Dynamics of subarcsecond bright dots in the transition region above sunspot and their relation to penumbral micro-jets

1.3 MB·

by  Tanmoy Samanta

ARXIVASTRO-PH.SRVERSIONJANUARY11,2017
AcceptedforpublicationinTheAstrophysicalJournalLetters
DYNAMICSOFSUBARCSECONDBRIGHTDOTSINTHETRANSITIONREGIONABOVESUNSPOTANDTHEIR
RELATIONTOPENUMBRALMICRO-JETS
TANMOYSAMANTA1,HUITIAN2,DIPANKARBANERJEE1,3,NICOLESCHANCHE4
1IndianInstituteofAstrophysics,Koramangala,Bangalore560034,India;[email protected]
2SchoolofEarthandSpaceSciences,PekingUniversity,China;[email protected]
3CenterofExcellenceinSpaceSciences,IISERKolkata,India;[email protected]
4Harvard-SmithsonianCenterforAstrophysics,Cambridge,USA;[email protected]
arXivastro-ph.SRversionJanuary11,2017
Abstract
7
1 Recenthigh-resolutionobservationsrevealthatsubarcsecondbrightdots(BDs)withsub-minutelifetimesap-
0 pearsubiquitouslyinthetransitionregion(TR)abovesunspotpenumbra. Thepresenceofpenumbralmicro-
2 jets(PMJs)inthechromospherehavealsobeenreportedearlier. ItwasproposedthatboththePMJsandBDs
areformedduetomagneticreconnectionprocessandmayplayanimportantroleinheatingofthepenumbra.
n
UsingsimultaneousobservationofthechromospherefromtheSolarOpticalTelescope(SOT)aboardHinode
a
J andtheTRfromtheInterfaceRegionImagingSpectrograph(IRIS),westudythedynamicsofBDsandtheir
relationwithPMJs. WefindtwotypesofBDs,onewhichisrelatedtoPMJsandtheotherswhichdonotshow
0
1 any visible dynamics in the SOT Ca II H images. From a statistical analysis we show that these two types
havedifferentproperties. TheBDswhicharerelatedtoPMJsalwaysappearatthetopofthePMJs, thevast
] majority of which show inward motion and originate before the generation of the PMJs. These results may
R indicatethatthereconnectionoccursatthelowercoronal/TRheightandinitiatesPMJsatthechromosphere.
S This formation mechanism is in contrast with the currently believed formation of PMJs by reconnection in
. the(upper)photospherebetweendifferentlyinclinedfields.
h
Keywords:Sun: corona—Sun: transitionregion—Sun: UVradiation—Sun: magnetictopology
p
-
o
r 1. INTRODUCTION Vissers et al. (2015) studied the multi-wavelength signa-
t tures of PMJs and found that PMJs show spatial offset from
s Sunspotsareregionsofconcentratedstrongmagneticfields
a comprising of dark central region, umbra surrounded by a the chromosphere to TR in the direction of the PMJ. Hence,
[ they proposed that PMJs may progressively heat up to TR
less darker region called the penumbra. Penumbral micro-
temperature. Tiwari et al. (2016) could not find noticeable
1 jets(PMJs)areoneoftheprominentfine-structuredynamical
signature of normal PMJs in any Atmospheric Imaging As-
v featuresobservedinthesunspotatthechromosphericheight.
