetal.:InversionsoftheHeI10830ÅmultipletinanARfilament 5
800G. In reviewing the literature, apart from 184, Third Advances in Solar Physics
ourstudy,no observationalevidencesatchro- Euroconference: Magnetic Fields and
mospheric levels for such strong magnetic Oscillations,ed.B.Schmieder,A.Hofmann,
fields in AR filaments have been found yet. &J.Staude,3–22
Is this common to all active region fila- Collados, M., Lagg, A., D´ıaz Garc´ı A, J. J.,
ments? Further spectropolarimetric observa- et al. 2007, in Astronomical Society of
tions, with high resolution, are needed to an- the Pacific Conference Series, Vol. 368,
swerthisquestion.Recentphotosphericobser- ThePhysicsofChromosphericPlasmas,ed.
vations of an AR filament, like the ones pre- P. Heinzel, I. Dorotovicˇ, & R. J. Rutten,
sentedbyOkamotoetal.(2008),showedhori- 611–+
zontalmagneticfieldsof650Gonaverageand Collados, M. V. 2003, in Society of Photo-
evidence of an emerging helical flux rope. If Optical Instrumentation Engineers (SPIE)
our filament would lay at a very low height Conference Series, Vol. 4843, SPIE
thenwecouldexpectsuchstrongfields. ConferenceSeries,ed.S.Fineschi,55–65
The inclination and azimuth maps have Kuckein, C., Centeno, R., Mart´ınez Pillet, V.,
shown thatthe magneticfield vectoris highly etal.2009,A&A,501,1113
transversalandin somepartsalignedwiththe Lites, B. W. 2009, Space Science Reviews,
darkstructuresofthefilament.Thisalignment 144,197
in the lower part of the AR would indicate Lo´pezAriste,A.&Casini,R.2002,ApJ,575,
that the field is inclined ∼ 45◦ with respect 529
to the axis of the filament. Since the velocity Okamoto,T.J.,Tsuneta,S.,Lites,B.W.,etal.
map of Fig. 2 shows upward motions for the 2008,ApJ,673,L215
filament, the emergingflux ropescenario(see Sasso, C., Lagg, A., Solanki, S. K., Aznar
Lites 2009) might be a valid explanation for Cuadrado, R., & Collados, M. 2007,
this case. Furthermore, Lites (2009) proposes in Astronomical Society of the Pacific
thatthefluxropeliesatlowheightsandthere- Conference Series, Vol. 368, The Physics
fore should become measurable using photo- of Chromospheric Plasmas, ed. P. Heinzel,
spheric spectral lines. Since our spectral win- I.Dorotovicˇ,&R.J.Rutten,467–+
dowcomprisesthephotosphericSiI10827.1Å Socas-Navarro, H. 2001, in Astronomical
line, it is mandatory to invert this line in or- Society of the Pacific Conference Series,
dertoretrieveinformationofitsmagneticfield Vol. 236, Advanced Solar Polarimetry –
vectorinthephotosphere. Theory, Observation, and Instrumentation,
ed.M.Sigwarth,487–+
Acknowledgements. Based on observations made Socas-Navarro,H.,TrujilloBueno,J.,&Landi
withtheVTToperatedontheislandofTenerifeby Degl’Innocenti,E.2004,ApJ,612,1175
the KIS in the Spanish Observatorio del Teide of
TrujilloBueno,J.&AsensioRamos,A.2007,
the Instituto de Astrof´ısica de Canarias. Christoph
ApJ,655,642
Kuckein would like to thank the LOC of the
Trujillo Bueno, J., Landi Degl’Innocenti, E.,
NSO Workshop #25 on Chromospheric Structure
Collados,M.,Merenda,L.,&MansoSainz,
and Dynamics for thetravel support. TheNational
R.2002,Nature,415,403
Center for Atmospheric Research is sponsored by
theNationalScienceFoundation. von der Luehe, O., Soltau, D., Berkefeld,
T., & Schelenz, T. 2003, in Society of
Photo-Optical Instrumentation Engineers
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