The Dynamics of Rapid Redshifted and Blueshifted Excursions in the Solar Halpha line

0.96 MB·English

by  D. Kuridze

DraftversionJanuary27,2015
PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
THEDYNAMICSOFRAPIDREDSHIFTEDANDBLUESHIFTEDEXCURSIONSINTHESOLARHαLINE
D.Kuridze1,4,V.Henriques1,M.Mathioudakis1,R.Erde´lyi2,T.V.Zaqarashvili3,4,S.Shelyag5,P.H.Keys1,2,andF.P.Keenan1
1AstrophysicsResearchCentre,SchoolofMathematicsandPhysics,Queen’sUniversity,Belfast,BT71NN,NorthernIreland,UK;e-mail:
[email protected]
2SolarPhysicsandSpacePlasmaResearchCentre(SP2RC),UniversityofSheffield,HicksBuilding,HounsfieldRoad,SheffieldS37RH,UK
3SpaceResearchInstitute,AustrianAcademyofSciences,Schmiedlstrasse6,8042Graz,Austria
4AbastumaniAstrophysicalObservatoryatIliaStateUniversity,3/5Cholokashviliavenue,0162Tbilisi,Georgiaand
5MonashCentreforAstrophysics,SchoolofMathematicalSciences,MonashUniversity,ClaytonVIC3800,Australia
(Dated:received/accepted)
DraftversionJanuary27,2015
5
1 ABSTRACT
0
Weanalysehightemporalandspatialresolutiontime-seriesofspectralscansoftheHαlineobtainedwiththe
2
CRisp Imaging SpectroPolarimeter (CRISP) instrument mounted on the Swedish Solar Telescope. The data

n reveal highly dynamic, dark, short-lived structures known as Rapid Redshifted and Blueshifted Excursions
a (RREs,RBEs)thatareon-diskabsorptionfeaturesobservedintheredandbluewingsofspectrallinesformed
J in the chromosphere. We study the dynamics of RREs and RBEs by tracking their evolution in space and

5 time, measuring the speed of the apparent motion, line-of-sight Doppler velocity, and transverse velocity of
2 individualstructures. AstatisticalstudyoftheirmeasuredpropertiesshowsthatRREsandRBEshavesimilar
occurrencerates,lifetimes,lengths,andwidths. Theyalsodisplaynon-periodic,non-lineartransversemotions

] perpendiculartotheiraxesatspeedsof4-31kms−1.Furthermore,bothtypesofstructureseitherappearashigh
R
speedjetsandblobsthataredirectedoutwardlyfromamagneticbrightpointwithspeedsof50-150kms−1,or
S emergewithinafewseconds.Astudyofthedifferentvelocitycomponentssuggeststhatthetransversemotions
h. along the line-of-sight of the chromospheric flux tubes are responsible for the formation and appearance of
p theseredshifted/blueshiftedstructures. TheshortlifetimeandfastdisappearanceoftheRREs/RBEssuggests
- that,similartotypeIIspicules,theyarerapidlyheatedtotransitionregionorevencoronaltemperatures. We
o speculate that the Kelvin-Helmholtz instability triggered by observed transverse motions of these structures
r maybeaviablemechanismfortheirheating.
t
s Keywords:Sun: Magneticfields—Sun: Atmosphere—Sun: chromosphere—Sun: oscillations—Waves—
a
magnetohydrodynamics(MHD)
[

1
v 1. INTRODUCTION characterisedbyablueshiftedspectrallineprofile,occurnear
5 networkboundariesmostlyinareaswhereHαrosettesarelo-
The highly inhomogeneous nature of the solar chromo-
0 cated. RosettesareclustersofelongatedHαmottlesexpand-
sphere has been known since the discovery of solar spicules
2 ing radially around a common centre over internetwork re-
almost140yearsago(Secchi1877).Spiculesaresmall-scale,
6 gions(Zachariadisetal. 2001;Tziotziouetal. 2003;Rouppe
jet-like plasma features observed ubiquitously at the solar
0 van der Voort et al. 2007). However, compared to mottles,
limbbetweenthephotosphereandthecorona(Roberts1945;
. RBEs are slender (∼200 km) and short-lived (∼40 s) with
1 Beckers 1968, 1972; Sterling 2000; Tsiropoula et al. 2012).
0 The traditional (type I) spicules have lifetimes ranging from higher apparent velocities (∼50-150 km s−1). Furthermore,
5 1-12minandarecharacterisedbyrisingandfallingmotions RBEs are characterised by rapid fading (within a few sec-
1 withspeedsof∼20-40kms−1. Subsequentobservationshave onds)inchromosphericspectrallineswithoutanydescending
v: revealed similar types of chromospheric fine structures such behaviour. Thisrapiddisappearancemaybetheresultoffast
heatingtotransitionregionandcoronaltemperatures(Rouppe
i asquietSunmottles(Suematsuetal.1995)andactiveregion
X fibrilsonthesolardisk. ObservationswiththeSolarOptical van der Voort et al. 2009). Some recent observational evi-
r Telescope (SOT) onboard Hinode have identified a different dence supports the heating hypothesis in limb spicules (De
a typeofspicule(typeII),whicharemoreenergeticandshort- Pontieuetal. 2011;Pereiraetal. 2014). However,themech-
anismforthisfastheatingremainsamystery.
