Plasma Parameters in a Post-Eruptive Arcade Observed with CORONAS-F/SPIRIT, Yohkoh/SXT ...

14 Pages·2006·0.96 MB·English

PASJ:Publ.Astron.Soc.Japan58,55–68,2006February25
(cid:1)c 2006.AstronomicalSocietyofJapan.
Plasma Parameters in a Post-Eruptive Arcade Observed with
CORONAS-F/SPIRIT, Yohkoh/SXT, SOHO/EIT, and in Microwaves
VictorV.GRECHNEV,ArkadiyM.URALOV,VasilyG.ZANDANOV,andGeorgeV.RUDENKO
InstituteofSolar-TerrestrialPhysics,Irkutsk664033,Russia
[email protected]
ValeryN.BOROVIKandIrinaY.GRIGORIEVA
MainAstronomicalObservatory,St.Petersburg(Pulkovo)196140,Russia
[email protected]
VladimirA.SLEMZIN,SergeyA.BOGACHEV,SergeiV.KUZIN, D
o
Igor’A.ZHITNIK,andAndreyA.PERTSOV w
n
P.N.LebedevPhysicalInstitute,Moscow117924,Russia lo
a
[email protected] d
e
d
KiyotoSHIBASAKI fro
NobeyamaRadioObservatory,Minamimaki,Minamisaku,Nagano384-1305 m
[email protected] http
and s
MoiseyA.LIVSHITS ://ac
a
InstituteofTerrestrialMagnetism,Ionosphere,andRadiowavePropagation(IZMIRAN),Troitsk,MoscowRegion142190,Russia d
e
[email protected] m
ic
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(Received2005March30;accepted2005September29) p
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Abstract m
/p
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The SPectroheliographIc X-Ray Imaging Telescope (SPIRIT) aboard the CORONAS-F spacecraft sometimes s
observesintheMgXII8.42A˚ line(Tmax∼9MK)large,brightfeaturesexistingformanyhourshighinthecorona. j/artic
Thisfactsuggeststhatplasmabetatheremightnotbesmall. Weidentifysuchafeatureobservedon2001October le
22ataheightof100Mmwithahotpartofapost-eruptivearcade. Usingmulti-spectraldataandradioastronomy -ab
s
methods, we estimate the plasma parameters inthe arcadeandrevealthe coronal magnetic configuration. Several tra
hoursaftertheeruption,thetemperatureinthearcadewas6–8MK,andtheplasmadensitywas(5–10)·109cm−3. ct/5
Weconfirmtheresultsbytheaccordofthequantitiesobtainedfromdifferentobservationsusingdifferentmethods 8/1
andbytheextrapolationofthephotosphericmagnetogramsintothecorona,fromwhichwehaveobtainedamagnetic /5
5
fieldstrengthofabout7Gatthatheight. Hence,indeedβ≥1inthepost-eruptivearcade. Thehigh-betapossibility /2
0
had previously been stated, e.g., by Ichimoto et al. (1994), but still not investigated. We address this issue in 25
8
termsofthe“standardflaremodel”elaboratedbyShibataandYokoyama. Duringthisevent,Yohkoh/SXTobserved 6
8
McKenzie–Hudson dark moving features. Those features are filled with cold material, if they are not empty, and b
y
mayaffectthearcade. gu
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Keywords:Sun: corona—Sun: magneticfields—Sun: radioradiation—Sun: UVradiation—Sun: X-rays, s
t o
gammarays n
1
5
A
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1. Introduction coronal plasma, which have not yet been comprehensively ril 2
investigated. 0
1
9
The CORONAS-F space solar observatory (Oraevsky, SPIRITsometimesobservesinthehigh-temperatureMgXII
Sobelman 2002; Oraevsky et al. 2003) was launched in 2001 8.42A˚ lineratherlargefeatures(withasizeofupto300Mm),
August. The CORONAS-F spacecraft operates in an orbit of which exist for many hours high in the corona (Zhitnik et al.
the Earth and contains several solar telescopes and spectro- 2003). Those features look like a “ball” or “spider” with
meterstorecordthermalandnon-thermalemissions. legs directeddownwards. Possible candidates to be identified
The SPectroheliographIc X-Ray Imaging Telescope withthosefeaturesarepost-eruptivearcades,well-knownsolar
(SPIRIT;Zhitniketal.2002)aboardtheCORONAS-Fcontains featuresobservedmanytimesbefore, e.g.,withYohkoh/SXT.
several extreme-ultraviolet (EUV) and soft X-ray telescopes WenoteinthisconnectionthedifferencebetweentheSPIRIT
and spectroheliographs, which provide full-disk solar images MgXII 8.42A˚ channel and the SXT (Tsuneta et al. 1991).
inseveralbands:175A˚ (FeIX–XI,Tmax∼1MK),284A˚,304A˚, ThetemperaturesensitivityrangeoftheSXTwasratherwide,
and 8.42A˚ (MgXII, Te ∼ 5–15MK, Tmax ≈ 9MK). SPIRIT roughly2.5–25MK.Therefore,thistelescopeshowedoverlap-
has been observing the Sun starting from 2001 August 15. ping different-temperature structures with no reference to a
Inadditiontothewell-knownEUVemissionlines,the8.42A˚ particular temperature. Using the filter ratio method (Hara
line images represent a new type of observations of hot etal.1992),itispossibletocomputetemperatureandcolumn
56 V.V.Grechnevetal. [Vol.58,
emissionmeasuremapsfromYohkoh/SXTimagesundersome abovenon-flaringactiveregions.
assumptions. By contrast, the SPIRIT MgXII 8.42A˚ data Ontheotherhand,observationsoflong-livingradiosources
supply monochromatic images, whose temperature sensitivity (some of them being off-limb at a height up to 170Mm)
range is narrower (5–15MK), and this range is centered on a in microwaves and millimeter waves have been reported in
highertemperatureofabout9MK. several papers (e.g., Urpo et al. 1989; Borovik et al. 1989).
