Upload
shilah
View
33
Download
0
Embed Size (px)
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 Presentation
Citation preview
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
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)
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
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
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
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
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
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
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 !!
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
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
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
Pressure Statistics
• The lobe field varies with solar wind pressure (ACE)
• We did not find other strong dependencies on solar wind parameters
August 21 2002 ca. 8 UT Canada Conjugacy Event
Active Period with good conjugacy to Churchill meridian at onset at 7.9 UT
Ground Pi2 also starts 7.9 UT (MCMU is to west)
Cluster Pi2 starts at 8.1 UT
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
B signatures: activation, flapping
Plasmoid: bipolar BZ
Flow direction changes
as neutral line
moves outward
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
Bifurcated Current Sheet?
• Bx several times went to 0 but not beyond
• Could bifurcated current sheet be related to Ps 6?
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
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