sembly(AIA)(Lemenetal.2012)passbands,althoughafew
1 Using the SOT (Tsuneta et al. 2008) Ca II H filter images,
strong and large PMJs show BD/BD-like signatures in the
3 Katsukawa et al. (2007) first reported that these micro-jets
5 occur ubiquitously above penumbra. They have lengths of 1600 A˚ and High-resolution Coronal imager (Hi-C) 193 A˚
2 1 to 4 Mm and lifetimes of up to a minute. They generally images. Alpertetal.(2016)foundthatBDsobservedinHi-C
0 move very fast and have apparent speeds over 100 km s−1. areonaverageslower,dimmer,largerinsizeandlongerlived
1. The magnetic field in the chromospheric penumbral region thanIRISpenumbralBDs. Theyalsofoundthatmostofthe
0 consists of a combination of spines (more vertical field) and BDs observed in Hi-C 193 A˚ may correspond to TR. Deng
7 interspines (more horizontal field) (Lites et al. 1993; Bel- et al. (2016) found that the locations of most of the penum-
1 lot Rubio et al. 2004; Scharmer et al. 2011; Scharmer & bral BDs show downflow. Their statistical analysis shows
: Henriques 2012; Henriques & Kiselman 2013; Tiwari et al. that BDs do not have consistent brightening response in the
v 2013,2015). Katsukawaetal.(2007)foundthatPMJsgener- chromosphere. Following Tian et al. (2014), they also sug-
i
X ally originate near the bright structures in between two dark gested that TR penumbral BDs are manifestation of falling
penumbral filaments and propagate upward along the direc- plasma from coronal heights along more vertical and dense
r
a tionofthespine. So,theyproposedthatPMJscouldoriginate magnetic loops or small-scale impulsive magnetic reconnec-
as a result of magnetic reconnection between the spine and tion at TR or higher heights. Kleint et al. (2014) and Chitta
interspinemagneticfields. et al. (2016) observed heating events which are associated
UsingtheIRIS(DePontieuetal.2014)observations,Tian with the strong downflows in the TR and proposed a similar
etal.(2014)foundthepresenceofsubarcsecondBrightDots scenario. Baietal.(2016)havestudiedsmalltransientbright-
(BDs)abovesunspotpenumbraintheTR.Theyappearubiq- eningeventsinthepenumbraandfoundthatitshowredshifts
uitously and mostly have a lifetime less than few minutes. in the Si IV 1402.77 A˚ line, an inward motion towards the
They sometimes appear slightly elongated along the penum- umbrainIRIS1400A˚ imagesandhaveamulti-thermalcom-
bralfilamentsandalsomovealongthefilamentswithspeeds ponent.Theyproposedthetriggeringmechanismasmagnetic
of10–40kms−1. Theyproposedthatsomeofthemcouldbe reconnection at low coronal heights. A component of the
duetoimpulsivereconnectionintheTRandchromosphereat plasma from the reconnection site may move downward and
footpointsofcoronalmagneticloopsandothersareprobably reachtheTR,whichisconfirmedbytheobservedredshiftsin
duetofallingplasma. ItisstillunclearhowBDsareformed the Si IV 1402.77 A˚ line and the inward motions as seen in
andiftheyshowanysignaturesinthelowerandupperatmo- theIRIS1400A˚ images. Finally,itreachesthechromosphere
sphere.
2 T.SAMANTAETAL.
Figure1. A-E:ImageofasunspotasseenbytheSOT,IRISandAIAaround08:57UTon19March2014.F-J:Showtherunningdifferenceimages.Thegreen
andbluecontours,derivedformatime-averagedsmoothedSOTimage,representtheumbraandpenumbraofthesunspot,respectively. Theyellowcontours
markthelocationsofbrightdots(BDs). EvolutionoftheBDinsidetheboxB1andB2(asmarkedinthe1400A˚)isshownintheFigure2(animatedfigures
correspondingtoB1,B2,B11-B13areavailableonline.)Ananimationofthisfullfigureisalsoavailable.
andappearsasribbon-likebrightening. pensateforthesolarrotation.ThepixelsizeofSOT,SJI’sand
In this work, while combining multi-wavelength observa- AIA are 0.109(cid:48)(cid:48), 0.166(cid:48)(cid:48) and 0.6(cid:48)(cid:48), respectively. We have in-
tions covering chromosphere, TR/corona we try to find the terpolatedSOTandAIAdatainspaceandtimetomatchthe
sourceoftheBDsandtheirrelationtoPMJs. IRISSJIcadence(10.50sec)andspatialresolution(0.33(cid:48)(cid:48) ).
The IRIS SJIs and SOT images were co-aligned using IRIS
2. DATAANALYSISANDRESULTS 2796 A˚ and SOT Ca II H images. The time difference be-
tweentheSOTandIRISimageisoftheorderofsub-seconds.
2.1. ObservationandDataReduction
AfterthatIRISandAIAwereco-alignedusingIRIS1400A˚
We use the data obtained from a coordinated observation andAIA1600A˚ images(Samantaetal.2015).
takenon19March2014,usingtheHINODE/SOT(HOP-250)
Figure 1 shows the image of a sunspot, where the bottom
(Tsuneta et al. 2008), IRIS (IRIS-3840007453) (De Pontieu
rows (G-K) correspond to running difference images. The
et al. 2014) and the AIA (Lemen et al. 2012) on board the
yellow contours show the location of BDs in the sunspot
Solar Dynamics Observatory. The coordinated observation
penumbra. Thesecontoursareobtainedfromthe1400A˚ im-
betweenalltheinstrumentswasperformedfrom08:51UTto
ageaftersubtractingthesameimagewitha8x8pixelsmooth-
09:46 UT. We selected a FOV of 37(cid:48)(cid:48)x50(cid:48)(cid:48) centered at 200(cid:48)(cid:48)
ing. Evolution of two BDs inside the box B1 and B2 (as
and 317(cid:48)(cid:48). The FOV is limited due to the SOT observations.