The formation of post-eruptive arcades often proceeds These sources were related to post-flare manifestations, but,
energetically, in a form of a post-eruptive flare with predom- because no regular soft X-ray observations were available at
inantemissionofacceleratedelectrons,relatedtomoreorless that time, they could not be directly identified with post-
strong magnetic fields rather low in the corona (cf. Sˇvestka eruptivearcades. Also,itwasnotpossibletoseparateinthose
2001). Later on, plasmas confined in the arcade undergo studies the temperature and emission measure contributions.
thermalization, and thermal emission of plasmas is assumed Therefore, Urpo et al. (1989) considered the possibilities of
to predominate. Analyses of post-eruptive arcades observed opticallythinemissionfromhotplasmaswithT =(5–10)MK,
e
in various spectral domains have been addressed in several which resulted in Ne =(8–11)·109cm−3, and optically thick Do
pAalptyenrsts(eev.ge.t, Iaclh.i1m9o9t9o;eGtraelc.h1n9e9v4;etHaalr.ra2-0M04u)r.nioTnheerteala.re19t9w8o; seemeins,siaosnsufmroimngctohledfpolramsmeraspowsistihbiTleity≈, o1n0e4Kob.tAaisnscaβn∼be1eawsiitlhy wnloa
d
circumstances in this connection, which have not yet been B∼20G. ed
completelyunderstood. Borovik et al. (1989) studied such an off-limb feature and fro
m
First, as Borovik et al. (1989) stated, the time scale of an found the microwave polarization to be very high, corre- h
off-limbpost-flarecoronalsourceobservedinmicrowavesand sponding for thermal bremsstrahlung to the magnetic field ttp
s
millimeters, severalhours, stronglysuggestsexcessiveenergy strengthofabout300Gataheightof50–100Mm.Suchstrong ://a
supply during the post-flare stage. McTiernan et al. (1993) magnetic fields seem to be unlikely at that height (cf. Dulk, ca
d
reportedthecoolingtimeobservedforhotpost-eruptive(post- McLean,1978);however,noconvincingconfirmationordenial e
m
flare) loops to be too long for the conductive cooling (which wasfound. Also,nodirectmagneticfieldmeasurementsinthe ic
.o
is known to work effectively at high temperatures), and that coronahavebeenavailableuptotherecenttime. u
p
the cooling rate is consistent with the radiative way. They It is well established that hot plasma in post-flare loops is .c
o
concluded that either the thermal conductivity is suppressed, evaporated from the chromosphere. Therefore, the measured m
/p
or a long-term heating takes place. Harra-Murnion et al. high plasma beta corresponds mostly to the evaporated a
s
(1998) alsostatedthat the cooling times deducedfrom obser- plasmas. We know only a few papers where the reconnec- j/a
vations of two other events are longer than the theoreti- tion rate and parameters of evaporated plasmas have been rtic
le
cally expected ones, and the discrepancy increases with time. addressed in a self-consistent way by the numerical solution -a
b
GetmanandLivshits(2000)andLivshits,Badalyan,andBelov of MHD equations. Yokoyama and Shibata investigated a stra
(2002)modeledthebehaviorofhotplasmasingiantpost-flare simplified model of a two-ribbon flare and its outcomes in a c
loops, and concluded again that a long-term heating is neces- seriesofpapers(e.g.,Yokoyama,Shibata1998,2001;Shibata, t/58
/1
sary to provide their observed properties. In principle, the Yokoyama, 1999). They showed the evaporated plasma beta /5
5
observedproperties are explainableinterms ofthe “CSHKP” to be rather high, even close to unity under typical coronal /2
0
(“standard”) flare model (Carmichael 1964; Sturrock 1966; conditions. However, they intuitively restricted its possible 2
5
Hirayama 1974; Kopp, Pneuman 1976) with magnetic recon- maximum value by about unity. They applied the model to 86
8
nectionabovetheloops. However,theheatingrateinthepost- theimpulsivephaseandtheearlydecayofaflare. Ontheother b
y
flare reconnection site would remain in this case sufficiently hand,theobservationsmentionedaboveandourssuggestthat g
u
highandlong-lastingtomaintaintheobservedplasmatemper- thestandardflaremodelelaboratedbyShibataandYokoyama e
s
atureanddensity. quantitatively describes parameters of post-flare loops, even t o
n
Second,asIchimotoetal.(1994)reported, estimatesofthe manyhoursaftertheimpulsiveflareenergyrelease. 1
5
relation between the gas and magnetic pressure in an arcade So, both these issues, the long-lasting heating and high A
p
oinbsaenrveudneaxfpteerctethdelyXh9ighflarpelasomf a19b9e2taNoofveβm=berP2 /rPesulte=d pnalatisomnasibnettearmatsloafrgtheeh“esitgahntdsairnd”thflearceomroondae,l.mNaoytehtahvaetnexoptlaal-l ril 20
gas mag 1
2nkT/(B2/8π) (cid:4) 1. Altyntsev et al. (1999) in their multi- solarastronomersbelieveintheapplicabilityofthismodelfor 9
spectral study of this event involved microwave data and theinitial,impulsivestageofasolarflare.
concluded that, under the usual assumption of optically thin In this paper, we first identify the large-scale hot feature
microwave free-free emission from the arcade, the magnetic observedwiththeCORONAS-F/SPIRITintheMgXIIline.We
field strength inferred from the microwave polarization data are interested in plasma parameters in those long-living high-
can be sufficient to balance the plasma pressure. However, temperature features, because even simple estimations really
as Grechnev et al. (2004) showed, even a small non-thermal show that the plasma beta in them might not be small. This
gyrosynchrotron emission can be responsible for the total followsjustfromthefactthatthefeaturewasbright,anditwas
polarization observed in a post-eruptive arcade. Shibasaki observed at 100Mm in the emission line with a maximum at
(1999, 2001) proposed that high-beta conditions are possible 9MK. Thus,plasmabetashouldbecarefullyaddressed.
