Current sheet structure around the nearEarth neutral line observed by Geotail

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Current sheet structure around the near-Earth
neutral line observed by Geotail
Asano, Y,. T., et al. JGR 2004
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• Introduction
• Case study thickness of the cross-tail current
  sheet
• Statistical study current sheet structure
• Statistical study current carriers
• Case study Hall current system
• Statistical study Hall current structure
• Conclusions

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Introduction Aims

• Study the current sheet dymanics around an X
  line whose thickness is less than the ion
  inertial length.
• Near Earth Neutral Line (NENL) estimated between
  X -20 and -30 RE.
• Geotail was designed to so that its orbit was to
  stay in this region.
• Therefore look for passage of NENLs (flow
  reversals).

Z
Current
Ions
Electrons
Dawnwards
Sunwards
X
Y
Introduction
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Introduction - sample
Introduction
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Case study thickness of the cross-tail current
sheet

• Reversal of BX
• Around substorm onset.
• Earthward flows.
• Tailward flows.
• Reversing BX and fast flows the sign of X-line
  observation.
• Large velocity separation ? large current.

Case study thickness of the cross-tail current
sheet
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• Between 1155 and 1205 UT particles are heated.
• Tailward flow.

Case study thickness of the cross-tail current
sheet
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• Current sheet half thickness becomes very small
  just before 1205.
• hcs BL/µ0jy
• BL (B2 2µ0nk(Ti Te))1/2
• Ion inertial length 720km, compared to current
  sheet 500km.
• Very thin current sheet, but hold on, didnt
  Geotail go into the lobe at this point?

Case study thickness of the cross-tail current
sheet
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• He says not, noting that BX never got high,
  and stayed within the inner plasma sheet (ß gt
  0.5).

Case study thickness of the cross-tail current
sheet
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• He measures the current in the Y direction, the
  cross tail current.
• fast dawnwards electrons and slow duskwards
  ions.
• A thin electron current sheet.

Case study thickness of the cross-tail current
sheet
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Field aligned Earthward
Electron ? velocity dawnward
Distributions inside a current enhancement

• Fast moving tail/duskwards ion population.
• Slow moving dawnwards ion population.
• Electrons are isotropic
• Overall, ion population is moving duskward
  current sheet.
• Cant really make out electron motion on this
  scale.

Case study thickness of the cross-tail current
sheet
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Field aligned Earthward
Distributions in outer plasma sheet

• Dawnward ion population still present
• High energy ions only in the central plasma
  sheet
• Dawnward ion population comparable to dawnward
  electron velocity E?B drift.

Electron ? velocity dawnward
Case study thickness of the cross-tail current
sheet
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B
E
E?B
B
E
B
Case study thickness of the cross-tail current
sheet
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Diffusion region and Hall current system
A Hall current system is set up by the thin
electron current sheet in this region.
Diffusion region the region in which
reconnection occurs. Here, the plasma frozen-in
condition does not hold and the full Ohms Law
must be used.
In the ion diffusion region ( ion inertial
length) ions can diffuse from the magnetic field.
Electrons, with lighter mass are still magnetised
and convect with the magnetic field.
Charge separation electric field
Case study thickness of the cross-tail current
sheet
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Calculate the E field towards the neutral sheet
(Ens) from electron velocity.

• Very large electric field 17 mVm-1, what is
  the cause?
• Note quite close to that lobe entry??

Ohms Law gives
Case study thickness of the cross-tail current
sheet
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Statistical study current sheet structure

• Look at all events and calculate hcs .
• Current sheet in thinner around X-line
  observation.

• Calculate current density profile.
• Bifurcated current sheets observed.

Central plasma sheet
Lobe
Statistical study current sheet structure
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Statistical study current carriers

• Near the neutral sheet the currents are
  comparable, but electron current dominates in the
  outer plasma sheet.
• Bifurcated electron current sheet,.
• Caused by E ? B drift.

Central plasma sheet
Lobe
Statistical study current carriers
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• Plot Ens ( EZ) and EY
• Ens much larger than EY, attributed to Hall
  term.
• Creates dawnward drift, increasing(decreasing)
  the electron(ion) current.
• This large could only occur if the current
  sheet thickness becomes comparable to the ion
  inertial length.

Central plasma sheet
Lobe
Statistical study current carriers
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Case study Hall current system

• Reversal of BX
• Earthward flows.
• Tailward flows.

Case study Hall current system
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Case study Hall current system
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• Excursions into the lobe/PSBL.
• Electrons move quicker in the X direction.
• At PSBL electrons move towards the X-line ?
  current away from X-line (anti-parallel).
• In central plasma sheet electrons move away from
  the X-line. VeX gt ViX ? current towards X-line.
• By oppositely directed before and after the
  X-line.

Case study Hall current system
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PSBL electron distributions
Earthward of X-line Flow is tailwards
Tailward of X-line Flow is Earthwards
Field aligned currents
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• Excursions into the lobe/PSBL.
• Electrons move quicker in the X direction.
• At PSBL electrons move towards the X-line ?
  current away from X-line.
• In central plasma sheet electrons move away from
  the X-line. VeX gt ViX ? current towards X-line.
• By oppositely directed before and after the
  X-line.

Case study Hall current system
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Statistical study Hall current structure
Central plasma sheet

• Outward flow BX/BL lt 0.85
• Inward flow BX/BL gt 0.85

Lobe
Ve,in X component of the velocity directed
toward the X-line
Statistical study Hall current structure
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Hall BY
Northern lobe
Southern lobe
Statistical study Hall current structure
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Conclusions

• Reported on the structure of the cross-tail
  current sheet around X-lines.
• Observed current sheet thicknesses less than the
  ion inertial length. Ions become demagnetised and
  decouple from the frozen in electrons.
• Current sheet becomes thicker away from the
  X-line.
• Intense current localised in the off-neutral
  sheet bifurcated current sheet.
• Electrons are current carriers.
• Large Ens caused by Hall term in Ohms law.
• Observed Hall current and BY system, in a case
  study and statistically.
• Geotail not able to resolve a electron diffusion
  region or plasma gradients.

Conclusions
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• Reversing BX/VX.
• Reversing BX and fast flows the sign of
  diffusion region observation.
• Spacecraft potential measurements calibrated
  with PEACE density above 20 eV used to deduce the
  ion density.
• Plasma sheet conditions.
• Current sheet normal is GSM Z direction.

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Northern lobe
Neutral sheet
BY gt 0 BY lt 0
Y
Southern lobe
Southern lobe
Hall BY is 50 to 75 of total magnetic field
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EZ points towards the neutral sheet
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Conclusions

• Observed reversing BX/VX
• Hall effects
• Quadrupolar magnetic field
• Large EZ towards the neutral sheet

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Occurrence rate of inward/outward current

• Inward current sheet in the off-neutral sheet

Inside diffusion region
Outside diffusion region
Statistical study Hall current structure