Implication%20of%20the%20post-midnight%20enhancement%20of%20the%20ring%20current%20flux%20-%20IMAGE/HENA

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Implication of the post-midnight enhancement of
the ring current flux- IMAGE/HENA

• Y. Ebihara 1 and M. C. Fok 2
• 1. USRA, NASA GSFC
• 2. NASA GSFC

Collaborators R. A. Wolf, P. Cson Brandt and
T.E.Moore
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Outline

1. HENA observation and CRCM simulation.Storm of
   August 2000
2. Current understanding of the post-midnight
   enhancement of flux and its association with
   ionospheric features (e.g., SAPS).
3. CRCM overview.
4. Testing of possibilities and its results.
5. Concluding remarks.

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HENA observation of flux enhancement
0920 UT, August 12, 2000
IMAGE/HENA, 27-39 keV
Cson Brandt et al. (GRL, 2002) reported that the
peak of the proton flux is found on the dawn side
in some storms. This departs substantially from
the traditional view.
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What HENA sees
?
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What HENA looks at is protons with low equatorial
pitch angles (field-aligned).
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Comparison of fluxes from HENA and CRCM for the
August 2000 storm
0800 UT, August 12, 2000
IMAGE/HENA, 27-39 keV
CRCM, 32 keV

• Comprehensive Ring Current Model (CRCM) flux
  peaks between midnight and dawn (Fok et al.,
  2003).

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Questions regarding PMEF

• We have many possible processes. After all, what
  is the dominant process for controlling peak of
  flux?
• What is the relationship between the
  post-midnight enhancement of flux (PMEF) and the
  ionospheric features?
• We tested some possible causes of PMEF by means
  of CRCM.

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Possible causes of PMEF

1. Longitudinal variation of distribution function
   in the plasma sheet.- Proton density is higher
   on dawn side than dusk.- Pitch angle is
   field-aligned on dawn side than dusk.
2. Temperature in the plasma sheet.
3. Longitudinal asymmetry of loss processes- EMIC
   wave activity is high on dusk side.
4. The twisting of convection by shielding.
5. The twisting of convection by gap between R2
   current and auroral conductance region.
6. Skew of convection by large IMF By.
7. Skew of convection by jump near terminator of
   conductivity.
8. Depression of local B-field by the ring current.

Source
Loss
Transport
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CRCM Kinetic equation solved
The kinetic equation was solved in terms of the
4-dimensional phase space (Fok et al., 2001).
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Block diagram of CRCM
Boundary condition fb
Charge exchange loss
Phase space density f
Grad-B and curvature currents
Kinetic equation
Magnetospheric E-field
Field-aligned current
Mapping
Poissons equation
Ionospheric E-field
Cross polar cap potential drop
Solar wind and IMF
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Physical models for this particular run

• Cross polar cap potential drop - Boyle et
  al.(1997), SW and IMF dependence
• Ionospheric auroral conductivity - Hardy et
  al.(1987), Kp-dependence
• Magnetic field - Tsyganenko 1996 (steady) -
  Dipole
• Boundary condition - Isotropic Maxwellian for
  particle distribution
• Initial condition - Sheldon and Hamilton (1993)

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Comparison of fluxes computed from Weimer and CRCM
Non-self-consistent(Weimer)
Self-consistent(CRCM)
0920 UT, August 12, 2000
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General explanation of the twisting of convection
Twisting of convection
Field-aligned current
Pedersen conductance
Ionospheric E-field
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The shrinking and extending of auroral oval
Field-aligned current
Pedersen conductance
Ionospheric E-field
Shrinkage
Extension
0920 UT, August 12, 2000
To see the effect of gap on the twisting, we
artificially shifted the auroral conductance with
respect to magnetic latitude.
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The shrinking and extending of auroral oval
Shrinkage
Extension
- The peak of flux resides on the dawn side,
regardless of latitude of auroral oval.- Both
the shrinking and extending of the oval produce
weaker SAPS.- The shielding is likely, but the
gap is unlikely for the contributor to the PMEF.
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Post-midnight enhancement of flux for other storms
March 20, 2001, 1100 UT
HENA, 27-39 keV
November 1, 2001, 1220 UT
HENA, 27-39 keV

• Not all storms show the post-midnight
  enhancement of flux.

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Comparison of fluxes from HENA and CRCM for other
storms
March 20, 2001, 1100 UT
HENA, 27-39 keV
CRCM, 32 keV
November 1, 2001, 1220 UT
HENA, 27-39 keV

• CRCM prefers to show the post-midnight
  enhancement.
• PMEF is not simply understood to the shielding.

CRCM, 32 keV
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Relationship between PMEF and plasma sheet
temperature
Number density was held constant.

• High plasma sheet temperature does not shift the
  peak of the flux.- SAPS is weakened as
  temperature increases.

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Local time dependence of plasma sheet density
2 hours

• The peak of flux depends on the longitudinal
  distribution of density.- Is the localized
  injection realistic?

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Skew of convection due to IMF By (Weimer model)
Northern Hemisphere
IMF Bygt0
IMF Bylt0
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Results

1. Longitudinal variation of distribution function
   in the plasma sheet.- Proton density higher on
   dawn side that dusk. (POSSIBLE)- Pitch angle is
   field-aligned on dawn side. (POSSIBLE)
2. Temperature in the plasma sheet. (UNLIKELY)
3. Longitudinal asymmetry of loss processes- EMIC
   wave activity is high on dusk side.
4. The twisting of convection by shielding. (MOST
   PROBABLE)
5. The twisting of convection by gap between R2
   current and auroral conductance region.
   (UNLIKELY)
6. Skew of convection by large IMF By. (NOT YET
   TESTED)
7. Skew of convection by jump near terminator of
   conductivity. (POSSIBLE? RCM)
8. Depression of B-field by ring current. (NOW UNDER
   RUNNING)

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Conclusion

• The post-midnight enhancement of flux is still in
  controversy.
• Realistic auroral conductance model is
  needed.(FUV will provide it.)
• Global distribution of the ionospheric ion drift
  will give us a clue.
• CRCM expects thatthe eastern edge of velocity
  reversal is in the vicinity of peak of flux.
• The relationship betweenSAPS and the peak
  offlux is unclear.

Eastern edge of drift velocity reversal
SubAuroral Polarization Streams (Foster et al.)
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END
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Energy dependence of PMEF
27-39 keV
39-50 keV
50-60 keV
16-27 keV
20 keV
40 keV
63 keV
100 keV
200 keV
0920 UT, August 12, 2000