for unstable features in the solar corona, and Getman and Wecanestimateplasmaparametersinthatfeaturefromsoft
Livshits (2000) showed that this is also possible for rather X-ray Yohkoh/SXT data, because observations with SPIRIT
stable features. Gary (2000) showed that the transition from were limited on that day, and its spectra are not available
β < 1 to β > 1 in the corona occurs at some hundreds Mm for this observation. Using microwave observations with the
No.1] PlasmaParametersinaPost-EruptiveArcade 57
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Fig.1. Post-eruptivearcadeobservedon2001October22invariousemissions.EUV:SPIRITat304A˚ (a)and175A˚ (b);EITat195A˚ (c)and284A˚ (d). p
Microwaves: NoRHat17GHz(e)andSSRTat5.7GHz(f). SoftX-rays: Yohkoh/SXT(g)andSPIRITat8.42A˚ (h). Allimagesareshownhereafter .co
m
non-linearlytoemphasizethefeaturesunderconsideration. /p
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j/a
Nobeyama Radioheliograph (NoRH, Nakajima et al. 1994) at from microwave observations with NoRH, RATAN-600, and rtic
le
17GHz, the Siberian Solar Radio Telescope (SSRT, Smolkov SSRT,aswellasEUVSOHO/EITdata.Weconfirmtheresults -a
b
et al. 1986; Grechnev et al. 2003) at 5.7GHz, and multi- by the agreement of estimates obtained using different obser- stra
frequencyone-dimensionalobservationsfrom2.8to15.6GHz vations and methods, and then discuss their physical implica- c
with the solar facility at the RATAN-600 Radio Telescope tions. We show that the SPIRIT MgXII 8.42A˚ observations t/58
/1
(Bogod et al. 1993), we can reveal coronal magnetic fields providepointingatfeaturesthatareofspecialinterestforsolar /5
5
by means of radio astronomy methods. We can alsocompare plasmaphysics. Section2ofthispaperdescribesobservations /2
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them with extrapolations of the photospheric magnetic fields withSPIRITandvariousotherinstrumentsindifferentspectral 2
5
(Rudenko2001). domains. Section 3 is devoted to estimations and simulations 86
8
Our approach is as follows. In studying solar phenomena, made from the observational data. Section 4 discusses the b
y
comprehensive data sets in various emissions put different results,andtheconclusionsaresummarizedinsection5. g
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observational constraints on the physical conditions and e
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scenarios considered. Analyzing them, one loses freedom 2. Observations t o
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to speculate, but gains confidence that the estimates obtained 1
5
fromvariousobservationsusingdifferentmethodsarereliable, Alarge-scalehotfeatureofourinterestwasobservedabove A
p
the interpretation is robust, and thus everything is consistent theSWlimbwiththeCORONAS-F/SPIRITcomplexat10:13– ril 2
withdifferentobservationalfacts. 10:22 (times hereafter are UT) on 2001 October 22 (observa- 0
1
For the initial analysis of data from the CORONAS-F soft tions in the SPIRIT MgXII 8.42A˚ channel were limited on 9
X-rayandEUVtelescopes,whichoperatesince2001August, that day). In 8.42A˚ images it “looks like a ball” (figure 1h,
it is reasonable to choose a post-eruptive arcade observed figure 2b). The height of its brightest part is about 115Mm.
simultaneouslywiththesoftX-raytelescope(SXT)onYohkoh Images obtained with other instruments within the same field
(operatedby2001December14),whichhadarelativelyclose ofviewinsoftX-ray,EUV,andmicrowaveemissionsshowan
temperature sensitivity range, and whose characteristics have arcade.
been well studied. This reason has determined the choice of
2.1. Identification
a feature observed on 2001 October 22 [this observation and
the results of its analysis were briefly reported by Borovik Differentinstrumentsshowinfigure1picturescorrespond-
et al. (2005)]. The present paper contains the first detailed ing to different temperature ranges. The hot MgXII feature
analysis of such a kind of SPIRIT observations (first briefly is spatially close to the arcade observed in soft X-rays with
reportedbyZhitniketal.2003)usingadditionaldataacquired Yohkoh/SXT, or coincides with it. The arcade observed in
inseveralspectralranges.WepresentSPIRITimages,estimate coolercoronalEUVlinesadjoinstothehotMgXIIfeatureand
the plasmaparametersfrom softX-rayYohkoh/SXT dataand the arcade in Yohkoh/SXT images from below. The arcade
58 V.V.Grechnevetal. [Vol.58,
was observed in the 304A˚ SPIRIT channel (figure 1a), likely
duetohottercoronallines, whichcontributetothis passband.
Thedissimilarappearanceofthearcadeindifferentemissions
is due to: 1) different temperature ranges; 2) different instru-
mental characteristics, in particular, the spatial resolution;
3) different observation times. Taking into account all of
thesefacts,weconcludethatthehotfeatureobservedwiththe
CORONAS-F/SPIRIT in the MgXII 8.42A˚ channel on 2001
October22isreallyassociatedwithhotloopsinthetoppartof
anarcade.
Thefull-diskYohkoh/SXTmovieshowsthatthehotarcade
existedmanyhoursbeforeandaftertheCORONAS-F/SPIRIT
D
observations. Itprobablyformedduringthecourseoferuptive o
w
events that occurred on that day. SOHO/LASCO recorded n
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three CMEs on October 22 at position angles approximately a
d
corresponding to the position of the arcade, i.e., at 00:50 (the ed
positionangle of260◦), at03:26(258◦), andat09:26 (252◦). fro
m
Theejectionat00:50likelyoccurredintheactiveregion9658 h
near or behind the western limb, and was accompanied by a ttp
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subflare of SF/M1.0 importance. The full-disk Yohkoh/SXT ://a
movieshowstheappearanceofalargebrightlooponthesouth ca
d
sideofthehotfeatureobservedwiththeCORONAS-F/SPIRIT e
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at about 02:40, and at about 05:30 the arcade takes shape ic
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similar to that observed around 09UT. However, we note u
p
that there are no full-disk Yohkoh/SXT images in the inter- .c
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valsof01:00–02:40and02:50–05:30. Hence,wecannotjudge m
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unambiguously the particular CME with which the appear- a
s
ance of the arcade was associated. All of them correspond j/a
to its position angle, and Yohkoh/SXT images do not provide rtic
le
decisiveinformation. -a
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2.2. Relative Position of the Arcade Observed in Different tra
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Emissions t/5
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Figure 21 shows the relative position of the arcade as /1/5
5
observed with different instruments in various emissions. In /2
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both panels, the gray-scale background shows a SOHO/EIT 2
5
195A˚ image, and white contours show the arcade observed 86
8
with Yohkoh/SXT. Top panel (a) also shows the NoRH Fig.2. Overlay of various images of the arcade. Background: b
image observed at 17GHz as a light-gray contour (level of SOHO/EIT 195A˚. White contours: Yohkoh/SXT. Light-gray con- y g
10000K). Bottom panel (b) also shows dark-gray contours tour(a): NoRH,StokesI at17GHz(104K). Dark-graycontours(b): ue
of the CORONAS-F/SPIRIT image in the MgXII line. The SshPoIwRsITth8e.4N2oA˚R.HThbeeasmm.allTehlleipasxeeisnsthhoewlowarecrselecftfrcoomrnethreinsoplaanreldi(sak) st on
coalignment accuracy of the MgXII image is not perfect, but center. 15
uncertaintiesdonotallowonetoshiftitsunward.Thus,thehot A
p
iMngsXofIIt fXea-rtauyresiwsiethithYeorhskpoahti/aSllXyTc,loosrestoomtehwehaartcahdieghoebrs.erTvhede one-dimensionalStokesI andV scansatthehighestfrequency ril 20
1
arcadeobservedincoolercoronalEUVlinescloselyadjoinsto available,15.6GHz, areshowninfigure3alongwithSPIRIT 9
the hot MgXII feature and the arcade in Yohkoh/SXT images dataobservedat175A˚ and8.42A˚.
