Comparison of Rice Model Results with IMAGE Data

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Comparison of Rice Model Results with IMAGE Data

• R. A. Wolf, Rice University

Unpublished results provided by Stanislav
Sazykin and Jerry Goldstein (Rice) Shin-Yi Su
(National Central Univ.) Pontus Brandt (JHU/APL)
and HENA Team Bill Sandel (U. Arizona) and EUV
Team Mei-Ching Fok (Goddard)
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Comparison of Rice Model Results with IMAGE Data

• Introduction and Outline
• Rice Convection Model
• Some Comparisons with IMAGE Data
• Overshielding and Plasmapause Shape
• Formation of Symmetric Ring Current in Recovery
  Phase
• Location of the Peak of the Main-Phase Ring
  Current
• Additional Issues Id Like to Address with IMAGE
  Data
• SubAuroral Plasma Streams (SAPS)
• Interchange Instability
• Summary

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Models of Convection E from 1960s and 1970s
Uniform Dawn- Dusk Field
Fact There is a dawn-to-dusk electric field
across the interior of the magnetosphere.
Without corotation
With corotation
Fact The inner edge of the plasma sheet tends to
shield the near-Earth region from the convection
E field.
Stern-Volland E Field
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Rice Convection Model Theoretical Approach

• Electric field calculated theoretically

• Models based on these equations imply that the
  inner edge of the plasma sheet tends to shield
  the inner magnetosphere from the main force of
  convection.

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Undershielding

• It takes a while for the inner edge of the plasma
  sheet to react to an increase in convection.
• Inner magnetosphere is poorly shielded for a
  while after a sudden increase in convection.
  Average dawn-dusk E is positive in inner
  magnetosphere.

Shielded again after 2 hours of strong convection
Shielding after long period of steady convection
Undershielding after sudden increase of convection
(Sazykin, 2000)
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Overshielding

• It takes a while for the inner edge of the plasma
  sheet to react to an decrease in convection.
• Inner magnetosphere is overshielded for a while
  after a sudden decrease in convection.

Shielded again after 1.5 hours of weak convection
Shielding after long period of strong convection
Overshielding after sudden decrease of convection
(Sazykin, 2000)
Note Overshielding was discovered by Kelley et
al. (1977) and well quantified by Fejer and
Scherliess (1995), both using measurements of
plasma flows in the equatorial ionosphere.
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Overshielding Observed at L4 by IMAGE

• A gradual northward turning of the IMF 14-19 UT
  on July 28 caused an outward motion of the
  plasmapause in the post-midnight sector.
• Caused a shoulder that had rotated to the day
  side by 3 UT on 29 July.

28 July
RCM-predicted overshielding pattern
(Goldstein et al., 2003)
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Cartoon of Plasmasphere Shoulder Formation
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Overshielding Subtlety

• Kelley et al. (1977) originally explained the
  overshielding phenomenon observed in the
  equatorial ionosphere in terms of reaction to a
  sudden reduction in polar-cap potential.
• In the RCM, it was difficult to explain the
  observed duration of overshielding (1- 2 hr),
  because the nightside plasma sheet requires 20
  min. to adjust to a change in potential drop
  (Spiro et al., 1988).
• Fejer et al. (1990) suggested that the duration
  of the equatorial response to a northward turning
  was due to magnetic reconfiguration (lessening of
  the magnetic flux in the magnetotail).
• The inner-magnetospheric response to the long,
  slow northward turning of July 28, 2000 must have
  been due to magnetic reconfiguration.

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RCM Plasmapause vs EUV Data

• Note
• Duskside slot extends eastward during period of
  weak convection (panels 1-3)
• Strong convection generates 2nd plume (panel 3).
• Dayside fills with cold plasma after hours of
  strong convection (panel 4)
• Reasonable model-data agreement
• RCM puts plasmapause too far out on dusk side.

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RCM Partial Pressure
Early Main Phase of March 31, 2001 Storm
HENA 39-60 keV Fluxes
Note Most ring current in dusk-midnight
quadrant. Little on day side
Data from Pontus Brandt, Model results Stanislav
Sazykin
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RCM Partial Pressure
Late Main Phase of March 31, 2001 Storm
HENA 39-60 keV Fluxes
Note Ring current has begun to penetrate to day
side.
Data from Pontus Brandt, Model results Stanislav
Sazykin
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RCM Partial Pressure
End of Main Phase of March 31, 2001 Storm
HENA 39-60 keV Fluxes

• Just after a strong decrease in convection, the
  ring current occupies most of the afternoon
  sector.

