Magnetic Tongues, Magnetic Helicity and Twist in Active Regions.
Magnetic Helicity Generation Inside the Sun
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Transcript of Magnetic Helicity Generation Inside the Sun
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Magnetic Helicity Magnetic Helicity Generation Inside the SunGeneration Inside the Sun
Dana LongcopeDana Longcope
Montana State UniversityMontana State University
Thanks: Alexei PevtsovThanks: Alexei Pevtsov
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Observations show a clear hemispheric Observations show a clear hemispheric asymmetry in the helicity of the coronal asymmetry in the helicity of the coronal magnetic field: magnetic field: HHRR < 0 in the North < 0 in the North
Q: Can we therefore conclude that field Q: Can we therefore conclude that field below the solar surface, and in the below the solar surface, and in the dynamo, has this same asymmetry?dynamo, has this same asymmetry?
Answer: NoAnswer: No
Magnetic Helicity Magnetic Helicity Generation Inside the SunGeneration Inside the Sun
Propagation fromPropagation from
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• Observed trends in photospheric twistObserved trends in photospheric twist• Implications for state of CZ flux tubesImplications for state of CZ flux tubes• Coupling of twist to coronal fieldCoupling of twist to coronal field• Observational evidence in emerging ARObservational evidence in emerging AR
Magnetic Helicity Propagation from Inside
the Sun
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Trend in photospheric Trend in photospheric twisttwist
466 ARs from Longcope & Pevtsov 2003
Trend:Trend:bestbest< 0< 0in Northin North
bestbest> 0> 0in Southin South
Correlation:Correlation:bestbest
w/ w/ latitudelatitude> 99.9999%> 99.9999%
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Fluctuations in twistFluctuations in twist
Large latitude-Large latitude-indep’t scatter indep’t scatter
created by created by turbulenceturbulence
Linear trend removed Linear trend removed (from Longcope, (from Longcope, Fisher & Pevtsov Fisher & Pevtsov
1998)1998)
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The origin of fluxThe origin of flux
(from Cauzzi et al. 1996)
Bipolar active regionBipolar active regionformed by emergence offormed by emergence ofFLUX TUBEFLUX TUBEfrom below photospherefrom below photosphere
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Twist in flux tubesTwist in flux tubes
Axis of tube:Axis of tube: xx(s)(s)satisfies thinsatisfies thinflux tubeflux tubeequationsequations(Spruit 1981)(Spruit 1981)
Field lines Field lines twisttwist about axis at a rate about axis at a rate q(s,t) “=“ d/ds
Plasma Plasma spinsspins about axis at rate about axis at rate(s,t) “=“ d/dt
ss
ss
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Dynamics of twistDynamics of twist
dt
da
as
q
dt
dA
2v2
Angular Angular momentum:momentum:
UnbalanceUnbalanced magneticd magnetic
torquetorque
(from Longcope & Klapper 1997)
q(s)q(s)
(s)(s)
ss
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Dynamics of twistDynamics of twist(from Longcope & Klapper 1997)
q(s)q(s)
(s)(s)
s
qssdt
dq
v
ksv
s ˆˆ
Differential spinningDifferential spinning
Field line Field line KinematicsKinematics ss
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Dynamics of twistDynamics of twist(from Longcope & Klapper 1997)
q(s)q(s)
(s)(s)
s
qssdt
dq
v
ksv
s ˆˆ
Differential spinningDifferential spinning
Field line Field line KinematicsKinematics ss
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s
qssdt
dq
v
ksv
s ˆˆ
Dynamics of twistDynamics of twist
2
22A2
2
vdt
d
s
Torsional Alven waves
dt
da
as
q
dt
dA
2v2
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Dynamics of twistDynamics of twist(from Longcope & Klapper 1997)
q(s)q(s)
vvss(s)(s)
ss
s
qssdt
dq
v
ksv
s ˆˆ
Field line Field line KinematicsKinematics
Axial stretchingAxial stretching
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Dynamics of twistDynamics of twist(from Longcope & Klapper 1997)
q(s)q(s)
vvss(s)(s)
ss
s
qssdt
dq
v
ksv
s ˆˆ
Field line Field line KinematicsKinematics
Axial stretchingAxial stretching
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s
qssdt
dq
v
ksv
s ˆˆ
Dynamics of twistDynamics of twist
Out-of-Out-of-planeplanemotion of motion of axisaxis
ss
indep. of q or indep. of q or
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Source of TwistSource of Twist
dt
dWr
dt
dTw
Increasing LHIncreasing LHwrithewrithe (dWr/dt <0 ) (dWr/dt <0 ) Increasing RHIncreasing RHtwisttwist (dTw/dt > 0) (dTw/dt > 0)
0)ˆ(
sdt
dq vks
Helicity Helicity ConservatiConservati
onon
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0 vv 0)ˆ(
s
vks
• Applies to Applies to mean mean fieldsfields• Creates Creates HelicityHelicity** • RHRH eddies eddies LH LH fieldfield
• Applies to Applies to flux flux tubestubes• Creates Creates TwistTwist • RHRH eddies eddies RHRH twisttwist
BB
JJ
RHRH
effecteffect effecteffect
* in the mean field* in the mean field
J
B
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Manifestation of -effect
• Simulation of Simulation of rising fluxrising flux tubestubes• Large scatter Large scatter • Latitude-Latitude-indep. indep.