from below. Moreover, the different-temperature soft X-ray 2.3.1. ObservationswithRATAN-600
and EUV images even interlace with each other beneath the The RATAN-600 high-sensitivity one-dimensional scans
hot arcade. This again shows that temperature gradients are show a weak off-limb source that looks like a small shoulder
high, and the temperature increases with height in the arcade approximatelycospatialwiththearcade. Thisrelationpersists
region,whichwasnoticedmorethanoncebefore(cf.Ichimoto at many frequencies from 2.8 to 15.6GHz around 09UT
etal.1994;Harra-Murnionetal.1998). (figure 3, bottom). There were no observations at higher
frequenciesonthatday.
2.3. MicrowaveObservations
The RATAN-600 data have two major advantages of very
The NoRH shows the arcade at 17GHz (figure 1e), and high sensitivity due to the huge effective area of the antenna
the SSRT shows it at 5.7GHz (figure 1f). The RATAN-600 system, and multi-frequency observations in a wide range
from 1.0 to 17GHz. However, there are two disadvantages:
1 ColorversionofthisfigurewaspresentedbyBoroviketal.(2005).
the ambiguity of the one-dimensional data and instrumental
No.1] PlasmaParametersinaPost-EruptiveArcade 59
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Fig.4. MicrowavespectrumofthearcadeaccordingtoRATAN-600, m
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solid:off-limbsource,dotted:limbsource. ttp
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regions close to the western solar limb, and we referred to e
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those observations. This allowed us to reliably separate both ic
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thelimbandtheoff-limbsourcesinthehigh-frequencypartof u
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theoperatingrangebothinthetotalintensityandthepolariza- .c
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tion. m
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The microwave spectrum measured from the RATAN-600 a
s
data is shown in figure 4. The error in flux measurements, j/a
±0.3sfu, was determined by the accuracyof both the separa- rtic
le
tionofasourceandthecalibrationtechnique. Thetotalfluxof -a
b
theoff-limbsourcedatagrowswithawavelengthfrom1.3sfu stra
at 1.9cm to 4.5sfu at 3.2cm, whereas the spectrum of the c
limb source is practically flat, about 3sfu in this range. It is t/58
difficult to separate the off-limb and limb sources correctly /1/5
5
at longer waves due to insufficient spatial resolution of the /2
telescope. Thetroughat11.3GHz(λ=2.67cm),visibleatthe 02
5
bothcurves,isobviouslyalsoduetoinstrumentalproblems. 86
8
Thelimbsource(likelyduetoemissionoflowcoronalloops b
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abovethetrailingpartofAR9658)steadilyexistedbeforeand g
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after the eruptions for at least some hours. Hence, it was not e
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relatedtotheeruptiveactivity. Whilewearenotinterestedin t o
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Fig.3. Observations with SPIRIT and RATAN-600. Top: SPIRIT thelimbsourceitself,weonlyuseitasaconvenientreference 1
175A˚ image(half-tone)overlaidwithcontoursoftheimageinMgXII sourcewithitspurelythermal,flatmicrowavespectruminour 5 A
line. The images are rotated to fit the orientation observed by p
RATAN-600, and the dotted line shows the polar axis of the Sun. studyofthemicrowavespectrumoftheoff-limbsource. ril 2
Bottom: raw RATAN-600 scans at 15.6GHz, Stokes V (solid) and The off-limb source in RATAN scans is distinctly polar- 01
StokesI (graydotted). Thedashedlineindicatesinbothpanelsthe ized, which promises the possibility to reveal the large-scale 9
polarizedoff-limbsourceobservedwithRATAN-600.Thedash-dotted magnetic configuration in the corona. This encouraged us to
linesindicatethephotosphericlimbandthesolardiskcenter.
also try to reveal the polarization in two-dimensional NoRH
data [it is not detectable in standard snapshot-mode images
contributions that are maximal at the solar limb. The instru- (integrationtimeof1s)].
mental polarization due to specific antenna configuration as 2.3.2. ObservationswithNoRH
the by-limb enhancement is well visible in Stokes V scan To detect very weak polarization in NoRH images, we
shown in figure 3b. This instrumental contribution can be producedthemwithanon-standard,verylongintegrationtime
eliminatedusingobservationsonotherdays,whenneitherby- of 120s. Furthermore, we averaged 4 images in the interval
limb,noroff-limbsourceswerepresent. Inaddition,thequiet of 06:11–06:17 and 7 images in the interval of 06:15–06:27.
