PRESSURE,%20CURRENTS%20AND%20THEIR%20RETRIEVABILITY:%20%20A%20STATUS%20REPORT

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PRESSURE, CURRENTS AND THEIR RETRIEVABILITY A
STATUS REPORT

• P. C. Brandt, R. DeMajistre, E. C. Roelof, D. G.
  Mitchell
• Acknowledgements N. Tsyganenko

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OUTLINE

• Magnetospheric current system(s)
• Measurement of currents
• Iridium
• Cluster
• HENA
• Why are currents important?
• Pressure driven currents from HENA
• Computation
• Important issues
• Iridium-HENA comparisons
• Intensity
• Latitude
• Longitude
• Conclusions
• Next steps

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MAGNETOSPHERIC CURRENT SYSTEM(S)

• Magnetopause current
• Force balance with solar wind pressure at
  magnetopause.
• Tail current
• Force balance with magnetic pressure in lobes.
• Region 1
• Complicated driver. Connected to flank
  magnetopause and tail current(?). Unclear if some
  is pressure driven by pressure gradients in the
  plasmasheet (very interesting, but not covered
  here).
• Ring current (region 2)
• Driven by pressure gradients in the ring current
  and plasma sheet. We use HENA pressures to derive
  the 3D current system.

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MAGNETOSPHERIC CURRENT SYSTEM(S)
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MAGNETOSPHERIC CURRENT SYSTEM(S)
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MEASUREMENTS OF CURRENTS

• Iridium
• 800 km polar orbiting satellites with
  magnetometers. Magnetic deviation can be used to
  compute FAC going in and out of the ionosphere.
  Present time resolution gt30 min. Future 10 min.
• Cluster
• examples of curlometer technique exists. Not
  validated yet.
• HENA
• Use global ring current pressure to compute
  currents.

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WHY ARE CURRENTS IMPORTANT?

• Pressure-driven currents relate the inner
  magnetosphere with the ionosphere.
• Ionospheric closure produces large potential
  drops, that modifies the electric field in the
  inner magnetosphere (SAPS, Not covered here).
• Pressure-driven currents in the Iridium derived
  FAC patterns are the best global information of
  pressure gradients.
• Iridium-HENA comparisons teaches us about the
  global pressure distributions.

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HOW TO COMPUTE CURRENTS

• APPROACH Use Euler potentials Q and P (currently
  only tested for isotropic pressure. Possible but
  trickier with anisotropic.)
• Retrieve pressure from ENA images.
• Assume magnetic field model.
• Compute differential flux tube volume
  (Q-function).
• Compute the FAC going in and out of the
  ionosphere.

(Dipole approximation)
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CHALLENGES

• Gradient of a measured pressure.
• Errors are magnified at higher latitudes.
• Constraints, constraints, constraints

(Dipole approximation)
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CONSTRAINTS?

• The inversion solution is (normally) an
  overdetermined equation system (higher number of
  ENA pixels than ion bins).
• Constraints are weighting matrices that force
  the solution to certain criteria.
• Smoothness in the first derivative (D1).
• Smoothness in the second derivative (D2).
• Be similar to nearest neighbor (Markhov).
• More
• Without constraints inversion solution goes
  unstable (well-known mathematical problem).
• After many trial and errors we have found that a
  combination of M and D2 works well. Results have
  been validated against Cluster in-situ ion
  measurements Vallat et al., JGR, in press,
  2004.
• Still work left to do

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IRIDIUM - HENA
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IRIDIUM - HENA
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IRIDIUM - HENA

• ASSUMPTIONS
• Region 2 can be identified and are pressure
  driven.
• ISSUES
• Iridium derived FACs uses spherical harmonic
  fits. Increases the risk of angular aliasing.

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2001 DOY 328(10-198 keV)
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DIFFERENCES

• INTENSITY
• HENA has only partial pressure.
• Energy range, species.
• LATITUDE
• Were using dipole approximation.
• What difference does T01-S make?
• LONGITUDE
• Iridium extends further to dayside.

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2001 DOY 328(10-60 keV)
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2001 DOY 328(10-198 keV)
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2001 DOY 328(10-60 keV)
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2001 DOY 328 (10-119 keV)
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2002 DOY 274(10-60 keV)
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2002 DOY 274(10-119 keV)
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2002 DOY 274(10-60 keV)
ALIASING?
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2002 DOY 274(10-119 keV)
ALIASING?
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2002 DOY 274(10-60 keV)
ALIASING?
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2002 DOY 274(10-119 keV)
ALIASING?
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NON-DIPOLAR FIELD

• Iridium R2 is often at lower latitudes (upto 10
  deg) than HENA.
• Why?
• We use dipolar field, while reality does not.
• Test
• Isotropic pressure is constant along field lines.
• Pick equatorial point and map it with dipole and
  T01-S and see what the difference is.

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NON-DIPOLAR FIELD
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CONCLUSIONS

• Intensity underestimated because of limited
  energy range (and species).
• Solution Validate and extrapolate energy
  spectrum!
• Latitude can vary 5-10 deg if inappropriate field
  model is used.
• Solution Use T01_S!
• Longitude Iridium R2 extends further to dayside.
• PAD?
• Inappropriate constraints at edges of FOV?

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NEXT STEPS

• T01-S Compute FAC in T01-S and compare to
  dipole.
• Energy spectrum validate HENA ion spectrum
  against in-situ.
• Constraints more systematic tests.
• Anisotropy Assimilate PADs and/or use multi
  vantage point observations.

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IRIDIUM - HENA
?
?
?
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2002 DOY 274(10-119 keV)
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2002 DOY 274(10-60 keV)
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2002 DOY 274
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2002 DOY 274(10-60 keV)
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2001 DOY 328 (10-60 keV)
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2001 DOY 328 (10-198 keV)
Not sure how valid this is. Not happy with
inversion here.