Multi-spectral analysis of fine scale aurora

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HANNADAHLGREN

Do toral Thesis

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TRITA-EE 2010:040

ISBN978-91-7415-749-9

SE-10044Sto kholm

SWEDEN

AkademiskavhandlingsommedtillståndavKunglTekniskahögskolanframlägges

tilloentliggranskningföravläggandeavDoktorsexamenifysikaliskelektroteknik

fredagenden22oktober2010klo kan13.15iH1,Teknikringen33,KTH,Sto kholm.

©HannaDahlgren,2010

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Abstra t

The Aurora Borealis is the visible manifestation of the omplex plasma in-

tera tionbetween thesolar windand theEarth's magnetosphereandionosphere.

Groundbasedandinsitu measurementsdemonstrateaprevalen eofdynami ne

stru ture within auroral displays, with spatial s ales down to tens of metres and

timevariations o urring onafra tion of ase ond. Thene-s ale morphology is

related to stru turing of auroral urrentsand ele tri elds and detailed spatial,

spe traland temporal observationsof theauroraare ru ialin understanding the

ele trodynami pro essestaking pla ein theionosphere andinits ouplingto the

magnetosphere.

In this thesis, the low-light opti al instrument ASK (Auroral Stru ture and

Kineti s)isusedtoimagesmall-s alestru turesin theauroraatveryhighspatial

andtemporalresolution. ASKisamulti-spe tralinstrument,imagingtheaurorain

threesele tedemissionssimultaneously. Thisprovidesinformationontheenergyof

thepre ipitatingele trons. TheSIF(Spe trographi ImagingFa ility)instrument

has been used in onjun tion with ASK, to givea more omplete pi ture of the

spe tral hara teristi softheaurora,andtodeterminethedegreeof ontaminating

emissions present in the same spe tral intervalas the emission lines observed by

ASK.

Data from ASKand SIF are usedto study therelation betweenthe morphol-

ogy and dynami s of small-s ale stru tures in the aurora and the energy of the

pre ipitating ele trons. By omparing ele tron density proles provided by EIS-

CAT (EuropeanIn oherentSCATter)radarmeasurementswithmodelingresults,

information on the hara teristi energy and the energy ux of thepre ipitating

ele trons anbeobtained. OneoftheASK hannels is imagingametastableO

+

emission,whi hhasalifetimeofabout5s. Bytra ingtheafterglowinthis hannel

opti allyadire tmeasureoftheE×^B^driftîsôbtained^from^whi
h^the^lo
alîono-

spheri ele tri eld anbe al ulated. ASKdatahasalsobeenusedtoanalysethe

propertiesofadistortedauroralar ,inwhi hauroralstru turingwasfoundtotake

pla e simultaneouslyat dierent spatial s ales. The smallestfeatures, `rus', are

undulationsfoundtodevelopontheedgeofanauroral url,foldorshear. Detailed

opti al studies of bla k aurora, in luding boththe type whi h is asso iated with

plasma shear motionsand no or weak shear motions were ondu ted from ASK

data,to investigatethespe tralpropertiesand nes alemorphologyof thebla k

stru turesand toshedlightonthepro essesbehindthisphenomenon.

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Sammanfattning

Aurora Borealis är den synbara manifestationen av en komplex plasmaväx-

elverkan mellan solvinden o h jordens magnetosfär o h jonosfär. Markbaserade

o h in situ mätningar har visat på enallmän förekomstav dynamisk nstruktur

inomnorrskenet,medskalstorlekarpånertilletttiotalmetero htemporalavaria-

tionerpåenbråkdelavensekund. Norrskenetsnskaligamorfologiärkopplattill

struktureringavnorrskensströmmaro helektriskafält. Förattförstådeelektrody-

namiskapro essernasompågårijonosfäreno hidesskopplingtillmagnetosfärenär

detaljeradeobservationermedhögrums-,tids-o hspektralupplösningnödvändiga.