Sun’s envelope makes it difficult to extract the total inten- The fluctuation sensitivity in those images is enhanced up to
sity source in many cases. However, for the observations on at least ∼ 20K. Using those high-sensitivity NoRH images,
2001 October 22, this problem is not crucial, because on the we computed the degree of polarization, while suppressing
two following days, October 23 and 24, there were no active unreliable regions in the same way as described by Grechnev
60 V.V.Grechnevetal. [Vol.58,
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Fig.5. Contours: microwavepolarizationofthearcadeenvironment c
observed with NoRH in the interval 06:11–06:17 (dashed positive, Fig.6. Hot part of the arcade observed with Yohkoh/SXT ade
dottednegative,levelsof±[0.5,1,2,4]%). Thehalf-tonebackground (background)withoverlaidmicrowavepolarization. Closedcontours m
show the degree of polarization observed at 17GHz in the interval ic
showscoronalmagneticfields(line-of-sightcomponent)extrapolated .o
isnitythoebpsoetrevnetdiawlaitphpNrooxRimHa(ti[o5n,.1T0,h1e5s]otlhidoucosanntoduKrs).showthetotalinten- 0(−6:[155,–70.65:,2170,]%d).ashTehde thpionsistiovleid l(i[n0e.2r,e2p,r1e0se]%nts),thedoStttoedkesnVegastciavne up.c
computed from NoRH image. The thick broken line represents om
the Stokes V scan observed with RATAN-600 at 15.6GHz. The /p
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orientationischangedtofitRATANobservations. s
et al. (2003). In the computation, all pixels, corresponding j/a
to those in both Stokes I and V maps with values below the rtic
le
sensitivity level (we specified (cid:4)40K) or below the dynamic whichsuggestsopticallythinfree-freeemission,butthearcade -a
b
range(−25dBinthiscase)withrespecttothebrightestsource top in the SSRT image is about 1.5-times brighter than one stra
in the whole full-disk image, were zeroed everywhere in the would expect from the squared frequency ratio. There is no c
map. Inaddition,theresultwassubjectedtotwo-dimensional detectablepolarizationinSSRTdata, whosesensitivityseems t/58
medianfilteringtosuppressanypossiblelocaldefectslessthan tobeinsufficient. /1/5
5
thebeamsize. 2.4. Dark Features Moving Downward Observed in Soft /20
Theresultsareshowninfigures5and6.Thedegreeofpolar- 2
X-Rays 5
izationshowsarelativelyshort-termappearanceofapositively 86
8
polarized region on the south side of the arcade (figure 5), AmoviecomposedfromseveralYohkoh/SXTimagesshows b
y
whereasthedominantnegativepolarizationonthenorthsideof dark features moving onto the arcade from above. It is rather g
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thearcade(StokesV (cid:4)60K,StokesI (cid:4)1000Kforthepatch difficult to show this phenomenon in a simple static picture. e
s
of maximum polarization) persists during the whole imaging Therefore, to reveal it, we produced difference frames by t o
n
interval(figure6). Thepolarizationwithinthearcadeisclose subtraction of the first two frames (separately for each filter, 1
5
tozero. 08:18:09 for AlMg and 08:18:29 for Al12) from eachcurrent A
p
Figure 6 also shows a RATAN-600 Stokes V scan (the frame. Figure 7 shows four of those frames representing ril 2
centraltimeis08:58:07)alongwiththeone-dimensionalscan dark features moving downward, and the most pronounced 0
1
computed from the two-dimensional NoRH Stokes V map, features are labeled 1 and 2. Those features likely represent 9
properly oriented and “de-rotated” to the observation time at the same phenomenon as that addressed by McKenzie and
the RATAN-600. The RATAN-600 scan is processed so as Hudson (1999), McKenzie (2000), Gallagher et al. (2002),
to suppress instrumental effects. Its by-limb part containing Innes, McKenzie, and Wang (2003), Sheeley, Warren, and
alargeinstrumentalcontributionisnotplotted. Thepolarized Wang(2004),andAsaietal.(2004).
off-limbdiffusesourcecoincidesinbothscans,withthepolar-
izedon-diskcompactsource,alsobeingcospatial.Theposition 3. EstimationsandSimulations
angleof25.◦7correspondstoobservationsattheRATAN-600.
3.1. TemperatureandDensity
2.3.3. ObservationswithSSRT
Figure1fshowsanSSRTimageofthearcadeintotalinten- Using soft X-ray Yohkoh/SXT data, we have estimated
sity averaged in the interval of 06:19–06:59. The 5.7GHz the temperature and emission measure in the arcade under
imagesare rather similartothose observedwiththe NoRHat the assumption of thermal equilibrium using the filter ratio
17GHz. The brightness temperature of the by-limb source at method (Hara et al. 1992; we used the AlMg and Al12
5.7GHzis≈10-timeshigherthanat17GHz(T ≤30000K), filters) eight hours after the first eruption following known
B17
No.1] PlasmaParametersinaPost-EruptiveArcade 61
recommendations (e.g., Doschek 1999). We used standard
Yohkoh software programs SXT PREP, SXT INTERP, and
SXT TEEM. The temperature distribution over the arcade
region is shown in figure 8a. The line filling shows the
regions where the estimates are not reliable (we apply the
tighteruncertaintylevelof0.3insteadof0.7usedbydefault).
The temperature of the main part of the arcade, including its
brightest portion, is 6MK, and its upper part has a tempera-
ture of up to 8MK. The column emission measure within a
2.(cid:3)(cid:3)46×2.(cid:3)(cid:3)46regionis2.5·1045cm−3forthebrightestpart.For
theresolvedlooplegsgoingdown,itisabout5·1044cm−3with
a minimum value of 2.5·1044cm−3. To estimate the plasma
D
density,onehastoovercometheambiguityofthedepth. Todo o
w
it,oneusuallyassumeseitherthattheemittingvolumeisequal n
tothevisibleareaatapowerof3/2,orthatthedepthisequalto loa
d
thewidthoftheemittingarea.Thiswayleadsustoanestimate ed
of the number density within the brightest part of the arcade fro
of 5·109cm−3. However, the high brightness of the central m h
portion of the arcade can also be due to an optical (projec- ttp
s
tion) effect. It is possible that the brightness of the loops is ://a
uniformatdifferentheights,butwhentheyaredirectednearly ca
d
along the line of sight, the column emission measure of the e
m
loop tops increases due to the integration. Under such condi- ic
.o
tions,weshouldestimatethedensityusingthedescendingloop u
p
leg;thedepthoftheemittingvolumeinthiscaseisequaltothe .c
o
widthoftheleg. Suchanestimateresultsin9·109cm−3. Both m
/p
estimatesarereasonablyclose,butthesecondwayseemstobe a
s
morerealistic. j/a
Withtheestimatedwidthofthelooplegandtemperature,we rtic
le
estimate the temperature gradient across the descending loop -a
b
legs. From figures 1 and 2 (and still better from figures in stra
Boroviketal.2005)onecanconcludethatthehot6MKplasma c
in the leg visible in soft X-rays is interlaced with a cooler t/58
1–3MKplasmainthelegvisibleinEUVat195A˚. Fromthis /1/5
5
fact,wecanestimatethetemperaturegradientsacrossthelegs /2
of the arcade loops to be of order 1Km−1. The closeness of 02
5
thebrightarcadetopvisibleinsoftX-raystothe“spine”ofthe 86
8
EUVarcadealsoleadstohightemperaturegradientsinthetop b
y
partofthearcade. g
u
Finally,infigure8weshowthetemperaturedistributionover e
s
thearcaderegionaccordingtocalculationsfromYohkoh/SXT t o
n
data (top) along with the dark features moving downward 1
5
(bottom). Theupperpaneldisplaysthetemperaturewithblack A
p
corresponding to 5MK, white corresponding to 8MK; the ril 2
contour levels are (5.5,6,6.5,7,7.5)MK. The lower panel 0
1
9
showsthesamecontoursofthetemperatureoverlaidontopof
the background representing the dark features moving down-
ward. The background image is computed as the minimum
along each pixel over the difference images from 08:42 to
09:04. The dark moving features show up as three short dark
lanes. The arrows point out the direction of their motion.
Fromthefigurewecanseethatthetemperaturesoftheregions
where they enter the arcade seem to be slightly less than in
theirvicinity.
3.2. SimulationofRadioImagefromSoftX-RayData
Fig.7. SXTdifferenceframesshowdarkfeaturesmovingdownward Wehavecomputedtheopticallythinfree-freeradioemission
ontothearcade. Twoofthemarelabeled1and2. Thebrightnessis from soft X-ray data. For this purpose, we used two-
limitedfrombothsidestoemphasizethedarkfeatures. dimensionaldistributionsofthecolumnemissionmeasureand
62 V.V.Grechnevetal. [Vol.58,
over a frame. To extend the area of our simulation to fainter
regions, we make use of the well-known fact that single-filter
Yohkoh/SXT images roughly represent the distribution of the
emission measure. In this way, we have normalized the thin-
filterAl.1imageof08:56:01totheobtainedbrightnesstemper-
aturedistributionusingthebrightesttoppartofthearcade.The
contribution of uncertainties in the electron temperature must
not be high, because the microwave brightness temperature
weakly depends on the electron temperature (TBν ∝ T−1/2),
and the temperature range of the observed corona is rather
limited.
Wehavethenaddedthe quietSun’s background, whichwe
D
foundfromstatisticalprocessingofNoRHimages. Afterthat, o
w
weconvolvedthesimulatedimagewiththeNoRHbeam. The n
lo
result of this simulation, shown in figure 9b, is similar to the a
d
observedmicrowaveimageat17GHz(figure9a). Thedissim- ed
ilarity is due to 1) continuing activity within the 2hr (which fro
m
showsup,inparticular,inalittlebitlargerheightofthearcade h
topinthesimulatedimage),and2)restrictionsofthissimula- ttp
s
tionsdiscussedabove. Noteagainthatthebrighterregionsare ://a
morereliable. ca
d
Thebrightnesstemperaturesobtainedinthissimulationare e
m
close to the observed values of (15–25)·103K at 17GHz. ic
.o
This confirms that we really deal with the optically thin case u
p
(τ ≈ 0.0015–0.0025). This also confirms our estimates of .c
17 o
plasma parameters. Moreover, the high temperature within m
/p
the arcadeisconfirmedbythe SPIRIT observationsinMgXII as
8.42A˚ channel (the temperature sensitivity range 5–15MK). j/a
Note, however, that the arcade top in the SSRT image at rtic
le
5.7GHz is about 1.5 times brighter than it could be expected -a
b
for the pure case of the optically thin thermal bremsstrahlung stra
(cf. figures1e,f). c
TheobservedbrightnesstemperaturesintheStokesI image t/58
/1
within the polarized regions are higher by 500–2500K than /5
5
those inthe simulatedimage, while inadjacentnon-polarized /2
0
regions they both do not exceed 800K, which is likely to be 2
5
duetoinstrumentaleffectsthatarepresentintheNoRHimage. 86
8
Fig.8. Temperaturedistributionoverthearcaderegionaccordingto Thesefactssupporttherealityofthepolarizationshownbythe b
y
the calculations from Yohkoh/SXT data (top, line filling marks the NoRHdata. g
regions where the estimates are not reliable) along with the dark SimilarsimulationsofmicrowaveimagesfromYohkoh/SXT ue
s
features moving downward (bottom, three short dark “lanes”). The imagesdatawereperformedbyGrechnevetal.(2003, 2004), t o
contoursinbothpanelsshowthetemperaturefrom5.5to7.5MK.The n
arrowspointoutthedirectionoftheirmotion. but in the one-dimensional case and without an absolute 15
calibration. The simulation considered in this subsection A
p
providesanindependentverificationoftheplasmaparameters ril 2
electron temperature calculated from Yohkoh/SXT full-disk inferred from soft X-ray data, and can be used to detect 0
1
images. For the optically thin limit, τ (cid:6) 1, the brightness possible gyromagnetic emissions, which visualize coronal 9
temperature,TBν≈Tτ,withτ≈0.2Ne2T−3/2ν−2L(e.g.,from magnetic fields and the probable presence of non-thermal
Dulk1985). Thus,weobtainTBν ≈0.2·EMcolT−1/2ν−2ρ−2 electrons, and instrumental contributions in the observed
withEM beingthecolumnemissionmeasureandρthepixel microwaveimages.
col
sizeoftheimage.