IRISSlit-jawimages(SJI)centeredat2796A˚ and1400A˚ are markedin1400A˚)areshownintheFigure2(alsoinonline
movie). TheBDsinsidetheboxB1includingboxB11,B12
dominated by the Mg II k and Si IV emission lines, respec-
andB13arerelatedtoPMJs. TheBDinsideB2isnotrelated
tively. The SOT filter is dominated by Ca II H line, which
toanyvisibledynamicsintheSOT.
forms at the lower chromosphere where the temperature is
below104 K.TheIRIS2796A˚ imagesaredominatedbythe
emission from a plasma at temperatures ∼10,000-15,000 K 2.2. TemporalevolutionofBDsandPMJs
and represent the middle chromosphere. The IRIS 1400 A˚ Figure 2 depicts the temporal evolution of two BDs.
passband is sensitive to TR ∼60,000-80,000 K. AIA filter- (cid:13)B1 shows the evolution of the BD inside the B1 box in the
gramimagescenteredat304A˚ (sensitiveto∼0.05MK)and Figure1. PanelA,CandEshowtheevolutionasseeninthe
193A˚ (∼1.25MK)aredominatedbyHe IIandFe XIIemis- IRIS1400A˚,SOTCa IIHandIRIS2796A˚,whereastheB,
sion lines, respectively. SOT Ca II H data was taken with DandFcorrespondtorunningdifferenceimages. Theclear
0.3 sec exposure and 1.58 sec cadence. IRIS SJI’s were ob- presenceofaBDisseenintheinthe1400A˚.Ajetlikefea-
tained with 4 sec exposure and a cadence of 10.5 sec. AIA tureisalsoseenintheCaIIHand2796A˚ images. Similarly,
304 A˚ and 193 A˚ observations were taken with 2 sec expo- (cid:13)B2 showtheevolutionoftheB2regionasmarkedintheFig-
sureand12seccadence. AIAdatawerethenco-alignedand ure1. Here, wecanclearlyfindthepresenceofa BDinthe
de-rotatedtothestarttime(08:51UT)ofobservationtocom- 1400 A˚ channel but no clear signature of intensity enhance-
DYNAMICSOFSMALLBRIGHTDOTSINTHETRANSITIONREGIONABOVESUNSPOT 3
Figure2. (cid:13)B1 showthetemporalevolutionofaBDandaPMJinsidetheB1region(asmarkedintheFigure1)asseenindifferentfiltergramimagesandtheir
runningdifferenceimages.ThegreencontourisderivedfromthetopmiddlepanelstoshowthelocationoftheBD.Yellowarrowindicatesthedirectionofthe
sunspotcenter.Similarly,(cid:13)B2 showthetemporalevolutionoftheB2region(asmarkedintheFigure1)
mentintheCa IIH.WehaveidentifiedseveralBDsandper- initiation of the BD. The long intensity strip at the center of
formedastatisticalanalysistocompilethepropertiesofthese theXTmapisduetohighintensityenhancementatthatpar-
BDs and their signatures in the other channels. At first, we ticulartimeframe.WefollowasimilarmethodfortheCaIIH
identifymanuallyanisolatedBDandlookforthetimeframe and 2796 A˚ channels (D and G). The intensity enhancement
whereitshowsmaximumintensityenhancement. Weusethat in these channels is only a few percent and very difficult to
asourcentralframeandmakeamovie(seeanimatedfigureof findthelocationoftheintensityenhancement. So,weusethe
Fig.2,coveringa5x5Mmregionforadurationof210sec)to runningdifferenceimagestoplaceourgreenandredcut(see
follow the BDs and their direction of propagation. The BDs EandH)ontheoriginalimages. TheXTmapisproducedaf-
oftenshowapparentmovementsalongthebrightfilamentary terremovingasmoothedbackgroundfromeachimages. An
structuresroughlyintheradialdirectionofthesunspot. elongatedbrightstructureisseenintheXTplot. Thesehave
NowtofindthepropertiesoftheBDs,wefollowasimilar beenconventionallyreferredaspenumbralmicro-jets(PMJs)
method as used by Tian et al. (2014). Two examples of the by Katsukawa et al. (2007). The lifetimes of these jets are
analysisareshowninFigure3(cid:13)&(cid:13). Wehavestudied180 verysmall. Mostofthemappearassuddenbrighteninginthe