Data from Pontus Brandt, Model results Stanislav
Sazykin
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RCM Partial Pressure
Early Recovery Phase of March 31, 2001 Storm
HENA 39-60 keV Fluxes
Note Ring current has curled around into morning
sector.
Data from Pontus Brandt, Model results Stanislav
Sazykin
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Later in Recovery Phase
RCM Partial Pressure
Note Model ring current is nearly symmetric
Model results Stanislav Sazykin
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Main Phase of August 12, 2000 Storm
IMAGE ENA, 27-39 keV
CRCM Model, 8 UT, 32 keV

• Note Ring current is concentrated between
  midnight and dawn, in both ENA data and model.
  Brandt et al. (2002) showed that this was very
  common.

Model results from Mei-Ching Fok. (Fok et al.,
2002)
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RCM Partial Pressure
Early Main Phase of March 31, 2001 Storm
HENA 39-60 keV Fluxes
Note Most ring current in dusk-midnight
quadrant. Little on day side
Data from Pontus Brandt, Model results Stanislav
Sazykin
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Equipotential Twisting
CRCM Run for 8/12/00
RCM Run for 3/31/01

• Maximum westward E peaks near dawn.
• SAPS extends well past midnight.

• Westward E extends further west.
• SAPS does not extend past midnight.

CRCM run by M.-C. Fok, RCM run by S. Sazykin
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Twisting of Inner Magnetospheric Equipotentials
Early main phase of 31 March 2001 storm
After modest increase in polar-cap potential

• Nightside equipotentials are much more twisted in
  the major-storm simulation than in a modest
  convection increase.
• Major storm exhibits clear SAPS in dusk sector.

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Physics of Equipotential Twisting

• We know at least two physical mechanisms that
  cause the nightside inner-magnetospheric electric
  field pattern to rotate counter-clockwise
• In a steady state, shielded magnetosphere, the
  inner magnetospheric penetration field is rotated
  90 east relative to the outer-magnetospheric
  pattern.
• The conductance changes at the terminator,
  combined with the Hall conductance, generates a
  tailward E in nightside inner magnetosphere
  (counter-clockwise rotation of pattern).
• The rotation of the inner magnetospheric pattern
  (particularly the westward penetration field)
  causes the ring current injection to rotate in
  the same sense and can make the 40-60 keV part of
  the ring current peak post-midnight.
• The amount of counter-clockwise rotation is
  highly variable both in the RCM and in Nature.
• We still dont understand the reasons for the
  variability
• Sensitive to details of the ionospheric-conductanc
  e pattern, particularly in the region just
  equatorward of the diffuse aurora.
• Related to the SAPS phenomenon
• May depend on other factors, such as plasma sheet
  temperature

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Other Points to be Investigated with IMAGE Data

• How does the SAPS affect the plasmapause in a
  storm?
• Does a SAPS bring the plasmapause closer to Earth
  in the dusk-midnight sector?

• Does IMAGE see evidence of ring-current
  interchange instability?
• Henderson et al. (Fall 2002 AGU) reported
  observation of giant undulations in the aurora.
  Was this ring-current interchange?

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RCM Plasmapause vs EUV Data

• Note
• Duskside slot extends eastward during period of
  weak convection (panels 1-3)
• Strong convection generates 2nd plume (panel 3).
• Dayside fills with cold plasma after hours of
  strong convection (panel 4)
• Reasonable model-data agreement
• RCM puts plasmapause too far out on dusk side.

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Sazykin et al., 2002
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Prediction of Interchange Instability

• If the main phase of a storm ends because of a
  sharp decrease in solar wind density, while the
  IMF remains southward, interchange instability
  should occur at the outer edge of the newly
  injected ring current.
• Henderson et al. (Fall 2002 AGU) reported seeing
  giant auroral undulations during a storm that
  occurred November 24, 2001.
• We hope to investigate that event to see if the
  undulations are due to interchange.

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Summary

• We think we have explained the plasmapause-shoulde
  r phenomenon in terms of overshielding.
• CRCM/RCM results are consistent with two HENA
  observations
• The HENA-range main-phase ring current often
  peaks post-midnight.
• The effect varies from storm to storm.
• In the model, this is related to equipotential
  twisting.
• We havent sorted out the physical mechanisms
  that govern the degree of twisting.
• Remaining to do
• Better quantitative agreement.
• Clarify the relationship between SAPS and the
  plasmapause shape and ring-current distribution.
• Does IMAGE ever see evidence of ring-current
  interchange instability?

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