SUMER spectral observations of postflare supra-arcade inflows

18 Pages·2003·0.71 MB·English

SUMERspectralobservationsofpostflaresupra-arcadeinflows
D.E.Innes ([email protected])
Max-Planck-Institutfu¨rAeronomie,Max-Planck-Str.2,37191Katlenburg-Lindau,Germany
D.E.McKenzie
DepartmentofPhysics,MontanaStateUniversity,P.O.Box173840,Bozeman,Montana
59717-3840
Tongjiang Wang
Max-Planck-Institutfu¨rAeronomie,Max-Planck-Str.2,37191Katlenburg-Lindau,Germany
Abstract. On 21 April 2002 a large eruptive flare on the west limb of the Sun developed
abright,verydynamic,postflarearcade.InTRACE195Åimages,aseriesofdark,sunward
movingflowswereseenagaintthebrightextremeultraviolet(EUV)arcade.SUMERobtained
aseriesofspectraofthedarkEUVflowsinthelinesC,FeandFeatafixedposition
above the limb. These spectra give spatially resolved line-of-sight velocities and emission
measuresforthearcadeplasmaoveratemperaturerange2×104to107K.Theflowsaredark
inallSUMERlines.TheUVcontinuumlongward(∼1350Å)andshortward(∼675Å)ofthe
Lymanlimitisusedtodetermine theamount ofabsorbing materialwithtemperaturebelow
2×104K.Thereissomeevidenceofcoldabsorbingmaterialjustbeforethearcadeemission
reachestheheightoftheSUMERobservations.Alongmostofthedarkchannelsthereisno
changeincontinuum ratioandwethereforeconclude, asoriginallysuggested byMcKenzie
andHudson(1999),thattheyareplasmavoids.
Keywords:Sun:flares–UVradiation
1. Introduction
LargepostflaresoftX-rayarcades(supra-arcades) risingover200arcsecinto
thesolar corona have been seen and described for anumber of long duration
solarflares.Thearcadeslooklikefansofcoronal raysdirected outwards into
the corona from the top of what appears to be a typical post-flare loop sys-
tem. In several of the supra-arcade events sunward moving structures have
been seen in SXT images (McKenzie and Hudson 1999; McKenzie 2000).
Alleventswithsunwardflowareassociated withlarge coronal massejection
(CME) events. The majority of these flows look like dark trails falling from
the corona through the flare arcade toward the Sun. They seem to slow and
stopastheyreachtheheightofthemainarcadeflareloops.Fromtheirappear-
ance and their association with CMEs one may think that the sunward flows
are ejected chromospheric material falling back to the Sun; however so far
not asingle one ofthem has been seen inHαorcold line images. McKenzie
and Hudson (1999) and McKenzie (2000) suggested, after analysing soft X-
ray images, that they are low density flux tubes contracting down to their
equilibrium position.
(cid:2)c 2003KluwerAcademicPublishers. PrintedintheNetherlands.
downflow.tex; 13/03/2003; 13:37; p.1
2
On 21 April 2002, a spectacular postflare coronal arcade of an X1.5 flare
was observed on the south west limb of the Sun. It was also associated with
a large CME.The flare and CMEwere observed by many instruments and a
number of papers on the event have already been written (Wang et al. 2002;
Caspi et al. 2002; Gallagher et al. 2002). RHESSI images detected soft X-
ray emission from the disk, AR9906, at the start of the event. The CME
was very fast. It was seen in LASCOC2 and by UVCS and had an outward
speedofabout2500kms−1.TRACEobtained highcadenceimagesin195Å
of the flare arcade dynamics and these have been coaligned to the SUMER
spectra by Wang et al. (2002) to reveal a 3-D view of some of the flows in
the flare corona. The evolution is very complex. Several large, hot loops are
seen expanding into the corona. The arcade is seen to move and restructure
inresponse tobothdiskandcoronal activity. Darksunwardmovingtrailsare
seenagainstthebrightEUVarcadeemissionabout30minafterthefirstflare
X-rays.
The spectrometer SUMER was pointing at a fixed position in the corona
above the active region when the flare erupted and obtained a time series of
spectra in the lines C, Fe and Fe. These three lines are formed at
approximately4×104,106and107Krespectivelyandthuscoveraverywide
temperaturerange.IfthedarkEUVflowsarecoolingloopmaterialonemight
expecttoseethemintheCline,oriftheyareassociated withreconnection
jets then they could show up as Doppler shifts in Fe or Fe. Therefore
a careful analyis of the spectra has the potential to unveil the origin of the
dark flows. The cadence was also short enough to see different sections of
the dark inflows as they cross the position of the SUMER slit. This paper
presents a first analysis of the SUMER spectra at the time of the inflows
and is directed at resolving the cause of the darkness: dense, cold absorbing
plasmaorplasma voids.