B1 B1
penumbral BDs and try to find if they have any signature in XT plot and hence it is very difficult to determine their di-
theSOTCa IIHandIRIS2796A˚ channels. Weusethecen- rectionofpropagationandthespeed. Inthisworkwedonot
tralimagetocomputetheintensityenhancement, lengthand focusonthespeedofthesejets. Usingearlierconventionwe
widthofaBD,asshowninFigure3(cid:13). Weplottheintensi- haveassumedthatthesebrighteningsarepropagatingoutward
B1
tiesalongthegreencutandredcut(panelA).Thegreencut formthesunspot,thoughitisnotclearlyestablishedfromour
isplacedalongtheradialdirectionandtheredcutisperpen- analysis that the bright structures are moving outward from
diculartoit. Theintensityvaluesareshownbyredandgreen thesunspot.Weusea2σintensitycontourtofindthelocation
diamondsymbols. AfittedGaussianisalsooverplottedwith andextentofthePMJ.Thebluelineisdrawnalongthedirec-
thesamecolor. TheFWHMofthegreenprofileprovidesan tion of intensity enhancement. The starting time of the blue
estimateofthelengthoftheBD,whereastheredprofilepro- lineisconsideredastheinitiationandthelengthofthelineis
vides the width. Percentage intensity enhancement is calcu- consideredasthelengthofthePMJ.Wecanclearlyseethat
latedfromthepeakintensityandthelinearbackgroundofthe theBDappearsbeforethePMJinthisexample.
fitted Gaussian. The smallest between the two values (green Similarly, in Figure 3(cid:13)B2, we study another BD. Follow-
andred)measurestheintensityenhancement. Weplotspace- ingsamemethodologywestudytheevolutionoftheBD.We
time(XT)diagram(panelC)tostudytheirtemporalbehavior couldnotfindaclearsignatureoftheBDintheCaIIHthough
andtomeasuretheirapparentspeed.A2σintensitycontouris afaintsignatureisseeninthe2796A˚ channel. Inthefollow-
drawnontheXTmap. Theinclinedbluelineisdrawninside ing subsection a statistical study is performed to find out if
the contour to measure the apparent velocity and lifetime of therearetwotypesBDspresentinourdata.
theBD.Thestartingtimeofthebluelineisconsideredasthe
4 T.SAMANTAETAL.
Figure3. (cid:13)B1 showtheprocedureofdeterminingthephysicalparametersoftheBDandthePMJinsidetheB1region(asmarkedintheFigure1).(A)showthe
BDenclosedbythebluecontourasseeninthe1400A˚.Thegreenslitandredslitisplacedalongtheradiallyoutwarddirectionanditsperpendiculardirection,
respectively. (B)showtheintensityprofilealongthegreenandredslitwithdiamondsymbol. ThesolidlineisaGaussianfittotheprofilestodeterminethe
widthandintensityenhancementoftheBD.(C)showthetemporalevolution(XTplot)alongthegreencut.Yellowcontourshowtheregionabove2σintensity
enhancement.TheslopofthebluelineisusedtodeterminetheplaneoftheskyvelocityoftheBD.(D)showCaIIHimage.Thegreenlineistheslittodetermine
XTplot.(E)showtheCaIIHimageaftersubtractingthepreviousframe.RedcontourshowthelocationofthePMJ.(F)showtheXTplotforthegreencutof
CaIIHimages.(G)show2796A˚ image.(H)-(I)aresimilarto(E)-(F),respectively,butforthe2796A˚ channel.Similarly,(cid:13)B2 showfortheBDinsideB2region
(asmarkedintheFigure1).
Figure4. (A-F)showthedistributionofdifferentphysicalparametersofBDs.RedrepresentstheBDswhichdonothaveanyconnectiontothePMJswhereas
bluerepresentsthosewhicharefoundonthetopofthePMJs. (A)showtheofthelength(FWHMoftheintensityprofilesalongthegreenslit),(B)thewidth
(FWHMoftheintensityprofilesalongtheredslit),(C)theratiooflengthandwidth,(D)thelifetime,(E)theintensityenhancementand(F)theplaneofthesky
speedoftheBDs.Theaveragevalueofeachparametersandalsotheirstandarddeviationareprinted(anrandare).