( Longcope, Fisher & Pevtsov 1998 )( Longcope, Fisher & Pevtsov 1998 )
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Coupling flux tube to Coupling flux tube to coronacorona
CZCZ: : >> 1 >> 1
coronacorona: : << 1 << 1
I=0I=0
I=0I=0
photospherephotosphere
surfacesurfacecurrentscurrents
(force-free field)(force-free field)
(thin flux tube)(thin flux tube)
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Coupling flux tube to Coupling flux tube to coronacorona
q(s)
(Longcope & Weslch 2000)
Radial shuntingRadial shunting
torquestorques = 0= 0
s
q
2Av
t
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Coupling flux tube to Coupling flux tube to coronacorona
q(s)
Low inertia Low inertia
torquestorques = 0= 0 Current matchesCurrent matches across interfaceacross interface
(Longcope & Weslch 2000)
02
zq
Coronal “twist”Coronal “twist”
Twist at end of FTTwist at end of FT
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Application to Emerging Application to Emerging ARAR(Longcope & Welsch 2000)(Longcope & Welsch 2000)
Model AssumptionsModel Assumptions
• Initial flux tube: uniformly twisted:Initial flux tube: uniformly twisted: qq(s)=(s)=• Poles separating:Poles separating: d(t) = dd(t) = d00 + v (t-t + v (t-t00))
Twist Twist propagates propagates into coronainto corona
(t)(t)d/vA ~ 1 day
Model AssumptionsModel Assumptions
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Application to Emerging Application to Emerging ARAR(Pevtsov, Maleev & Longcope 2003)(Pevtsov, Maleev & Longcope 2003)
)1(
0
11
)(
d
dt
• Initial flux tube: uniformly twisted: qInitial flux tube: uniformly twisted: q(s)=(s)=• Poles separating:Poles separating: d(t) = d d(t) = d00 + v (t-t + v (t-t00))
• Uniform Alfven speed in tubeUniform Alfven speed in tube: : vvAA= = vv• Coronal helicity:Coronal helicity: H = H = d d 22
Model AssumptionsModel Assumptions
SolutionSolution
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Observational EvidenceObservational Evidence(Pevtsov, Maleev & Longcope 2003)(Pevtsov, Maleev & Longcope 2003)
• Study 6 ARs during emergenceStudy 6 ARs during emergence• FindFind d(t)d(t)• (t)(t) 8/19 12:478/19 12:47 8/19 20:478/19 20:47 8/20 4:478/20 4:47
8/20 12:478/20 12:47 8/20 20:478/20 20:47 8/21 4:478/21 4:47
dd
AR9139AR9139SOHO MDISOHO MDI2000-8-19 2000-8-19
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Observational EvidenceObservational Evidence(Pevtsov, Maleev & Longcope 2003)(Pevtsov, Maleev & Longcope 2003)
Fit Model to DataFit Model to Data
)1(
0
11
)(
d
dt
v=264 m/sv=264 m/s
= 2 10= 2 10-8-8 m m-1-1
vvAA = 158 m/s = 158 m/s
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Observational EvidenceObservational Evidence(Pevtsov, Maleev & Longcope 2003)(Pevtsov, Maleev & Longcope 2003)
AR8582AR8582
AR8817AR8817
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Implications of modelImplications of model• TwistTwist exists exists beforebefore emergence emergence (i.e. rising tube is twisted) (i.e. rising tube is twisted) • Tube Tube TwistTwist propagates into corona propagates into corona Coronal HelicityCoronal Helicity
II
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Implications of modelImplications of model• Twist Helicity Twist Helicity q(s) q(s) 22 ~ ~ I(s)I(s) uniform uniform• Twist Twist fills infills in lengthening region lengthening region It It DOES NOTDOES NOT favor wider portion favor wider portion
Parker 1979Parker 1979
• Assumes p(r)=constantAssumes p(r)=constant• Predates Berger & Field Predates Berger & Field • No BG coronal fieldNo BG coronal field
Longcope & Welsch 2000Longcope & Welsch 2000
• Assumes Assumes >>1 >>1 <<1<<1• Conserves HelicityConserves Helicity• Includes BG coronal fieldIncludes BG coronal field