livedfeaturescharacterisedonlybyupwardapparentmotions
Anotherimportantdynamicalcharacteristicofthesestruc-
(DePontieuetal.2007a).
tures is their transverse displacements. Like spicular struc-
Anotherclassoffine-scalechromosphericstructures,called
tures, RBEs undergo transverse motions perpendicular to
the Rapid Blueshifted Excursions (RBEs), was found on the
their longitudinal axis (Rouppe van der Voort et al. 2009).
solar disk using high resolution, ground-based spectroscopic
In relatively long-lived structures, such as type I spicules
observations (Langangen et al. 2008). These are absorption
and mottles, these motions are usually observed as periodic
featuresdetectedinthebluewingsoftheCaII8542ÅandHα
transverse displacements, and interpreted as magnetohydro-
6563 Å lines (Langangen et al. 2008; Rouppe van der Voort
dynamic(MHD)kinkwaves(Zaqarashvilietal.2007;Pietar-
etal.2009). ThemeasuredpropertiesofRBEsaresimilarto
ila et al. 2011; Kuridze et al. 2012; Morton et al. 2012; Jess
those reported for type II spicules, supporting the idea that et al. 2012). However, it is extremely difficult to detect any
they are the same phenomenon viewed from different obser-
suchperiodicityinRBEsduetotheirshortlifetime.
vational angles, i.e. limb vs disk. (Langangen et al. 2008;
Sekseetal.(2013a)reportedthepresenceofRBE-likeab-
Rouppe van der Voort et al. 2009). These dynamic events,
2 Kuridzeetal.
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Figure1. Hαcoreand±0.906Åimagestogetherwiththephotospheric,line-of-sightmagnetogramobtainedfromFe6302ÅStokesVprofiles. Redandblue
arrowsindicatetypicalRREsandRBEsobservedintheHαlinewings. TherosetteregionswheremostoftheRBEs/RREsaredetectedarehighlightedwith
whitedottedcircles. TheimagesshowthatthefootpointsoftheRBEs/RREscorrespondtophotosphericbrightpointsandstrongmagneticfluxconcentrations.
Thecolourscaleinthemagnetogramindicatesthemagneticfieldstrengthinkilogauss.
sorption features in the red wing of the Ca II 8542 Å and benefitedfromtheadaptive-opticscorrection.
Hα6563Ålines,referredtoasRapidRedshiftedExcursions AlldatawerereconstructedwithMulti-ObjectMulti-Frame
(RREs). They are detected in the same regions, along chro- Blind Deconvolution (MOMFBD; van Noort et al. 2005;
mosphericmagneticfluxconcentrations,whereRBEsandHα Lo¨fdahletal.2011),using51Karhunen-Loe`vemodessorted
mottlesareusuallyobserved,butwithaloweroccurrencerate. byorderofatmosphericsignificanceandusing88×88pixel
Sekseetal.(2013a)suggestedthatthreekindsofmotioninthe subfields in the red. A prototype of the code published by
chromospheric flux tube, i. e. field-aligned flows, swaying de la Cruz Rodr´ıguez et al. (2014) was used before and af-
motionsandtorsionalmotions,canresultinanetredshiftthat ter MOMFBD. Reflectivity fitting, due to the broadness of
ismanifestedasRREs. Lipartitoetal.(2014),havereported H-alpha,wasnotusedasjustifiedindelaCruzRodr´ıguezet
similarstructuresintheHαwing(blue-andred-wingfibrils) al. (2013). The images, reconstructed from the narrowband
near the solar limb. These structures occur with comparable wavelengths,werealignedatthesmallestscalesbyusingthe
occurrencerateswithoutanyevidencefortorsionalmotions. method described by Henriques et al. (2013). This employs
Lipartito et al. (2014) have presented arguments that at least cross-correlation between auxiliary wide-band channels, ob-
some of the detected structures may correspond to warps in tained from an extended MOMFBD scheme, to account for
two-dimensionalsheets,whichisanalternativeinterpretation different residual small-scale seeing distortions and different
fordynamictypeIIspicular-likefeatures(Judgeetal.2011). reconstruction PSFs. Data were composed into time-series
Here,wepresenthighspatialandtemporalresolutionimag- havingbeende-rotated,aligned,andfinallydestretchedasin
ingspectroscopyofRREsandRBEsintheHαon-disk,quiet Shineetal.(1994).