DYNAMICSOFSMALLBRIGHTDOTSINTHETRANSITIONREGIONABOVESUNSPOT 5
Figure5. (A):TheyellowarrowsshowthelocationofthePMJsasfoundintheCaIIHimagesandthedots(red/blue)showsthelocationoftheBDsasseenin
the1400A˚ channelsovertheCaIIHimage.ReddotsrepresenttheBDswhichmovesinwarddirection(oppositeofthePMJ’sdirection)andbluerepresentthe
BDswhichmovesoutward(alongthedirectionofPMJs).(B):Theyellowarrowandthedots(red/blue)showthelocationofCaIIHPMJsand1400A˚ BDsover
the2796A˚ SJI.ThegreenarrowsrepresentthePMJsasobservedinthe2796A˚ images.(C):TheBDswhichdonothaveanyvisiblecounterpartsintheCaIIH
imagesareshownoverthe1400A˚ image. (D)showthedistributionofthelengthofthePMJs. (E)showthedistributionofthedistancebetweenthefootpoint
ofthePMJsandstartinglocationoftheBDs. (F)showthedistributionofthetimedelayoftheoriginationofBDafterPMJ.AnegativevaluemeansthataBD
originatesearlierthanaPMJ.AllthePMJsandBDsareobservedoverthetotalobservingwindowwhereasthebackgroundimagesaretakenatt=0. Themean
valuesofeachparameterandalsotheirstandarddeviationareprinted.TheasoandairrepresentsthemeanvalueasobtainedfromtheSOTCaIIHandIRIS
2796A˚,respectively.
6 T.SAMANTAETAL.
2.3. StatisticalbehavioroftheBDs reaches the chromosphere and appears as ribbon-like bright-
ening(PMJ)whichcouldexplainthetimedelayintheappear-
We analyze the properties of many BDs and their signa-
anceofthePMJsandtheirshortlifetime. Klimchuk(2006),
tureintheotherchannels. Outofthetotal180events,90are
Jiangetal.(2012)andWinebargeretal.(2013)proposedthat
identifiedashavingacorrelatedsignatureintheCaIIHand
this kind of small magnetic reconnection occurs in the low
2796 A˚ channels, while the other 90 do not. We have sepa-
corona. Thisisincontrasttothereconnectionmodelaspro-
ratedthesetwoclassessoastofindoutiftheyhaveanysignif-
posed by Katsukawa et al. (2007). The progressive heating
icantdifferencesintheirphysicalproperties. Figure4(A)-(F)
mechanismofthePMJsasproposedbyVissersetal.(2015)
show the distributions of the observed parameters. The red
alsomaynotexplaintheinwardmotionsoftheBDs,appear-
colorinthehistogramsrepresenttheBDswhichdonothave
anceofBDsbeforePMJsandthelongerlifetimeoftheBDs.
a counterpart in the Ca II H images whereas the blue repre-
Itisstillunclearhowthenon-relatedBDsoriginate.
sent the BDs which are related to the Ca II H PMJs. The
mean value of each of the parameters for both the types are
alsoprinted. WefindthattheBDswhicharerelatedtoPMJs,
WethankYukioKatsukawaforusefuldiscussionsandsug-
generally,arelongerandhavehigherintensityenhancements.
gestions. H. Tian is supported by the Recruitment Program
They are also more elongated (the ratio between length and
of Global Experts of China, NSFC under grant 41574166,
width is higher). One of the distinguishable feature is that
and the Max Planck Partner Group program. H.Tian thanks
mostoftheBDswhicharerelatedtoPMJsshownegativeve-
ISSI Bern for the support to the team “Solar UV bursts: a
locities(driftstowardsthecenterofthesunspot).
new insight to magnetic reconnection”. We thank the Hin-
ode, IRIS and SDO team for proving the data in the pub-
2.4. StatisticalbehaviorofPMJsandtheirconnectiontoBDs
lic domain. Hinode is a Japanese mission developed and
WehavealsocalculatedafewphysicalparametersofPMJs. launchedbyISAS/JAXA,withNAOJasdomesticpartnerand
The position and length of each PMJs are measured in both NASA and STFC (UK) as international partners. It is oper-
the 2796 A˚ and Ca II H channels. The starting point of the ated by these agencies in co-operation with ESA and NSC
PMJs is considered as the location close to the center of the (Norway). IRISisaNASAsmallexplorermissiondeveloped
sunspot. ThelocationsofthePMJasseenintheCa IIHand and operated by LMSAL with mission operations executed
2796 A˚are marked (yellow arrows) in the Figures 5A, B re- at NASA Ames Research center and major contributions to
spectively. The length of the arrow measures the length of downlink communications funded by the Norwegian Space
thePMJs. TheBDswhicharerelatedtothesePMJsarealso Center(NSC,Norway)throughanESAPRODEXcontract.
shownbythesmalldots.Redrepresentsthedotswhichmoves
inwardandbluerepresentsthosethatmoveoutwardfromthe
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