onsets and flapping studied using a dynamic harris sheet model
DESCRIPTION
Onsets and Flapping Studied using a Dynamic Harris Sheet Model. M. Connors 1 , R. Lerner 1 , G. Jaugey 2 , B. Lavraud 3 , M. Volwerk 4 , R. L. McPherron 5 1 Athabasca U., 2 U. Joseph Fourier, 3 LANL, 4 Österreichische Ak. Wissensch. , 5 UCLA. - PowerPoint PPT PresentationTRANSCRIPT
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Onsets and Flapping Studied using a Dynamic Harris Sheet Model
M. Connors1, R. Lerner1, G. Jaugey2,
B. Lavraud3, M. Volwerk4, R. L. McPherron5
1Athabasca U., 2U. Joseph Fourier, 3LANL, 4Österreichische Ak. Wissensch., 5UCLA
STAMMS-2 Orleans, France September 2007 Image: Mikko Syrjäsuo
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Athabasca University Geophysical Observatory (AUGO)
54.72 N, 246.7 ECGM (2005) 62.0, 306.5L=4.55
Founded 2002(UCLA mag 1998)
A comprehensive observatory in western Canada ideally located for THEMIS conjunctions (see poster)
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Harris modelHarris model
Magnetic field :
where By=Bz=0
and Bo = Blobes
Current density :
derived from
Plasma sheet - Harris modelPlasma sheet - Harris model
0 zx JJ
Plasma sheet : hot plasma
n = 3 cm-3
Eth ~ 4200 eV
B ~ 10 nT
β ~ 6
z>0 => Bx>0 , z<0 => Bx<0
)tanh(L
ZoZBB ox
)²(sec)(0
0
L
ZoZh
L
BJy
)(0 BJ curl
Bo
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Method for Harris sheet fitting
• The Levenberg-Marquardt nonlinear fitting method was used for BX(Z0,L,B0) during the slow traversal of Cluster through the plasma sheet
• The Levenberg-Marquardt nonlinear fitting method was used for BX(Z0,L) with B0 held constant during flapping
• Thompson et al. (2005 JGR) used a similar fitting technique
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Cluster Plasma sheet crossing September 22th, 2001
Plasma sheet crossing at Plasma sheet crossing at ~ 5 UT~ 5 UT
B reverse
low |B|
more energized particles
Harris form
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Harris fitHarris fitAugust 3August 3rdrd, 2003, 2003
Results
Bo = B(lobes) = 26.8 nT
=> realistic ( Theory ~30 nT )
Zo = -1.64 Re
=> not centred (z axis)
=> movement of the plasma sheet in the Z direction => flapping
L = 0.63 Re
=> thickness of the plasma sheet
Questions
Are the parameters related ?
What influences the parameters ?
Bo
Zo
)tanh(L
ZoZBB ox
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Flapping studyFlapping study
flapping real crossing
Harris flapping study :
fit again using Z separation of the spacecraft rather than the Z motion over time
Bo is known ( Harris crossing study)
Harris fit to find L, Zo
awaited results : L constant and Zo function of time
Flapping affects fits during slow passage of Cluster through plasma sheet
BUT,
Plasma sheet crossing is too long flapping are very fast => Good Harris study could come from flap fits
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Flapping studyFlapping studyAugust 23August 23rdrd , 2002 , 2002
L ~ cst during flapping
Zo(t) => velocity of the flapping Vz
ResultsResults
L ~ 0.89 Re
Zo is a function of time => Vz
Questions
Vz same as Cluster data ?
The current is Harris current ?
=> Vz and J
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Flapping studyFlapping studyAugust 23August 23rdrd , 2002 , 2002
Vz (cluster data) > Vz fit
11 UT => +60 km.s-1
Current fit realistic (compared to the data in black)
Jfit(blue)
Vy ≈ or > Vz !! => First hypothesis (flap only in Z direction) false
our fit needs some changes !!
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Conclusions on FlappingConclusions on Flapping
Our method for the fit gives good results for the plasma sheet thickness, the Zo evolution and also the current.
Nevertheless, our hypothesis of only Z motion of the plasma sheet is false. It’s moving also at least in the Y direction.
We are working to improve our fit assuming that the plasma sheet can be tilted yet planar (3-d Harris fit).
This approach would differ from least-variance methods
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Statistical Trends
• 71 plasma sheet crossings in the public data set were fit
• The only significant trend was of B0 vs P
• L(B0) seemed to show the opposite behaviour than expected, thicker for higher lobe field
• The lack of relationships is likely due to use of overly long time periods
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Pressure Variation
• Beff is the lobe field as calculated from the measured B and particle data
• Ptotal also arises from this and shows an influence on B0
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Pressure Statistics
• The lobe field varies with solar wind pressure (ACE)
• We did not find other strong dependencies on solar wind parameters
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August 21 2002 ca. 8 UT Canada Conjugacy Event
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Active Period with good conjugacy to Churchill meridian at onset at 7.9 UT
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Ground Pi2 also starts 7.9 UT (MCMU is to west)
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Cluster Pi2 starts at 8.1 UT
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Features of 21 Aug 2002 event
• Plasmoid signature in Bz at onset time
• Flow reversal from –X bursts to +X bursts indicating tailward motion of the X-line
• Flapping after onset
• Current enhancement at onset
• Very clear activations of all signatures at onset
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B signatures: activation, flapping
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Plasmoid: bipolar BZ
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Flow direction changes
as neutral line
moves outward
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July 27 2003: Inference from Flapping
• Unusual evening Ps6/omega band event recorded by magnetometer and THEMIS test camera at Athabasca
• Cluster in morning sector; ground data sparse; spacecraft spacing very close
• Multiple flappings where BX goes to 0 but not beyond
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Bifurcated Current Sheet?
• Bx several times went to 0 but not beyond
• Could bifurcated current sheet be related to Ps 6?
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Summary
• Enhanced facilities in Canada benefit THEMIS and Cluster
• 1-d Harris fit can be applied for PS crossing or for flapping
• 3-d Harris fit may be needed and is being worked on
• Conjugate studies can now be more frequent and with better data
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Acknowledgements
• Canadian Space Agency and University of Alberta for CARISMA data
• A. Balogh, ICSTM, for Cluster FGM data
• This work funded by Canada Research Chairs, Canada Foundation for Innovation, AU, and NSERC