solar chromosphere. The structures are detected at symmet- Spatialsamplingis0(cid:48)(cid:48).0592pixel−1 andthespatialresolu-
ricwingpositionslocatedat±0.906ÅfromtheHαlinecore. tionupto0(cid:48)(cid:48).16inHαoverthefield-of-view(FOV)of41×41
Mm. TheHαlinescanconsistof7positions(-0.906,-0.543,
Someofthemarehigh-speed,dark,blob-likefeaturesmoving
upwardlyalongmagneticfluxtubes. Toinvestigatetheirdy- -0.362, 0.000, 0.362, 0.543, +0.906Åfromlinecore)corre-
namicalcharacteristicsandspatio-temporalevolutionweper- spondingtoavelocityrangeof-41to+41kms−1invelocity.
formdetailmeasurementsoftheirline-of-sight(LOS)veloci- Following reconstruction, the cadence of a full spectral scan
ties,transversevelocitiesintheimageplaneandapparentve- was1.34s. CRISPalsoundertookphotosphericFe6301and
locitiesoftheindividualstructures.Usingthemeasuredprop- 6302 Å spectral scans with 16 wavelength positions per line
erties,theinterrelationsbetweenRREsandRBEsandthefor- every ∼5 minutes over the same FOV to obtain full Stokes
mationoftheirspectrallineasymmetriesareinvestigated.We profiles. TheStokesVcomponentwasusedtoconstructLOS
discuss which mechanism could be responsible for the rapid magnetogramsemployingthecentreofgravitymethod(Rees
heating, and hence the observed fast disappearance of these &Semel1979;Uitenbroek2003).
structures,aswellasshowthepresenceofsimilarfeaturesin The widget-based CRIsp SPectral EXplorer (CRISPEX;
numericalMHDsimulationsofthesolaratmosphere. Vissers&vanderVoort2012)andTimesliceANAlysistools
(TANAT)wereusedtoperformthedataanalysisandthemea-
2. OBSERVATIONSANDDATAREDUCTION
surementspresentedinthisstudy.
Observations were undertaken between 09:06 and 09:35
UT on 2013 May 5 at disk centre, on the edge of a 3. ANALYSISANDRESULTS
coronal hole, with the CRisp Imaging SpectroPolarimeter Figure 1 shows images in the Hα line core and ±0.906 Å
(CRISP; Scharmer 2006; Scharmer et al. 2008) instrument, from line core together with the photospheric LOS magne-
mountedonthe1-mSwedishSolarTelescope(SST;Scharmer togram obtained from the Fe 6302 Å Stokes V profiles. The
et al. 2003a). The dataset includes spectral imaging in the Hαlinecoreimagecontainstwolargerosettestructures,high-
Hα6563ÅandFe6302Ålines. Adaptiveopticswereused lighted with white dotted circles (second panel of Figure 1),
throughouttheobservationsconsistingofatip-tiltmirrorand where most of the chromospheric flux tubes, and hence the
a85-electrodedeformablemirrorsetupthatisanupgradeof fine-scalestructures,areconcentrated.Therootsoftherosette
thesystemdescribedinScharmeretal.(2003a). Thissystem structures are co-spatial with strong magnetic field concen-
providedanearconstant“lock”andallreconstructedframes trations that outline the intergranular lanes. The LOS mag-
DynamicsofchromosphericRapidRedshiftedandBlueshiftedExcursions 3
Median =41.6±25.2 s Median =3.0±1.3 Mm Median =257.0±69.2 km
RBE RBE RBE
20 Median =35.1±16.9 s Median =3.4±1.5 Mm Median =264.0±85.5 km
RRE RRE RRE
15
e
c
n
e
urr
cc10
O
5
0
20 40 60 80 100 120 2 3 4 5 6 7 8 9 100 200 300 400 500 600
Lifetime (s) Length (Mm) Width (km)
Figure2. Thelifetime(left),length(middle),andwidth(right),forthe70RREs(redlines)and58RBEs(bluelines)analysedinthisstudy.Medianvalueswith
standarddeviationsarealsogiven.