3.3. CoronalMagneticConfiguration
Limitations of the dynamic range in measurements of the
columnemissionmeasureandtemperaturefromYohkoh/SXT Toclarifythesituationwiththemagneticfields,weconsider
data using the filter ratio method put severe restrictions on photospheric magnetograms. Because the active region of
opportunities of such a simulation due to uncertainties in the interestwaslocatednearthelimb,wehavetodealwithearlier
measurements of EM and T . The uncertainties become data. ConsideringSOHO/MDImagnetogramsforseveraldays
col e
quite large when the signal levels are low. An additional prior to the event along with Yohkoh/SXT and SOHO/EIT
restriction, as Yoshida and Tsuneta (1996) showed, is due to 284A˚ data, we come to the conclusion that the overall large-
the scattered light in the SXT, which limits the possibilities scalemagneticconfigurationaroundtheactiveregionpersisted
of correct measurements by 10% of the maximum brightness almostunchangedforalongtime. Itwasmainlybipolarwith
No.1] PlasmaParametersinaPost-EruptiveArcade 63
the degree of polarization map computed from NoRH data
averaged within the interval of 06:11–06:17. The overall
configurationcorrespondswelltothepolarizationobservedin
microwaves,withthepredominantleft-handedpolarizationand
ashort-termappearanceoftheright-handedpolarizationinthe
southwardregionofthearcade.Themagnitudeofthemagnetic
fieldextrapolatedtoaheightof100Mmisabout7G.Notethat
wecanhardlyexpectaperfectcoincidenceoftheextrapolated
valueswiththeobservations,because1)themagnetogramused
fortheextrapolationwasobservedthreedaysbeforetheobser-
vations; 2) the results of the potential extrapolationcan differ
fromthereality.
D
o
w
4. Discussion n
lo
a
d
SoftX-rayYohkoh/SXTandmicrowaveNoRHimagesshow ed
thattheformationofthearcadeon2001October22probably fro
m
started after the first eruption (00:50), and the arcade took h
theshapeobservedbytheCORONAS-F/SPIRIT(10:13)after ttp
s
the second eruption (03:26). Hence, the arcade existed for at ://a
least 7 hr keeping its configuration without any conspicuous ca
d
changes. The visible shape of the high-temperature arcade e
m
in the CORONAS-F/SPIRIT image is due to the instrumental ic
.o
characteristics of the telescope (in particular, the astigmatism u
p
of the optical system). An additional instrumental feature is .c
o
thatthe8.42A˚ lineisactuallyadoubletwithdifferentfocusing, m
/p
andweseethesuperpositionoftwoslightlydisplacedimages. a
s
Furthermore, thewide temperature sensitivityrange resultsin j/a
anadditionalsmearingthestructuresofdifferenttemperatures rtic
le
intheimages. -a
b
Taking into account these instrumental particularities and stra
comparing the relative positions of the images produced c
with SPIRIT at 8.42A˚ and Yohkoh/SXT in figures 1g,h and t/58
/1
figure 2b (note again that the coalignment accuracy is not /5
5
perfect) with figure 8, which shows the temperature distribu- /2
0
tionobtainedfromYohkoh/SXTdata,onecanseethat: 2
5
8
• both telescopes show the same arcade, although some 68
b
dissimilaritiesarepresent,whichcanbeduetoboththe y
g
different instrumental characteristics of the telescopes u
e
andthedifferentobservationaltimes; st o
Fig.9. NoRH image observed at 06:28–06:30UT (a) and a “radio • the lower-temperature by-limb loops look fainter in the n 1
image”simulatedusingEM andTe inferredfromYohkoh/SXTdata SPIRIT8.42A˚ imagethanintheYohkoh/SXTimage, 5 A
2hrlater(b).Onlysufficientlybrightfeaturesarereliable. • thehighest-temperaturepartofthearcadeinitstopwith p
T ≥7.5MK,whichisclosetothetemperaturesensitivity ril 2
e 0
the N-polarity at the eastern part andS-polarity at its western maximum of the MgXII 8.42A˚ line, roughly coincides 19
withthebrightestfeatureintheSPIRIT8.42A˚ image,
part,withaweakeroppositemagneticfieldinthesoutheastern
• theupperenvironmentofthearcadedisposedtothenorth
part of the large-scale activity complex. The direction of the
andwestthatisbrighterintheSPIRIT8.42A˚ imagethan
coronalloopsvisibleinhigh-temperatureemissionsabovethis
in the Yohkoh/SXT one suggests its enhancedtempera-
complex was predominantly a latitudinal one, so the config-
ture,whichcannotbemeasuredfromSXTimagesusing
uration observed on the limb corresponds to the previously
thefilterratiomethodduetoitsinsufficientbrightness.
observedconfiguration,withthemicrowavepolarizationsense
correspondingtothex-modeemission. Thus, the SPIRIT 8.42A˚ images show hot loops in the
diverse-temperature arcade structure. All these properties are
3.4. ExtrapolationofMagneticFieldsintoCorona
just what can be expected from the difference of the temper-
Weextrapolatedthemagneticfieldsintothecoronafromthe ature sensitivity ranges of the two instruments, and support
SOHO/MDIphotosphericmagnetogramof2001October19in bothourresultsandconclusionspreviouslymadeonthebasis
apotentialapproximationusingamethodbyRudenko(2001). of Yohkoh/SXT data analyses. In particular, all of them
The line-of-sight component is shown in figure 5 along with confirm the fact that the upper parts of arcades have higher
64 V.V.Grechnevetal. [Vol.58,
temperatures, which was employed many times to support fromthemicrowavepolarizationdataunderthethermalcondi-
magneticreconnectionhighinthecorona.Thisalsoshowsthat tions, nevertheless, the polarization represents the large-scale
the combination of data from the SXT and SPIRIT enhances magneticconfigurationhighinthecorona.
thereliabilityofinformationaboutcoronalstructures.