Inanaccompanying paper(Innesetal.2003)asecond aspectofthespec-
tra, 1000 km s−1 flows in the Fe line associated with the dark inflows,
is reported. A third paper is planned to discuss the structure and oscillations
seenintheTRACEimagesofthedarkinflows.Variousmodelsoftheinflows
havebeensuggested byMcKenzieandHudson(1999)andMcKenzie(2000).
These and other alternatives will be investigated in more detail in a fourth
paper.
2. Observations anddataanalysis
The SUMER spectrograph was pointing at a 4 × 300 arcsec2 long narrow
strip ofthesolar corona about 100 arcsec above thelimb centered atSunco-
ordinates (981,-184). Stigmatic images were taken of the C 1335 Å (T ∼
4×104 K),Fe 1349 Å (T ∼ 106 K)and Fe 1354 Å(T ∼ 107 K)lines
downflow.tex; 13/03/2003; 13:37; p.2
Spectraofpostflareinflows 3
e
r
u
s
a
e
m
m) Fe XII Fe XXI
n
o e
si sr/ C II
s2/
mi m
e c
r s/
e /
p h
p
y (
t
i
s
n
e
t
n
I
Log Temperature
Figure1. LineintensitycontributionfunctionsforthelinesobservedbySUMER.Ionization
equilibriumandSuncoronalabundanceshavebeenused.
withacadenceof50s.Thecontribution functionscomputedusingCHIANTI
(Dereetal.1997) ofthe three lines areshownin Fig1.Formostofthe time,
only 2 Å centered on the lines were transmitted. Each hour a full spectral
window from 1333-1373 Å was transmitted. Direct measurement of the line
intensities can give constraints on the plasma emission measure at various
temperatures using a model of the ionization and the element abundances.
Thelineshiftsgiveconstraints ontheplasmaflowvelocities.
Thecontinuumemissionaroundeachlineisacombinationoffirstandsec-
ond order radiation and may provide valuable clues to the origin of the dark
EUV flows. In the wavelength region observed, the first order (∼ 1350 Å)
emission is above the Lyman limit and the second order (∼ 675 Å) is below.
So second order photons are absorbed by H along the line-of-sight and the
firstordernot.Thus,ifthereiscoldmaterialalongtheline-of-sight, thismay
showupasdecrease inthesecond tofirstordercontinuum ratio.
Thetwoorderscanbedisentangledbyusingtheknowndetectorresponse.
Asdescribed byWilhelmetal.(1997), thecentral partoftheSUMERdetec-
tor is coated with KBr. This essentially increases the detector sensitivity to
photons byabout anorder ofmagnitude atwavelengths greater than 1100 Å.
For example, at 1350 Å the KBr is almost 20 times more sensitive than the
downflow.tex; 13/03/2003; 13:37; p.3
4
C II Fe XXI
Fe XII
c)
e
s
c
r
a
Y (
n-
u
S
bare
bare
KBr
Wavelength
Figure2. TheSUMERspectrum taken at1:26:33 UT,showing the change indetector sen-
sitivitydue tothe KBr coating on the middle section. The profile isthe spatially integrated
emissionbetweenthedottedlines.Thedashedlinesshowthethreespectralregionstransmitted
thoughoutmosttheobservingsequence.
uncoated part of the detector. Atwavelengths less than about 800 Åboth the
bareandtheKBrarecomparable. Oneneedsobservations onboththecoated
(KBr) and uncoated (bare) part of the detector. The C was on the bare part
of the detector and the Fe was on the KBr. As can be seen in Fig. 2, the
continuum around the C is appreciably below the continuum around the
FeandFe.Foreachwindow,thereisanequation relating theobserved
countratetothefirstandsecondorderfluxesandthedetectorsensitivity. The
two equations can be solved to obtain the first and second order continuum
levels.