netogram shows that the FOV consists mostly of unipolar The detected structures either appear as upward-directed
patcheswithmagneticfieldstrengthinthekilogaussrange. high speed jets and blobs, or emerge suddenly within a few
We selected dark absorption features manually (i.e by vi- time resolution elements (∼1-5 s). Many of them (13 RREs
sualinspection)inthefarredandbluewingsatHα±0.906Å and 22 RBEs) display transverse motions perpendicular to
thatarelongerthan1.5Mmforatleast10seconds,andhave theiraxisintheimageplane. Attheendoftheirlifetimethe
anaspectratio(length/width)greaterthan∼5atmaximumex- structures disappear extremely quickly without any apparent
tension. Wehaveexcludedgranularboundariesandso-called downwardmotion. WenotethatRREs/RBEsdetectedinthe
swirls,characterisedbyvortex-likebehaviour,fromourselec- ±0.906Å images, are normally also visible in neighbouring
tion.Basedonthesecriteria,wedetected70featuresinthefar ±0.543Åimages. Theyappearanddisappearsimultaneously
redwingatHα+0.906Åand58featuresinthefarbluewing inbothwavelengthpositions. Transversevelocitiesandtrans-
at Hα−0.906 Å, identified as RREs and RBEs respectively. verse displacements are measured using time-distance (t-d)
Examplesofthesedarkabsorbingstructures,originatingfrom diagramsgeneratedalongthecutsperpendiculartothestruc-
magneticbrightpointsalongchromosphericfluxtubes,arein- ture’saxes.Figure3illustratesatypicalexampleoftransverse
dicatedbytheredandbluearrowsintheHαlinewingimages displacementofaselectedRBE.We do not see periodicbe-
(Figure1). ItshouldbenotedthatthenumberofRBEs/RREs haviourforthe detected transverse motions (i. e.,thereare
detectedinthisworkismuchsmallerthanthenumberofde- noswingsfromsidetoside).However,itshouldbenotedthat
tectionsbyanautomatedalgorithmusedbyotherauthors(see the structures disappear in the Hα images before the trans-
e.g., Sekse et al. 2012). However, our selection is restricted versemotionscometoahalt. Transversevelocitiesareinthe
to features that are clearly distinguishable, fully observable rangeof4-22kms−1,withdisplacementsbetween250and
fromedgetoedge,andfullytraceableintimeinthefarwing 1200 km (Figure 4). It is clear that the transverse displace-
imagesoftheHαtimesequence. Thisreducesthenumberof mentsaregreaterthanthewidthofthestructures(∼250km),
detectionsbutgivesusamorerobustdatasettoinvestigatethe implyingthatthemotionsarestronglynonlinear.
differentvelocitycomponentsandthespatial-temporalevolu- The apparent velocities of the RREs and RBEs projected
tionwhichistheultimategoalofthiswork. Wealsowantto ontheimageplanearedeterminedusingt-dplots,generated
notethatthetotalnumberofdetectedRREsishigherthanthe along the path of these motions. Velocity estimates are in
numberofRBEs(70vs58),webelievethatRREsandRBEs therange50-150kms−1 (Figure4)whichareAlfve´nicand
appearherewithasimilaroccurrencerate. Thedifferencein super-Alfve´nic in the chromosphere. The apparent absolute
numbersmaybeattributedtothelowercontrastbetweenthe velocitiesofRREsandRBEsareinasimilarrange,although
backgroundandRBEs. the number of structures where reliable measurements were
Wehavemeasuredeachstructure’slifetimeasthetimein- possible are different (36 vs 5 for RREs and RBEs, respec-
terval between its first appearance and complete disappear- tively).WebelievethatthehighercontrastoftheRREsmakes
ance; the maximum projected length as the maximum visi- theapparentmotionsinthemmoreeasilymeasurablethanin
bleextensionwithmanuallyidentifiedlowerandupperends RBEs.TheHαdatapresentedhereshowsnoevidenceforany
of the structure during its lifetime; the width as the visible downwardmotionseitherintheRBEsorintheRREs.
diameter of the central part of the detected structure at the Some of the detected RREs/RBEs appear as high-speed
time of maximum extension. The width measured with this blobsmovingupwardlyalongthesamepathwhereotherchro-
methodagreesverywellwithfull-widthathalf-maximumof mospheric fine structures are detected. Figure 5 shows the
Gaussian-fit trials performed on the cross-section of the flux temporal evolution of a typical dark blob detected in the red
profiles. wing at Hα+0.906 Å. The blob starts from the bright point
The results show that RREs and RBEs have almost iden- andmovesupwardalongacurvedtrajectorywithanapparent
tical lifetimes, widths and lengths (Figure 2). Despite these propagationspeedof∼110kms−1.
similarities,theanalysisdoesnotshowthattheappearanceor Thelargeredandbluelineprofileasymmetry,whichisthe
disappearance of RREs is correlated with the appearance or maincharacteristicofRREsandRBEs,meansthattheyhave
disappearanceofRBEssuggestingthattheyareindependent aLOSvelocitycomponent.ValuesofLOSvelocitiesforeach
features. Similarly, there is no one-to-one relation between individual feature are calculated using Doppler signals pro-
themandHαlinecoremottles. vided by their Hα spectral profile. For these measurements
4 Kuridzeetal.
8 t=0 s t=7.8 s t=15.6 s
6
t=18 s
m t=24 s
M 4
2
0
8
t=23.4 s t=31.2 s t=39 s
6
m
M 4
2
0
0 5 10 15
Mm
Figure3. Atypicalexampleofthenon-periodictransversedisplacementofanRBEwithadisplacementof∼500kmandatransversevelocityof∼13kms−1.