4.2. MagneticFieldsintheCorona
We now discuss further consequences, which follow from
an analysis of the multi-spectral observations that we have The arcade environment is negatively polarized in NoRH
presented. 17GHz images as well as in RATAN-600 scans at a close
frequency of 15.6GHz, thus delineating the magnetic config-
4.1. MicrowavePolarization
uration (x-mode, figure 5). The short-term appearance of
Because the microwave polarization computed from the the positive polarization in NoRH images from the southern
NoRH data is very faint, well below the instrumental limita- boundary of the arcade toward the south shows the presence
tions of the ordinary NoRH imaging mode, the instrumental in that region of an opposite large-scale magnetic field with
polarization clearlyshows up inthose V maps. Nevertheless, respecttothenorthwardregionofthearcade(N-polarity). The Do
w
we are confident concerning the reality of the polarization proximity of the opposite large-scale magnetic fields explains n
lo
associated with the arcade region, because 1) the bright- the absence of detectable polarization in the brightest part of a
d
ness temperatures are rather high in the polarization maps the arcade. However, the contour of the negative polarization ed
(>500K), well exceeding the fluctuation sensitivity level penetratingtheregionofthearcadeinfigure 5showsthatthe fro
m
of those images (20K), 2) its configuration is significantly arcade region is polarized in microwaves in the sense of the h
different from the instrumental polarization patterns, 3) the predominantmagneticfield(x-mode). Thelarge-scalecoronal ttp
s
polarizedregionkeepsitsshapeandpositioninseveralframes, configurationisinaccordwiththeobservationsontheprevious ://a
4)thepolarizationinNoRHimagesisinaccordwiththepolar- days(seefigure10),whichsuggestsitsrelativestability. ca
d
izationmeasuredwiththeRATAN-600(figure6),5)thepolar- As noted in the previous subsection, the magnetic field in e
m
izationstructureisinaccordwiththemagneticfieldconfigura- the corona is weak at 100Mm, about 7G, in accord with our ic
.o
tionextrapolatedfromaphotosphericmagnetogram(figure5), simulation and all above considerations. However, if we take u
p
6) the relation between the brightness temperatures in the account of the possibility of strong local electric currents and .c
o
microwaveStokesI imagesimulatedfromsoftX-raydataand corresponding magnetic fields in compact regions, this could m
/p
inthereallyobservedonealsoshowsenhancementsabovethe drastically increase our estimate. There are, however, some a
s
sensitivity level in the polarized regions. All of these facts circumstanceswhichruleoutsuchapossibilityinourcase. j/a
supporttherealityofthepolarizationshownbytheNoRHdata. First, the gyrosynchrotron emissivity is known to depend rtic
le
It seems from the first glance that the polarization of the stronglyonthe magneticfieldstrength(see, e.g., Dulk1985). -a
b
microwave emission is due to the optically thin free-free Thus, the presence of a strong magnetic field even in a stra
emission in the magnetic field. If this is really so, then the compactregionwoulddefinitelyresultinhigh-intensitygyro- c
d2ecgorseθe(oνfBp/oνl)ar(iDzautlikon19c8an5)b,eadnedtefrrmominethdisbyexthpereesxspiornestshioenlirnce=- ssytantciohnroartryoenffeemctisssrieolna,tewdhtiochthweamsangonteotibcsefirevledds., eSxepceocntda,bnleonin- t/58/1/5
of-sightmagneticfieldstrengthisB(cid:4)≈3·103rc,withrcbeing theimpulsivephaseofaflare,wouldresultinobservableshort- 5/2
0
thedegreeofpolarizationat17GHz. FromtheNoRHdatawe term manifestations, which are incompatible with the long- 2
5
obtain that the maximum degree of the negative polarization term steady conditions in the post-eruptive arcade. The latter 86
8
onthenorthsideofthearcadeis10to12%,whichwouldlead factisalsoincompatiblewiththelargeexcessofthemagnetic b
toincomprehensiblemagneticfieldsof∼300Gatsuchagreat fields of electric currents above the potential fields. Finally, y g
u
height.Thiswouldstronglycontradicttheclassicalestimateby a pinching of electric currents is hardly expected in the post- e
s
Dulk and McLean (1978), B ∼0.5[(R/R(cid:5))−1]−1.5 ≈10G, eruptivearcadeunderhigh-betaconditions(seefarther). Thus, t on
and disagree with the result of our extrapolation. We note in onehastorecognizethattheweakmagneticfieldofabout7G 1
5
this connection that recently Lin, Penn, and Tomczyk (2000) corresponds to its possible total value. Strong local electric A
p
and Lin, Kuhn, and Coulter (2004) reported direct measure- currentscanbeimportantduringtheimpulsivephaseinregions ril 2
mentsofmagneticfieldsinthecoronausingtheZeemansplit- ofstrongmagneticfields,wherethemajorflareenergyrelease 0
1
ting of the FeXIII 10747A˚ line. They found line-of-sight is observed, unlike the late post-eruptive situation far from 9
magnetic field strengths of 10G at 84Mm, 33G at 105Mm, thoseregions,whichwedealwith.
and 4G at 72Mm above three active regions. These results
4.3. ConditionsintheArcade
supportourassumptionthatthemagneticfieldstrengthof7G
ismuchmorerealisticthan300Gat100Mm. The cooling of a single loop in the arcade must first be
We must therefore recognize that there was a non-thermal dominated in a conductive way, with the characteristic time
contribution:aseeminglynegligibleadditionof40%-polarized scale being determined by the expression τ ∼ 3N kTL/q
c e c
gyrosynchrotron emission with a brightness temperature as with q ∼ CT5/2(T/L) [ergs−1cm−2] thermal flux per unit
c
low as 250K could be responsible for the total polarization square, C (cid:4) 10−6 [ergs−1cm−1K−7/2], and L the charac-
observed. Inourcase,themicrowavespectrumoftheoff-limb teristic length. This results in our case in τ ∼ 3hr. Then,
c
source measured from the RATAN-600 data grows with the when the temperature becomes low enough, the radiative
wavelength(seefigure4). Thus,thehigh-frequencypartofthe cooling becomes appreciable (cf. Aschwanden, Alexander
off-limbsourcespectrumrulesoutitsthermalorigin. Although 2001), whose time scale for an optically thin coronal plasma
we cannot quantitatively estimate the magnetic field strength with T >0.5MK is τ ∼3N kT/Q with Q=N2R(T), and
r e e