I dennaavhandlinganvändsdetljuskänsligaoptiskainstrumentet ASK(Auro-

ralStru tureand Kineti s)för attavbilda norrskenets småskaligastrukturer med

väldigt hög rumslig o h temporalupplösning. ASK ärett multi-spektraltinstru-

mentsomavbildar norrskeneti treutvaldaemissionersamtidigt. Detta gerinfor-

mationomenergiinnehållethosdepre ipiterandeelektronernasomgerupphovtill

norrskenet. SIF (Spe trographi Imaging Fa ility) instrumentet har använtstill-

sammansmed ASK, för att ge en merkomplettbild av spektralakaraktären hos

norrskenet,o hförattbestämmagradenavkontaminationiASKdatatfrånandra

emissioner.

DatafrånASKo hSIFanvändsförattstuderaförhållandetmellannorrskenets

morfologio h dynamikpå småskalor o h den energi som depre ipiterande elek-

tronerna besitter. Genom att jämföra elektrontäthetsproler som fås från mät-

ningarmedEISCAT(EuropeanIn oherentSCATter)radarmedmodellprolerav

elektrontäthetenärdet möjligtatt bestämma den karaktäristiskaenergin o h en-

ergiödet hos de pre ipiterande elektronerna. En av ASKs kanaler avbildar en

metastabil emission från O

+

, som har en livslängd på omkring 5 s. Genom att

optiskt spåra det emitterande plasmatfås ett direkt mått påE ×^B ^driften ^o
h

därmedävenpådetlokalaelektriskafältetijonosfären. DatafrånASK haro kså

använts för att analyseraegenskaperna hos en förvrängd norrskensbåge som up-

pvisadestrukturering påolikarumsligaskalor samtidigt. Deminstastrukturerna,

`rus',ärvågigaformersomutve klaspåkantenavnorrskensformersåsom` urls',

`folds'o h områdenmed starkamotriktade plasmaöden. Detaljerade studier av

svartnorrsken,såväl detsom ärkopplat, o h detsom saknar koppling till starka

motriktadeplasmaödenharkunnat görasmed datafrån ASK, förattundersöka

despektralaegenskapernao hdennskaligamorfologinhossvartanorrskensstruk-

turero hförattbelysadepro essersomgerupphovtill dettafenomen.

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Thisthesisisbasedontheworkpresentedinthefollowingpapers.

I. H. Dahlgren, N.Iv henko,J.Sullivan,B.S. Lan hester,G.Marklund and

D.Whiter

Morphologyanddynami sofauroraatnes ale: rstresultsfromtheASK

instrument

Ann. Geophys.,26,1041-1048,2008

II. H. Dahlgren, N. Iv henko, B. S. Lan hester, J. Sullivan, D. Whiter,

G.MarklundandA.Strømme

Using spe tral hara teristi sto interpret auroralimagingin the731.9 nm

O

+

line

Ann. Geophys.,26,1905-1917,2008

III. H. Dahlgren, N. Iv henko, B. S. Lan hester, M. Ashra, D. Whiter,

G.MarklundandJ.Sullivan

Firstdire topti alobservationsofplasmaowsinthemetastableO

+

ionin

dis reteaurora

J. Atmos. Solar-Terr. Phys.,71,228-238,2009

IV. H. Dahlgren, A. Aikio, K. Kaila, N. Iv henko, B. S. Lan hester,

D.K.WhiterandG.T.Marklund

Simultaneousobservationsofsmallmulti-s ale stru turesinanauroralar

J. Atmos. Solar-Terr. Phys.,72,633-637,2010

V. J.Ar her,H. Dahlgren,N.Iv henko,B.S.Lan hesterandG.T.Marklund

Dynami sand hara teristi sofbla kauroraasobservedbyhighresolution

ground-basedimagersandradar

A eptedby Int. J. RemoteSens.,April 2010

HannaDahlgrenhad themain responsibilityforPapersI,II,III andIV.This

in ludedtakingpartinrunningandmaintainingtheinstruments, alibrationofthe

opti aldataandperformingthedataanalysis,aswellastakingmainresponsibility

fordis ussingtheresultsandwritingthepapers. PaperVistheresultofthemaster

thesisproje tbyJennyAr her,forwhi hHannaDahlgrenwasthesupervisorand

parti ipatedin thedataanalysis,interpretationands ienti dis ussions.