Bluelinesshowtheinitialpositionofthestructureforreference.
we use the first moment with respect to wavelength method theirfootprintsarealmostco-spatial,itmaybeexpectedthat
described by Rouppe van der Voort et al. (2009). A box is theywillhavethesamewavedrivers. Thuswaveswithsim-
definedforeachmanuallydetectedRRE/RBEthatcoversthe ilar periods may be expected to be excited in them as well.
wholestructurewhenithasmaximumextensionof±5pixels, However, RREs/RBEs are short-lived, compared to mottles,
and calculate the v (x,y,i), where x and y are transverse with mean lifetimes much lower than the typical periods of
LOS
and longitudinal coordinates of this box, and i = j,...,k so transversekinkwavesinchromosphericfinestructures. This
that jandkarethenumberofframeswherethestructurefirst makes it extremely challenging to determine the periodicity
appearsanddisappearscompletely,respectively. Dopplerve- ofthesemotionsinRREsandRBEs. Wenotethatsomeev-
locitieswerecompensatedaftercomputationforetaloncavity idence of periodic transverse motions in RBEs has been re-
errors. Our analysis shows that the LOS velocities are ap- portedbySekseetal.(2013b). Thetransversemotionsanal-
proximately constant along the lengths of RREs/RBEs. Fur- ysedinthisworkdonotshowfullswings(i.e.,motionsfrom
thermore,theyarealmostregularanddonotshowanytypeof left to right and back again) that would be the manifestation
periodic behaviour over their lifetime. An absolute value of of kink waves. RREs and RBEs display a nonlinear trans-
thesevelocities, averagedalongthestructureslengths, arein verse motion in only one direction that does not come to a
therangeof21-34kms−1. ThedistributionofLOSvelocities halt before the structure disappears. This suggests that the
is shown in Figure 4 (bottom right panel) where again both reasonforthelackofafullwavecycleisduetotherelatively
typeofstructureshavesimilarLOSvelocities. shortlifetimesoftheRREsandRBEs. Itshouldbenotedthat
We inspected the locations where RBEs or RREs were the strong damping of these motions could be an alternative
detected a few minutes before and after the appear- explanation for their non-periodic nature. However, scaling
ance/disappearanceofeachstructureandlookedforevidence lawsofexistingdampingmodels, suchase. g. resonantab-
in both the LOS Doppler velocity signal and intensity in the sorption,phasemixing,waveleakageormodeconversion,in-
line wings. We found no significant signal and therefore no dicatethatthedampingtimescalesshouldbelongerthanthe
evidenceofreturningorperiodicmotions. wave periods (Goossens et al. 2002; Ofman & Aschwanden
2002;Goossensetal.2014). Thissuggeststhataperiodicity
4. DISCUSSION could still have been seen. It could be argued that the ob-
4.1. Transversedisplacements served transverse motions are not waves, but just transverse
displacementofthefluxtubeasawholefromitsinitialposi-
Awiderangeoftransversemotionshavebeenstudiedthor-
tion. Hasan&Kalkofen(1999)haveshownthatwhentrans-
oughly in chromospheric fine structures such as type I and
versemotionsareexcitedbyaphotosphericgranule,andthe
II spicules (Zaqarashvili et al. 2007; Zaqarashvili & Erde´lyi
interactiontimescalesbetweenthegranuleandthefluxtubeis
2009; Okamoto & de Pontieu 2011), mottles (Kuridze et al.
comparable or greater than the chromospheric cutoff period,
2012;Mortonetal.2012;Kuridzeetal.2013),fibrils(Pietar-
thereisnokinkwave. Instead,thewholetubeissimplydis-
ilaetal.2011),andRBEs(RouppevanderVoortetal.2009;
placed from its initial location at all heights. Unfortunately,
Sekse et al. 2013a). These motions frequently reveal peri-
the dataset analysed in this work does not allow us to deter-
odicbehaviourwhichisattributedtoMHDkinkwaves(Spruit
mine the excitation mechanisms, and hence the exact nature
1982). Reported wave periods in the relatively long-lived
ofthedetectedtransversemotions.