The C is a doublet and except for a few transient events, the emission
is stray light from the disk. It results in two faint emission lines along the
spectrometer slit (Fig.3a).Thepositions of the lines do not change although
their strength can vary depending on the active region disk brightness. We
therefore average the intensity from specific pixels to determine the contin-
uum. The pixels chosen to obtain continuum levels around the C on the
bare part of the detector are shown in Fig. 3a. The pixels used to obtain the
continuum count rate on the KBr coated part, around the Fe, are shown
in Fig. 3b. Here we make a distinction between continuum on the blue and
downflow.tex; 13/03/2003; 13:37; p.4
Spectraofpostflareinflows 5
a) b) Fe XII 1349
C II 1335
red
blue
pixel no. pixel no.
Figure 3. The average line profiles in the a) C and b) Fe windows. The pixels used to
estimatethe continuum aremarked withsolidhorizontal lines.Asdiscussed inthetext,the
continuumontheleft(markedblue)andright(markedred)behavedifferently.
redside of Fe.Aswillbeseen, there are phases inthe evolution whenthe
red-to-blue intensity ratiochanges duetoeither FeorFeline emission
shifting intothecontinuum windows.
TheSUMERspectrahavebeendividedbythemostrecentflat-field,taken
inNov2001.Thereissomeevidencethatthisnolongerrepresentsthepresent
small scale structure of the detector so that caution must be exercised when
interpreting stucture with a regular 5-6 pixel pattern. The other important
correction applied to the SUMER data was the geometric correction which
converts the raw ‘inverse cushion’ images to ‘rectangular grid’ images. The
standard corrections fordeadtime andlocalgainwerealsoapplied.
TRACE195Åimageswereobtained atfullresolution (pixelsize0.5arc-
sec) and with a cadence of approximatly 20 s. There were short interuptions
in TRACEdata around 0:50, 1:10, 1:40 and 2:00 UT.The images have been
processed with the solarsoft routine trace prep to remove the dark pedestal
andccdcharacteristics.
Imagesat304Å,171Å,195Åand284Åwereobtained byEITbetween
1:00and1:20UT.Theimagesat171Å,195Åand284Åshowthesameflare
arcadestructures astheTRACEimage.The304Åimageshowscolderloops
extending intothecoronaat1:19UTattheposition ofbrightCemissionin
downflow.tex; 13/03/2003; 13:37; p.5
6
SUMERspectra. The304 Åimagewascalibrated using thesolarsoft routine
eit prep.
The TRACE, EIT and SUMER images were first coaligned to within 5
arcsecusingthegivencoordinatesandpixelsizesintheimageheaders.Then,
within that range, weadjusted the overlay for the best correspondence of the
emissions. Using TRACE and SUMER time series between 0:30 and 4:00
UT,alignment towithin1arcsecwasachieved.
3. OverviewoftheEvent
The three phases of the flare evolution have been outlined by Wang et al.
(2002). The initial phase starts with the ejection of disk material. First two
jets can be seen in the TRACE images and a few min later hot loops erupt
and expand through the corona above the flare site. In the corona, SUMER
first detects the jet in C with a blue shift of 170 km s−1. The subsequent
loop ejection, can be seen in TRACE as an expanding front with plane-of-
sky speed about 120 km s−1. The loop reaches the height of the SUMER
observations just before 1:00 UT. The next phase, starting at about 1:10 UT,
istheinflowphase(Fig.4c,d).
4. SUMERobservations
SUMERintensities in the three windows are shown in Fig. 5 along with the
corresponding time-space map of TRACE intensities. The TRACE image is
constructedbyextractingthetimeseriesof195Åintensitiesatthepositionof
theSUMERslit(Sun-X=981and−346 ≤Sun-Y≤ −50).EachoftheSUMER
frames in this figure is the sum of continuum and line intensity. The initial
Fe hot, hook shaped structure is, as explained in Wang et al. (2002), the
risingloop inFig.4b.‘jet-1’ canonly beseen intheCimage.
TheFedataisalmostfeatureless.Theintensityincreasesafter1:25UT
when the flare arcade rises into the SUMER field-of-view. This general pat-
tern is reflected in the TRACE. The TRACE passband, however, includes
an Fe line as well as Fe and Bremstrahlung continuum so the inten-
sity distribution also shows features seen in Fe such as the dark channels
around (1:40,-300) cutting intothebright arcade emission.
4.1. TFe
After 1:15 UT, much of the emission in the Fe and C windows is con-
tinuum.Thecontinuum isBremstrahlung emission fromthehotflarearcade.