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Papers that are not in luded in this thesis

Refereed Journals

Chronologi al order

VI. H. Dahlgren,P.CarlqvistandG.F.Gahm

Filamentarystru turesinplanetarynebulae

Astrophys. Spa e S i.,310,Issue1-2,65-72,2007

VII. J. M.Sullivan, M. Lo kwood, B. S. Lan hester, E. P. Kontar, N. Iv henko,

H. DahlgrenandD.K.Whiter

An opti al study of multiple NEIAL events driven by low energy ele tron

pre ipitation

Ann. Geophys.,26,2435-2447,2008

VIII. D. K.Whiter, B.S. Lan hester, B.Gustavsson,N. Iv henko,J.M. Sullivan

andH. Dahlgren

Small-s alestru turesin i keringaurora

Geophys. Res. Lett.,35,L23103,2008

IX. R.Cosgrove,M.Ni olls,H.Dahlgren,S. Ranjan,E.San hezandR.Doe

Radar dete tion of a lo alized 1.4 Hz pulsation in auroral plasma, simul-

taneous with pulsating opti al emissions, and Pi1B magneti pulsations, at

substormonset

SubmittedtoAnn. Geophys.,2010

X. D. K. Whiter, B. S. Lan hester, B. Gustavsson, N. Iv henko and

H. Dahlgren

Usingmultispe tralopti alobservationsto identifythea elerationme ha-

nismresponsiblefori keringaurora

A eptedby J.Geophys. Res.,2010

XI. K.A.Lyn h,M.Mella,D.Hampton,H.C.Stenbaek-Nielsen,H.Dahlgren,

M.Disbrow,P.M.Kintner,M.LessardandE.Lundberg

Stru ture and dynami s of auroralele tron pre ipitation: sounding ro ket

observationsoftallraysat thenightsidepolewardboundary

SubmittedtoGeophys. Res. Lett.,2010

XII. M. Mella, K. A. Lyn h, P. M. Kintner, E. T. Lundberg, M. Lessard,

D.L.Hampton,H.C.Stenbaek-NielsenandH. Dahlgren

Soundingro ket studyofanauroralpolewardboundaryintensi ation

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Conferen e Pro eedings

XIII. O.Jokiaho,N. Iv henko,H. Dahlgren,D. WhiterandM.Alaniz

Auroraldiagnosti sforPoGOLiteastrophysi alballoon

Pro eeding for19th ESAsymposium,2009

Popular S ien e Journals

XIV. H. Dahlgren

Jaktenpåsvartnorrsken

PopulärAstronomi, 2,12-17,June2010

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List ofPapers v

Refereed Journals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Conferen ePro eedings . . . . . . . . . . . . . . . . . . . . . . . . . vii

PopularS ien eJournals. . . . . . . . . . . . . . . . . . . . . . . . . vii

Contents viii ListofFigures x ListofA ronyms xv A knowledgements xvii 1 Introdu tion 1 2 Spa e PlasmaPhysi s 3 2.1 Earth'sMagnetosphere. . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Earth'sIonosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Magnetospheri Conve tionandCurrentSystems . . . . . . . . . . . 8

2.4 Ele tri FieldsAroundanAuroralAr . . . . . . . . . . . . . . . . . 13

2.5 A elerationme hanisms. . . . . . . . . . . . . . . . . . . . . . . . . 14

2.5.1 Inverted-Vstru tures . . . . . . . . . . . . . . . . . . . . . . 15

2.5.2 Alfvénwaves . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3 The Aurora 21 3.1 Physi al Pro essesintheHigh-LatitudeIonosphere . . . . . . . . . . 21