chromospheric features, such as type I spicules, mottles and
fibrils,areintherange1-5minutes. Themeanperiodofthe
kink waves in the Hα line core mottles are ∼3 min (Kuridze 4.2. ApparentandLOSVelocities
et al. 2012; Morton et al. 2012). RREs/RBEs have similar TheobservationspresentedhereshowthatRREsandRBEs
geometries and spatial scales (length and width) as the Hα either display apparent motions that are directed outwardly
mottles. Furthermore, as they occur at the same place and from the magnetic bright point, or emerge suddenly within
DynamicsofchromosphericRapidRedshiftedandBlueshiftedExcursions 5
6
Median =13.9±4.8 km/s Median =536±238 km
5 RBE RBE
MedianRRE=17.0±4.8 km/s 5 MedianRRE=472±148 km
4
4
e
c
en3
urr 3
c
c
O2
2
1 1
0 0
5 10 15 20 25 200 400 600 800 1000 1200 1400
Transverse velocity (km/s) Transverse displacement (km)
6 12
Median =79±37 km/s Median =28±3.0 km/s
RBE RBE
5 MedianRRE=94±28 km/s 10 MedianRRE=27±2.6 km/s
4 8
e
c
n
e
urr3 6
c
c
O
2 4
1 2
0 0
60 80 100 120 140 160 20 25 30 35
Apparent velocity (km/s) LOS velocity (km/s)
Figure4. DistributionoftheaverageLOSvelocity(absolutevalue),apparentvelocity,transversevelocity(intheimageplane)andtransversedisplacement
amplitudefortheanalysedRREs,RBEs(redandbluelines,respectively).Medianvalueswithstandarddeviationsarealsogiven.
a few seconds. Our very high cadence (1.3 s) allowed us to have θ <90◦ at the disk centre. Thus, flows can only pro-
estimate apparent motions for some of the RREs and RBEs duceblueshiftedsignalswhereastransversemotionscangen-
with speeds ranging between 50 - 150 km s−1 (Figure 4). eratebothredandblueDoppler-shiftswithoutanypreference.
Similar behaviour in RREs and type II spicules has been re- Hence, if the apparent motions are plasma flows, then there
ported by other authors (Rouppe van der Voort et al. 2009; aremoresourcesablueshiftedDopplersignals. Thereshould
Sekse et al. 2013a). The Alfve´nic/super-Alfve´nic apparent thereforebeaprevalenceofRBEsandtheabsolutevaluesof
velocities measured here, and the sudden appearance of the theDopplervelocitiesfortheRBEsshouldbehigherthanof
structures, are their most puzzling aspects. One possible ex- RREs. However, we could find no evidence that would sub-
planation is that they are formed as a result of upward di- stantiate this scenario. By contrast, the number of detected
rectedplasmaflowsalongchromosphericfluxtubes. Anum- RREsishigherthanthatofRBEsandtheyhavesimilarabso-
berofrecentnumericalsimulationshaveshownthatthehigh- lutevaluesofLOSvelocities(bottomrightpanelofFigure4).
speed upflow patterns in the chromosphere can be gener- Thisindicatesthatfieldalignedflowsisanunlikelyexplana-
atedbyphotosphericvortexmotionsinmagneticfeaturesand tionforthefastappearanceoftheRREsandRBEs.
Lorentzforces(Goodman2012;Kitiashvilietal.2013).How- An alternative interpretation for the high apparent veloci-
ever,theextremelyfastappearance(withspeedsofmorethan ties and rapid, sudden appearance of these features is based
∼700kms−1)isstillunexplainedbythisflowscenario. Fur- on a wave scenario. The basic idea is that upward prop-
thermore, it does not explain the appearance of large num- agating kink waves/disturbances along the flux tube can be
bers of redshifted features (RREs) at the solar disk centre. seenasDopplersignalsintheformofRREs/RBEsappearing
ThesourcesoftheDopplersignalsofthechromosphericflux with Alfve´nic speed. Indeed, the observed high-speed red-
tube could be field-aligned flows and/or the transverse mo- shifted/blueshifted jet- and blob-like features may be inter-
tions along the LOS. It must be noted that torsional motions pretedintermsofpropagatingtransversepulses/disturbances
canalsocontributetothelineshift,butwehavenotfoundany travellingwithaphasespeedwhichcouldbeclosetothechro-
evidence of torsional motions in our dataset. Field-aligned mosphericAlfve´nspeed. Ontheotherhand,superpositionof
flowsthatappearfromthebrightpointsandmoveoutwardly the opposite-directed kink waves may result in a wave with
along a magnetic flux tube can produce blueshifted Doppler averyhigh/infinitephasespeed(standingwaves)thatcanbe
signals, v → blue, ifthetiltangle(θ)withrespecttothe manifested as the sudden appearance of RREs/RBEs. Both
dopp
observer is <90◦, v → 0 if θ ≈90◦, and v → red if propagating (upward and downward) and standing, trans-
dopp dopp
θ >90◦. As the observed quiet Sun region is almost unipo- verseoscillationshavebeenreportedintypeIIspiculesatthe
lar (right panel of Figure 1) it is expected that the region is limb (Okamoto & de Pontieu 2011) and mottles on the disk
dominatedbyopenfluxtubeswhichsuggeststhattheydonot (Kuridzeetal.2013). However,thereisnoevidenceofdown-
