Characteristics of the flapping current sheet as inferred from CLUSTER observation'

1
Characteristics of the flapping current sheet as
inferred from CLUSTER observation.

• S. Dubyagin1, V. Sergeev1, S. Apatenkov1, D.
  Sormakov1
• A. Runov2, R. Nakamura2, W. Baumjohann2
• J.-A. Sauvaud3
• (1) Institute of Physics of St.Petersburg State
  University, St. Petersburg, Russia
• (2) Space Research Institute, Austrian Academy of
  Science, Graz, Austria
• (3) CESR/CNRS, Toulouse, France
• supported by INTAS 03-51-3738 grant

2
What is flapping ? First observations.
Flapping motions ? large-amplitude (vertical)
motions of tail plasma sheet

• Known for 40 years (Ness 1965, Toyichi,Miyazaki
  1976, Fairfield et al 1981, Lui et al.,1984,
  Sergeev et al.,1993,1998, Sanny et 1994, Bauer et
  al.,1995....)
• Prevailing view due to flapping induced by
  solar wind blow or substorm effects (not
  conclusive)
• properties/physics not well-known because of
  difficulties of interpretation of single SC
  observations
• flapping oscillations as important tool to probe
  CS structure

3
Methods of determination of current sheet
orientation

• Timing analysis allows to estimate Vn.
• Probably the most reliable method.
• Assumption 1D current sheet
• We used delays between observations of the
  certain B levels at different SC.
• Req. results must be the same for different B
  levels.

• MVA works if CS has a variation in guide
  component (it is not always the case) Single
  spacecraft method - allows to control quality by
  intercomparison between different satellites
  (Sonnerup and Scheible, 1998).

• Curlometer MVA (requires accurate j
  determination)
• N3 N1 x rot B / rot B N1 - maximum
  B variance direction

Local current sheet coordinate system N1
direction of the maximum variance (MVA)
directed to the Earth N3 normal to the CS, N3
Vn / Vn here, Vn timing velocity N2
N3 x N1 collinear with j
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Accuracy of the methods

• Sergeev et al., AG 2006 compare three different
  methods of evaluation of current sheet normal

• Selecting subset when NTIM and NCUR agree to
  within 15º and suggesting that NTIM was a true
  normal they obtained a criterion for MVA
  accuracy

With r23 ?2/ ?3 gt 4 most of normals are expected
to be accurate to within 20-30º
5
Timing normals and propagation velocities

• Propagation from center to flanks, perpendicular
  to magnetic field plane
• Zone of mixed direction near midnight (esp.
  premidnight) where X-line observations are
  most frequent at substorm onset (Nagai et 1998)

• The normal velocity value varies from 10 300
  km/s. In 50 of cases 20lt Vn lt60 km/s
• Propagation pattern reminds the pattern of
  cross-tail average flow, could be structures
  standing in the flow???

VT
(Runov et al., AG 2005)
nVn/Vn morning sector evening sector
Bmain
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Occurrence of Fast CS crossings

• Sergeev et al., 2006, Geotail
• 3 years data set, 3s B-field from DARTS
• 1270 fast crossings
• MVA with ?2/ ?3 gt4 to get normals (480 events)
  

• Results
• n3 ? 0, nY, nZ like at Cluster ? confirm
  kink mode
• occurrence frequency increasing downtail and
  increased in Y 0..10 Re,
• occurrence resembles occurrence of BBFs

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Activity Dependence

• Dependence on AE and auroral activity
• appear at any activity, with/without auroral
  brightenings (In five of 50 events with good UV
  data the flapping motions were not accompanied by
  any auroral manifestations. )
• Dependence on Local activity
• propagation speed is large in/near the BBFs,
• VT is small (tens km/s) in quiet plasma sheet

Sergeev et al., 2006, Geotail
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CS profiles, comparison to Harris distributions

• Three kinds of CS profile
• central embedded sheet 55
• bifurcated sheet 10
• asymmetric sheet 35
• Results
• Non-Harris distributions !!
• Density and j- profiles are different, flat
  density distributions

Petrukovich et al., AG 2006
BL PB2 /2µ0
(Runov et al., AnnGeo 2006)
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Summary of previous studies
Properties of the flapping waves