3.2 Someimportantopti alemissionsinaurora . . . . . . . . . . . . . . 24

3.2.1 Atomi oxygenemissionat5577Åthegreenline . . . . . . 24

3.2.2 Atomi oxygenemissionat6300Åtheredline . . . . . . . 25

3.2.3 Mole ularnitrogen emissionat 4278Åtheblueline . . . . 26

3.2.4 Mole ularoxygenemissionat5632Å. . . . . . . . . . . . . . 26

3.2.5 Atomi oxygenemissionat7319/7330Å . . . . . . . . . . . . 26

3.2.6 Mole ularnitrogen emissionat 6730Å . . . . . . . . . . . . . 29

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3.2.7 Atomi oxygenemissionat7774Å . . . . . . . . . . . . . . . 29

3.3 Global AuroralMorphology . . . . . . . . . . . . . . . . . . . . . . . 29

3.4 TheAuroralSubstorm . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.5 Morphologyofsmall-s aled aurora . . . . . . . . . . . . . . . . . . . 30

3.5.1 Curls,folds andrus. . . . . . . . . . . . . . . . . . . . . . . 31

3.5.2 Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.5.3 Bla kaurora . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.5.4 Auroralthin laments . . . . . . . . . . . . . . . . . . . . . . 37

4 Instrumentation 41 4.1 AuroralStru tureandKineti s-ASK . . . . . . . . . . . . . . . . . 41

4.1.1 InstrumentalDesignofASK. . . . . . . . . . . . . . . . . . . 42

4.1.2 Spe tralsele tionbyinterferen elters . . . . . . . . . . . . 43

4.2 Spe trographi ImagingFa ility-SIF . . . . . . . . . . . . . . . . . 45

4.3 EuropeanIn oherentS atterRadar-EISCAT . . . . . . . . . . . . 48

5 Data Analysis Tools 53 5.1 OperationStrategy andDataHandling. . . . . . . . . . . . . . . . . 53

5.2 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.2.1 EMCCDandSNR . . . . . . . . . . . . . . . . . . . . . . . . 55

5.2.2 AbsoluteIntensityCalibrationforASK . . . . . . . . . . . . 57

5.3 Opti alFlowAnalysis . . . . . . . . . . . . . . . . . . . . . . . . . . 59

5.4 Ele tronTransportandIon- hemistryModelling . . . . . . . . . . . 61

5.5 Two-dimensionalEnergyMapping . . . . . . . . . . . . . . . . . . . 62

6 Resultsand Dis ussion 67 6.1 PaperI-Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

6.2 PaperII -Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

6.3 PaperIII -Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

6.4 PaperIV-Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

6.5 PaperV-Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

7 Future work 79

Bibliography 83

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1.1 The magni ent display of aurora, viewed from Toolik Field Station,

Alaska. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Earth'smagneti environmentandsomeof itsintrinsi regions. Image

redit: EuropeanSpa eAgen y,ESA. . . . . . . . . . . . . . . . . . . . 7

2.2 Heightvariationoftemperature,iondensityand neutraldensityin the

atmosphere,afterRees(1989). . . . . . . . . . . . . . . . . . . . . . . . 8

2.3 Typi alele trondensityprolesintheionosphereduringsolarminimum

andmaximum,fordaytimeandnight-time. FromRi hmond(1987). . . 9

2.4 Magnetospheri onve tionofplasma,drivenbyre onne tion. Thenum-

bersindi ate the timeevolutionof thegeomagneti eld linesafter re-

onne tionofanIMFeldline(1')atthefrontsidemagnetopause. The

ionospheri footprintoftheeldlinesatthedierenttimesarealsogiven

asthe orresponding numbersin the inserted gurein thelowerright

orner. Figure fromKivelson&Russell(1995). . . . . . . . . . . . . . . 11

2.5 Sket hofhorizontalandeld-aligned urrentsintheionosphere. . . . . 12

2.6 Auroralele tri eld lassi ation. FromMarklund(1984). . . . . . . . 15

2.7 Anegatively hargedpotentialstru ture(left)isa eleratingele trons,

produ ing inverted-Vaurora. A positively harged potentialstru ture

(right),representativeoftheauroralreturn urrent,isa eleratingthe

ele tronsaway from Earth, to lose the auroral urrent ir uit. From

Marklundetal.(2001). . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1 Ionisation rates as a fun tion of altitude in the ionosphere aused by