6 Kuridzeetal.
t=0 s t=7.8 s t=15.6 s t=23.4 s t=31.2 s t=39 s t=46.8 s
5 80
4
Mm23 Time (s)40
1
0 0
0 2 4 6 8 0 2 4 6
Mm Mm
Figure5. Time-sequenceofatypicaldarkblobdetectedintheredwingatHα+0.906Å(42kms−1).Theblobstartsfromthenetworkbrighteningandmoves
upwardalongthecurvedtrajectoryhighlightedwiththereddottedline.Thecorrespondingt-ddiagram(rightpanel)estimatesanapparentpropagationspeedof
∼110kms−1.
ward propagating signals in RBEs and RREs. Furthermore, nificantlyenhancethegrowthrateofKHI.
as noted by Lipartito et al. (2014), standing waves require a ThegrowthrateofKHIofatransversallymovingmagnetic
finite time to set up. This time is estimated as t ∼ l/v s structure can be easily estimated in the case of slab geom-
A
(Judgeetal.2012),wherelandv isthestructure’slengthand etry. The dispersion relation of antisymmetric incompress-
A
chromospheric Alfve´n speed, respectively. RREs and RBEs ibleperturbationsforamagneticslabofhalf-widthdmoving
havetypicallengthsof∼3Mm,andv isestimatedas∼20-80 transversallyinitsplanecanbewrittenas(Zaqarashvili2011)
A
km s−1 in the chromosphere. Hence t is ∼40 - 150 s which (cid:104) (cid:105)
iscomparable/greaterthantheirtypicallifetime(∼40s). Itis ρ (ω−k·U)2−(k·V )2 coth(kd)+
0 A0
therefore doubtful that standing wave patterns, generated by
(cid:104) (cid:105)
thesuperpositionofoppositelydirectedwaves, areresponsi- +ρ ω2−(k·V )2 =0 (1)
blefortheobservedfastappearanceoftheRREs/RBEs. e Ae
ThemainaspectofthereportedtransversemotionsinRREs where ρ , ρ , and V , V are the densities and Alfve´n
0 e A0 Ae
and RBEs is that the structure is displaced as a whole, i. e. speeds inside and outside the slab, respectively, U is the un-
every single part of their visible length is moved in phase, perturbed velocity (transverse velocity of flux tube in our
in the same direction and with the same speed. These mo- case) and k is the wave vector. For symmetric perturbations
tions (vtr⊥) are detected in the image plane. However, it is coth(kd) should be replaced by tanh(kd). For |kd| > 1, both
expected that the motions should also have a LOS compo- coth(kd) and tanh(kd) are constant and equal to unity. Then
nent (vtrLOS) and some should also be polarised in the LOS Eq. (1)issimplifiedandusingk·VA0 =k·VAe ≈0wegetthe
plane. IfthefluxtubesstarttomovealongtheLOSinasim- imaginarypartofperturbations,whichisactuallythegrowth
ilar way to what is detected in the image plane (Figure 3), rate, as ω ≈ kU(cid:112)ρ /ρ . Thus the growth time of KHI de-
then they should appear suddenly in the Hα line wings as i e 0
pendsonthetransversevelocity,thedensitycontrastandthe
redshifted or blueshifted features depending on the direction
spatialscaleofperturbations. Atransversevelocityof20km
of the LOS velocity with respect to the observer. We pro- s−1, aspatial scale of theorder of tube widthof 200 km and
pose that the observed RREs and RBEs are manifestations thedensitycontrastofρ /ρ =0.1leadstothegrowthtimeof
e 0
ofthechromosphericfluxtubestransversemotionsalongthe
5s.
LOS. Similar to the transverse motions in the image plane,
The generated KH vortices may lead to enhanced MHD
LOS Doppler signals and thus the associated transverse mo-
turbulence near the tube boundaries through nonlinear cas-
tions do not show any evidence of periodic behaviour, e.g.
cade. Shear flow energies then can be rapidly transferred to
periodic variation or the change of the flux tubes Doppler-
small scales, where it will dissipate and heat the surround-
shift sign. It must be noted that the obtained median values
ing plasma. This explains the fast heating of the chromo-
of v (∼27-28 km s−1 for RREs and RBEs, respectively)
trLOS sphericfluxtubesandhenceobservedsuddendisappearance
are higher compared to the median of vtr⊥ (∼14-17 km s−1 of RREs/RBEs. However, this is a speculative interpretation
forRREsandRBEs,respectively). However,astheanalysed anddetailednumericalsimulationsarerequiredtoassessthe
structuresaredetectedatthefarredandbluewingpositions importanceofKHIonthestabilityofthesefeatures.