TheseparateFeandClineandcontinuumintensitiesareshowninFig.6.
downflow.tex; 13/03/2003; 13:37; p.6
Spectraofpostflareinflows 7
a)
b)
jet-1
jet-1
hot loop
jet-2
c) d)
Figure4. TRACE195Åfilterimagesshowingthedevelopmentoftheflaringregion.(a)The
jetsatthetimeofflareonset(b)Differenceimageshowingthejetsandtheejectedloopatflare
onset(c)Theinitialflarearcade(d)thesupra-arcadeandinflows.ThepositionoftheSUMER
observationsismarkedwithawhiteverticalline.
Here, the line emission is that remaining after continuum subtraction. The
continuum levels are computed from the pixels indicated in Fig. 3. The first
frameshowstheFelineintensity.ThereisclearlylessFeafter1:15UT.
The disappearence of the Fe is seen more clearly in Fig. 6b, the running
difference image. It can be seen that Fe disappeared from the whole slit
area a little earlier at 1:10 UT. This unfortunately is exactly the time of a
TRACEdata gap. The front is also picked up in the Fe continuum on the
blue side of the line. It is marked ‘Feshift’ on Fig.6c because it is due to
Fe line emission shifted into the ‘blue’ continuum window. The Doppler
shift associated with the front is at least 250 km s−1 and it moves up the slit
withaspeed 600−1000 kms−1.Thecontinuum ontheother sideoftheline
is shown in Fig.6d. This has basically the same structure but there is no fast
frontat1:10UT.Threeareashavebeenmarked‘Feshift’becauseanalysis
of the SUMERspectra at these times suggests that Fe line emission with
downflow.tex; 13/03/2003; 13:37; p.7
8
a) Fe XXI b) Fe XII
hot
loop
c) C II d) TRACE
jet-1
Figure5. ThetimeevolutionofSUMERintensitiesobservedatthepositionmarkedinFig4,
comparedtotheTRACE195Åintensityatthesameposition.Thewhitecontoursoutlinethe
mainFeemission.Allintensitiesareintegratedoverthe2Åwidewindowsanddisplayed
withalogarithmicscale.Thewhiterectangleshowsthepositionofthedarkinflowsanalysed
inSect.5.
Doppler velocity ∼ 1000 km s−1 is shifted into the Fe window at these
times(PaperII).
4.2. TC
For the interpretation of the dark flows, the C line emission is probably
mostinteresting. There are4mainemission structures inthefirsthour ofthe
flare.Thefirstisrelated to‘jet-1’. Thesecond andthird,asshownintheEIT
He304Åimagetakenat1:19:25UT,arealsorelatedtodisklooporjet-like
structures. Fig. 7 shows there were cold structures reaching as far out as the
SUMER slit at this time. SUMER spectra show they formed at this height
around 1:10UTandlasted until1:30 UT.
The amount of material in the cold structures along the line-of-sight can
be estimated from the C and EIT 304 intensities. As seen in Fig. 7, the
spatial overlap between the two is very good and it is quite likely that the
two lines are essentially coming from the same plasma. A lower limit to the
downflow.tex; 13/03/2003; 13:37; p.8
Spectraofpostflareinflows 9
a) Fe XII line b) Fe XII line difference
c) Fe XII Fe XII shift d) Fe XII
blue red
Fe XXI shift
e) C II line f) C II continuum
hard X-ray
Figure6. ThetimeevolutionoftheSUMERlineintensitiesandcontinuumaroundtheline,
scaled logarithically. Contours outline the position of main Fe emission as in Fig 5. a)
Fe line intensity b) Fe running difference with 5 min time lag c) Fe continuum on
thebluesideoftheline.FeaturesofhighFelineshiftaremarked.d)Fecontinuumon
theredsideoftheline.FeaturesofhighFelineshiftaremarked. e)Clineintensityf)
ContinuumaroundC.
downflow.tex; 13/03/2003; 13:37; p.9
10
a)
C II Fe XII Fe XXI
)
c
e
s
c
r
a
(
Y
-
n
u
S
EIT SUMER
Sun-X (arcsec)
b)
C II (photospheric)
C II (coronal)
e EIT
r
u
s
a
e
M
n
o
si
s
mi
E
Sun-Y (arcsec)
Figure7. SUMERspectraatthetimeoftheEIT304ÅimageshowingtheCstructuresin
theactiveregion.a)EIT304imagealignedwiththeSUMERC,FeandFespectrab)
EmissionmeasurefromtheEIT304intensitiesatthepositionoftheSUMERslitcompared
to the emission measure computed from C intensities assuming photospheric and coronal
carbonabundance.
downflow.tex; 13/03/2003; 13:37; p.10