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Implications of modelImplications of model• Tube Tube WritheWrithe: : irrelevant irrelevant to coronato corona• Helicity Helicity dearth dearth propagates downwardpropagates downward
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Summary• Observed:Observed: Hemispheric trend Hemispheric trend in p-spheric twist in p-spheric twist coronal Hcoronal HRR
• Coronal HCoronal HR R fixed byfixed by TWISTTWIST of anchoring tube of anchoring tube
• -effect produces -effect produces TWISTTWIST in rising FT in rising FT BUT leaves BUT leaves helicityhelicity unchanged unchanged
• Observed:Observed: Helicity evolution in Helicity evolution in emerging AR consistent w/ thisemerging AR consistent w/ this
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Dynamics of twistDynamics of twist(from Longcope & Klapper 1997)
q(s)q(s)(s)(s)
ss
dt
da
as
q
dt
dA
2v2
Angular Angular momentum:momentum:
Changing tube Changing tube radiusradius(Michelle Kwan (Michelle Kwan effect)effect)
aa
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Coupling flux tube to Coupling flux tube to coronacorona
LowLow-- coronalcoronalEquilibrium: Equilibrium: FFFFFF
HighHigh-- CZCZField: twistedField: twistedThin flux tubeThin flux tube
InterfaceInterface
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Possible sources of twistPossible sources of twist1.1. Initial state of flux tube: Initial state of flux tube: q(s,0)q(s,0)
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Possible sources of twist1.1. Initial state of flux tube: Initial state of flux tube: q(s,0)q(s,0)
2.2. External flow External flow “twirls”“twirls” tube segment tube segment
Creates regions of opposing twistCreates regions of opposing twist
Requires anomalous “friction”Requires anomalous “friction” across flux tube surfaceacross flux tube surface
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Possible sources of twistPossible sources of twist1.1. Initial state of flux tube: Initial state of flux tube: q(s,0)q(s,0)
2.2. External flow External flow “twirls”“twirls” tube segment tube segment
3.3. Net current driven along flux tubeNet current driven along flux tube
Violates assumption of isolated flux tubeViolates assumption of isolated flux tube Cannot be a “thin flux tube”Cannot be a “thin flux tube”
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Axis-twist couplingAxis-twist coupling
sdt
dq
vks )ˆ(
Term required to conserve Term required to conserve H = Tw + WrH = Tw + Wr
dssqTw )(2
2
Function of twistFunction of twist
dssdt
dWr
vks )ˆ(
2
2
Function of axisFunction of axis
Kinematic eq. for twistKinematic eq. for twist depends on axis motiondepends on axis motion
dt
dWr
dt
dTw
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Photospheric twist w/o Helicity*Photospheric twist w/o Helicity*
* From the emergence of a flux tube with no net helicty* From the emergence of a flux tube with no net helicty
•Begin w/ straight untwisted tubeBegin w/ straight untwisted tube (H=0)(H=0)•External flows induce LH writheExternal flows induce LH writhe (dH/dt =0)(dH/dt =0)•Coupling term Coupling term RH twistRH twist
•Tube crosses photosphereTube crosses photosphere•Helicity is transported intoHelicity is transported into coronal fieldcoronal field•Current in coronal fieldCurrent in coronal field matches twsit in flux tubematches twsit in flux tube
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Writhe from Turbulence: The Writhe from Turbulence: The -effect-effect
s
e
vks )(
dkkFkc )(5
12
dkkFa cee )(
3
1vv
Twist sourceTwist source
Averaging over turbulence:Averaging over turbulence:
Spectrum of kinetic helicity
Compare to Compare to -effect:-effect:
2222 )(|| dkkEkkVariance of twist source:Variance of twist source: