Transcript Cross-Meridian Currents in Substorms and Sawtooth Events

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)
A comprehensive observatory in western Canada ideally located for
THEMIS conjunctions (see poster)
54.72 N, 246.7 E
CGM (2005)
62.0, 306.5
L=4.55
Founded 2002
(UCLA mag 1998)
Plasma sheet - Harris model
Plasma sheet : hot plasma
 n = 3 cm-3
 Eth ~ 4200 eV
 B ~ 10 nT
β~6
 z>0 => Bx>0 ,
Bo
Harris model
 Magnetic field :
where By=Bz=0
Z  Zo
Bx  Bo tanh(
)
L
and Bo = Blobes
Current density :
derived from
Jx  Jz  0
Jy  (
 0J  curl(B)
z<0 => Bx<0
B0
Z  Zo
) sec h²(
)
 0L
L
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 ~ 5 UT
 B reverse
 low |B|
 more energized particles
 Harris form
Harris fit
August 3rd, 2003
Results
Bo
 Bo = B(lobes) = 26.8 nT
=> realistic ( Theory ~30 nT )
Z  Zo
Bx  Bo tanh(
)
L
 Zo = -1.64 Re
=> not centred (z axis)
=> movement of the plasma sheet in
the Z direction => flapping
 L = 0.63 Re
Zo
=> thickness of the plasma sheet
Questions
 Are the parameters related ?
 What influences the parameters ?
Flapping study
 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
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
flapping
real crossing
 awaited results : L constant and Zo
function of time
Flapping study
August 23rd , 2002
Results
 L ~ 0.89 Re
 Zo is a function of
time => Vz
Zo(t) => velocity of
the flapping Vz
Questions
 Vz same as Cluster data ?
 The current is Harris current ?
=> Vz and J
L ~ cst during
flapping
Flapping study
August 23rd , 2002
Jfit(blue)
 Vz (cluster data) > Vz fit
11 UT => +60 km.s-1
 Current fit realistic (compared to
the data in black)
 Vy ≈ or > Vz !! => First hypothesis
(flap only in Z direction) false
our fit needs some changes !!
Conclusions 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