• The structures are polarized in the ZY plane.
• The structures tend to move from the center to
  flanks with velocities several tens km/s (Sergeev
  et al., GRL, 2004)
• The current sheet corrugation can extend over
  tens RE along X direction (Petrukovich et al.,
  JGR, 2003)
• The thick current sheet can be flapping as well
  as thin one
• Flapping current sheets are non-Harris
• Flapping current sheets often have negligibly
  small normal component of magnetic field and
  strong guide/shear component (Runov et al.,
  AG, 2006)
• The proton contribution to the total current is
  not the dominant contribution (proton current
  often has opposite sign) (Runov et al.,
  AG, 2006)

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Selection criteria
186189 cases during 20012004 tail seasons
available at http//geo.phys.spbu.ru/runov/Cluste
r/2001_xings_survey/ http//geo.phys.spbu.ru/runo
v/Cluster/2004_xings_survey/ 30 cases in 2001
with reliable normals (detailed)
http//geo.phys.spbu.ru/runov/Cluster/2001_xings
_survey/db_2001_30.html

• Timing results (direction of normal) are stable
  during the crossing
• Angle between timing normal and one determined
  from at least one of the other methods lt 15
• Cluster barycenter crosses Bx0 region
• The amplitude of crossing ?Bx is larger than 15
  nT
• Time of crossing not longer than 5 min
• Magnetogram Bx shapes are similar at 4 spacecraft
• These criteria have automatically removed events
  with fast flows.
• max Vx 310 km/s
• Finally we had 57 current sheet crossings to
  analyse.

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The role of guide-component of magnetic filed in
the flapping current sheets
B vector is averaged over -3nT lt BX lt3nT
region. a - angle between n and Z GSM
The flapping current sheets probably are
essentially different from undisturbed (standard)
sheets. B guide component can play important role
in their formation.
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Can strong BY IMF be the cause of a strong guide
component in the tilted current sheet ?
BY IMF can penetrate into magnetosphere ? BY
can turn into BZ guide as a result of current
sheet tilt during the flapping. If it is the case
then By IMF gt 0 gt Bj gt 0 By IMF lt 0 gt Bj lt 0
BIMF is time shifted to CLUSTER position and
averaged over 15 min before t0
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Possible mechanism of formation of current sheet
with a strong B guide

• Petrukovich et al., AnnGeo 2006 analysed series
  of current sheet crossings and found that
• orientation of magnetic field remained the same
  for all kinks.
• The current density is proportional to the
  current sheet steepness

Interpretation Dynamic structure produced by
almost vertical slippage motion of neighboring
magnetic flux tubes. This interpretation agrees
with our result (Bn/BGUIDE is smaller for highly
inclined current sheets).
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The test of Petrukovich et al., hypothesis
B vector is averaged over neutral sheet region
(-3 nT lt BX lt3 nT). a - angle between current
sheet and YX GSM plane
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Comparison between normal velocity of the kink
front and normal ion bulk velocity
Plasma bulk velocities Vp in the neutral sheet
(-3 nTlt B1 lt 3 nT) 2001 CIS/CODIF
measurements from spacecraft4 2004 HIA
measurements from spacecraft1 and 3 (HIA does not
distinguish O ions therefore, storm time
events were excluded) Kink front velocity Vf
was evaluated from timing analysis
The good agreement between plasma and kink front
velocity demonstrates high accuracy of the
moments computed from Cluster spectrometers
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Curlometer and proton current discrepancy

• Expected Ti/Tegtgt1 ? jpgtgtje and main contribution
  from jp to jTOT
• Observed large discrepancies for N gt 7 for most
  cases
• different signs jC and jplt0, jp gt jC
• implies electrons as main current carriers
• Explanation 1 dawnward convection, implies
  converging En and negative e-charge in the sheet
  
• Explanation 2 Bn/Bguide ltlt1 ? currents are
  field-aligned.
• Ve gtgtVi ? jTOT jelectron
• both ion and electron moments are required to
  know !!!

(Runov et al., AnnGeo 2006)
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RESULTS AND CONCLUSIONS

• The strong guide component is a common feature of
  the vertically oriented current sheets. In the
  majority of the cases it points in the positive Z
  direction.
• For vertical CSs in contrast with horizontal
  sheets guide component does not depend on IMF BY.
• We suggest that vertical current sheets are by no
  means horizontal sheet tilted at right angle.
  This structure can appear due to vertical
  displacement of adjacent magnetic flux tubes of
  standard planar sheet, as was suggested by
  Petrukovich et al., 2003, 2006
• The currents in flapping sheets are essentially
  field-aligned, therefore the knowledge of
  electron moments is critical for understanding of
  their physics.