pre ipitatingele tronswithauxof10

8

ele trons m

−2

s

−1

anddierent

initialenergies. FromPas hmannetal.(2003),afterRees(1963). . . . 22

3.2 PartialenergyleveldiagramsforO,illustratingthe5577Åand6300/6364Ådou-

bletdis ussedinthisthesis. After(Chamberlain,1961). . . . . . . . . . 25

3.3 Volumeemissionrateinphotons/s/ m

3

oftheO

+

2 ^1N(1,0)^band^system. ²⁷

3.4 O

+

2 ^1N^band,ⁱⁿ^the^spe
tral^region⁵⁶⁰⁰⁵⁶⁴⁰^Å. ^Spe
trum^re
orded

byHiTIES,on26January2006. . . . . . . . . . . . . . . . . . . . . . . 27

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3.5 Grotrian diagram of 7319/7330Å emission lines, originatingfrom the

O

+ 2

Pstate(afterStephanet al.(2003)andChamberlain(1961)). . . 28

3.6 CurlformationobservedbyVogtetal.(1999)on7De ember2004with

asmall-angle(21

◦ ×¹⁵^◦⁾âmera. ^Theâmera^wasôperatedât²⁵ ^Hz,

andshownherefrom leftto right,toptobottom areimagesfrom 25 s

ofdata(oneframefromea hse ond). . . . . . . . . . . . . . . . . . . . 32

3.7 Distributionofwavelengthsofauroral url systems. FromTrondsen&

Cogger(1998). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.8 ASK3 image, showingthe edge of a fold, whi h is moving to the left

(markedby thebla k arrow),and thesmallerrus moving tothe east

(white arrows). A full imagesequen e ispresentedin paperIVand in

thepagefooterofthisthesis. . . . . . . . . . . . . . . . . . . . . . . . . 34

3.9 Auroralrays. Theraysareextendinglikea oronaalongthegeomagneti

eld lines from the lo al magneti zenith. The image is taken on 20

Mar h2009atToolikFieldStation,Alaska,withanAVIGuppy amera

witha6450Åpassbandlter,to utoutlonglivedemissionsat shorter

wavelengths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.10 Time sequen efrom theASK3imager, showingthepassageofabla k

auroralring o urring on 9Mar h 2007. Timeruns from left to right,

toptobottom. The ontrastbetweenthebla kstru tureandtheba k-

groundemission anbeseentoin reasewithtime. . . . . . . . . . . . . 37

3.11 Distributionofmeasurementsofauroralthi knesses,measuredbyMaggs

&Davis(1968). Theresolutionlimitis70m. . . . . . . . . . . . . . . . 38

3.12 Fines aleaurorain ASK3(O), apturedfromTromsøon15De ember

2006. The narrowest features have widths of less than 70 m, when

assuminganauroralaltitudeof105km. . . . . . . . . . . . . . . . . . . 39

4.1 S hemati oftheASKsystem. Fordetails,seethetext. . . . . . . . . . 43

4.2 a)Asket hoftheASKassembly,withthebu ketmountedonasteerable

pivotontopofthe omputerbox. The omputerbox ontainsthethree

omputers onne tedtothe ameras. Afourth omputer,themaster,is

onne tedtothese in theEISCATtransmitter hall. b)A sket h ofthe

ASKbu ket,withtheAndor iXonimager marked inyellow,theKowa

lensand interferen elterin green,Galilean onverterin blueand the

twophotometersinred. . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.3 Theltertransmission urves(red),forthefourASKltersusedinthis

thesis. Filter 1: TheO

+

2 ^1N^(1,0)^band^emissions^measured^by^SIF^are

plottedinblue. Filter2:ModelledN2^1P^(5,2)^and^N2^1P^(4,1)^bands^are

shown, together with the onvolved proles of these bands (blue solid

line). With thanks to Olli Jokiaho for supplying the modelled data.

Filter 3: The blue spe tra is measured by SIF during a time period

withlowenergypre ipitationwiththeoxygenlinesaround7320Åand

7330Åevident. Filter 4: Theoxygenemission lineat7774Åisin fa t