ofHα(±0.906Åcorrespondingto±41kms−1),v derived
trLOS
from spectroscopic observations are biased toward detecting 4.4. Numericalmodelling
higher velocity events, which may be the reason for the dis-
We carried out numerical modelling using the radiative
crepancybetweentheaveragevaluesofv andv .
trLOS tr⊥ MHD code MuRAM (Vo¨gler et al. 2005). In a numerical
domain, which covers the top 2 Mm of the convection zone
4.3. Stability
and the bottom 1.2 Mm of the solar atmosphere, transparent
TransversemotionsoftheRREs/RBEscancreatevelocity bottom and semi-transparent (allowing outflows) top bound-
discontinuities between the surface of the flux tube and sur- ary conditions were used. A non-magnetic convection snap-
roundingmedia,whichmaytriggertheKelvin-HelmholtzIn- shot was employed as an initial configuration, where 200 G
stability (KHI). The component of the magnetic field paral- uniform unipolar vertical magnetic field was injected. The
lel to the discontinuity tends to stabilize sub-Alfve´nic flows numerical domain was then let to develop for about 20 min-
(Chandrasekhar 1961), while the perpendicular component utes of physical time. One of the developed snapshots of
hasnoeffectonKHI(Sen1963). Thissuggeststhatthetube thesimulationwasusedtocarryoutHαdiagnosticswiththe
moving with any speed at an angle to the magnetic field is NICOLEcodeinnon-LTE(Socas-Navarroetal.2014). The
unstabletoKHIforsufficientlyhighazimuthalwavenumber diagnostics showed elongated features similar to RBEs with
m (Zaqarashvili et al. 2014). The observed velocities of the lifetimes of ∼ 40 s and a length of few Mm, naturally ap-
LOStransversemotionsoftheRREs/RBEs(∼21-34kms−1) pearing in the simulations (see Figure 6, first panel). These
couldbehigherthanthelocalAlfve´nspeed, whichmaysig- featuresareproducedbystrong,short-livedlocalisedplasma
DynamicsofchromosphericRapidRedshiftedandBlueshiftedExcursions 7
Figure6. ExampleofasimulatedRBE.Firstpanel: IntensityintheHαbluewing. Secondpanel: line-of-sightvelocityfortheaverageRBE(solidcurve)
comparedtoregionsnotshowingtheRBE(dashedcurve).Thirdpanel:temperatureprofilesforthesameaverages.Fourthpanel:averageHαStokes-Iprofiles
forRBE(solidcurve)andnon-RBE(dashedcurve). Theaveragingwascarriedoutfor50randomRBEandnon-RBElocationsinthesimulationdomain. The
horizontalflowstructurearoundthesimulatedRBEisshownwitharrowsinthefirstpanel.
motionsupwardalongthefieldlinesofmagneticfluxconcen- (F-CHROMA).ThisresearchwassupportedbytheSOLAR-
trations in the low-β plasma (Figure 6, second panel). The NET project (www.solarnet-east.eu), funded by the Euro-
RBEsobservedinthesimulationsdonotappearalignedwith pean Commissions FP7 Capacities Program under the Grant
themagneticfield,howevertheylieatconstantAlfve´nicsur- Agreement 312495. The work of T.Z. was supported by
faces and are possible signatures of dissipation of torsional FP7-PEOPLE-2010-IRSES-269299 project- SOLSPANET,
Alfve´n waves (Shelyag et al. 2013; Shelyag & Przybylski by Shota Rustaveli National Science Foundation grant
2014, also Figure 6, third panel). Despite some qualitative DI/14/6-310/12 and by the Austrian Fonds zur Frderung der
similarities between the observations and the simulated Hα wissenschaftlichenForschungunderprojectP26181-N27. Dr
lineprofiles(seeFigure6,fourthpanel)aswellastheirtem- Shelyag gratefully thanks Centre for Astrophysics & Su-
poral and spatial characteristics, it is still not fully clear if percomputing of Swinburne University of Technology (Aus-
the features seen in the simulations are the observed RBEs. tralia), NCI National Facility systems at the Australian Na-
There is a significant difference between the shape of simu- tionalUniversity,supportedbyAstronomyAustraliaLimited,
lated and observed RBEs, which may be a result of forcing and the Multi-modal Australian ScienceS Imaging and Vi-
the magnetic field to be vertical at the top of the simulation sualisation Environment (MASSIVE) for the computational
domain. However, if it is assumed that the observed RBE resources provided. Dr Shelyag is the recipient of an Aus-
structurestrackthefieldlinesinstronglyinclined(relativeto tralianResearchCouncilsFutureFellowship(projectnumber
the LOS) magnetic field concentrations, the torsional veloc- FT120100057).
ity component would cause the red- and blueshift of the Hα
profile. Atorsionalvelocitycomponentispresentintheflux
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