Self-Consistent Modeling of Inner Magnetospheric Electric Fields: Recent Results

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Self-Consistent Modeling of Inner Magnetospheric
Electric FieldsRecent Results

• S. Sazykin, R. A. Wolf, R. W. Spiro, F.
  Toffoletto
• (Rice Univ.)
• and
• N. Tsyganenko (NASA/Goddard)

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Outline

• This presentation will focus on using the RCM to
  model at the March 31, 2001 storm.
• We have done different kinds of runs through the
  event and learned a lot of different things.
• What governs the creation of SAPS and the local
  time location of the pressure peak?
• Location of main-phase-ring-current pressure peak
• 4 runs through an idealized storm
• RCM runs using Tsyganenko major-storm magnetic
  field model and Tsyganenko-Mukai plasma-sheet
  model.
• Use of new magnetic field model puts SAPS at more
  realistic latitude.
• Use of Tsyganenko-Mukai with new B-model leads to
  Golovchanskaya-Maltsev interchange instability.
• Use of new magnetic field model improves the
  plasmapause location.
• Summary

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Event March 31, 2001 magnetic storm

• A CME and related interplanetary shocks caused a
  large magnetic storm early on 3/31/2001, with a
  minimum Dst of 400 nT. The polar cap potential
  reached 200 kV, and the magnetopause was
  compressed to within the geosync. orbit.
• During the storm, IMF Bz repeatedly became large
  negative (southward) driving strong
  magnetospheric convection, and then positive
  (northward) resulting in convection decreases.
• Large subauroral electric fields (SAPS) were
  observed by DMSP and Millstone Hill incoherent
  scatter radar. IMAGE HENA instrument provided
  spatial images of ring current particle fluxes in
  the inner magnetosphere.
• RCM was run though the 2-day event
  (3/30/2001-3/31/2001). Time-dependent inputs were
  based on ACE lagged data as well as DMSP polar
  cap potential estimates.

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• SAPS lies in region of downward Birkeland
  current.
• It lies on, and just equatorward of, the
  equatorward edge of auroral conductance
  enhancement.
• Pressure peak location in good agreement with
  HENA.

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Questions raised by the RCM run

1. What causes the SAPS?
2. What causes the main-phase-ring-current pressure
   to peak around midnight?
3. Why does the RCM get SAPS (and equatorward edge
   of electron aurora) at too high a latitude?

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Runs Through Idealized Version of March 31 Storm

• The RCM runs consistently show two features that
  have recently been observed
• SAPS and near-midnight pressure peak. We are
  doing these runs to try to determine what
  physical inputs affect those observed features.
• Objective is to explore cause-and-effect
  relationships involving
• SAPS
• Location of pressure peak
• Idealized inputs.
• Idealized storm has only one period of strong
  convection, whereas real storm had two.
• 4 runs
• Uniform dawn-dusk E
• Self-consistent RCM electric field but with
  uniform conductance.
• Add auroral enhancement
• Full-up RCM with conductance model that includes
  both auroral enhancement and day-night asymmetry

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RCM with uniform dawn-to-dusk electric field
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RCM with uniform total conductances
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RCM with uniform background conductances but
with an auroral zone
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Full RCM
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Summary SAPS and the location of Pmax

• Results from the RCM show that having a
  self-consistent E-field and a consistent auroral
  zone conductance is essential to re-producing
  SAPS and twisting of equipotentials in modeling.
  (and thus, the location of Pmax ).
• The run that had an auroral zone but no dayside
  ionization produced a SAPS that stretches across
  the dusk side but doesnt extend to local
  midnight. SAPS are actually not observed much in
  early afternoon but are often observed near and
  past the midnight region.

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Physical Interpretation of SAPS

• In the pre-midnight sector, plasma-sheet ions
  penetrate closer to Earth than electrons.
• Electrons mostly control ionospheric conductance.
  
• Therefore, in the premidnight sector, the inner
  edge of the plasma-sheet ions lies at lower L
  than the auroral conductance enhancement.
• Most of the shielding (region-2) current is
  driven by ions, because they carry most of the
  pressure.
• Therefore, some region-2 current flows into
  low-conductance, subauroral ionospheric region in
  the pre-midnight sector. Causing the strong
  electric field in SAPS in the RCM.

Electrons
Ions
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Physical Interpretation of SAPS
Arrows represent Pedersen and Birkeland currents.
Colors represent conductance.

• SAPS occur where substantial poleward currents
  have to flow across regions of low conductance
  (blue).
• The RCMs SAPS events occur by the mechanism that
  Southwood and Wolf (JGR, 83, 5227, 2978) proposed
  to explain SAID events (aka polarization jets)
• Separation between inner edges of plasma sheet
  ions and electrons is greater for major storms
  than for shorter bursts of strong convection.

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Role of the Magnetic field model

• Traditionally, Hilmer and Voigt JGR, 5613, 1995
  was used with the RCM.
• SAPS, as well as other structures, have
  frequently been correctly predicted, but at wrong
  locations. We suspect that much of this
  discrepancy is due to inaccurate mappings of
  field lines from the ionosphere to the equatorial
  plane.
• Role of B-field (through flux-tube volume V)is
  to affect distribution of P
• Recently, Tsyganenko et al. JGR, 2003 published
  a new, data-based storm time B-field model that
  was specifically designed to account for severe
  distortion of the inner (lt10 Re) magnetosphere
  during large storms.
• Two RCM runs for the storm are compared, with HV
  and T03 magnetic fields

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T03 mapping for March 31, 2001 (from N.
Tsyganenko)
Distortion of B-field line of constant ? that
maps out to 3RE before the storm, will map to 9
RE midnight and gt 9 Re in the dusk sector during
the time Dst is rapidly decreasing, affecting the
location and shielding ability of the region-2
field-aligned currents.
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Comparison with DMSP using T03 magnetic field
model Sample results

• On the next 2 slides plots RCM computed drifts
  are compared to DMSP drift data
• Results show that while there has been
  improvement, the RCM places the SAPS location
  poleward of where the data indicate
• Also the magnitudes of the RCM-computed electric
  fields are in many cases too high
• The plots also show the locations of the
  RCM-computed and measured equatorward edge of the
  electron precipitation
• The model places them approximately correctly
  relative these boundaries.
• Although in several cases the data indicate that
  there is significant displacement between between
  the precipitation boundary and the SAPS location.

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Northern Hemisphere pass
MLT 21
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Southern Hemisphere Pass
Southern Hemisphere pass
MLT 21
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Comments

• We in the process of evaluating the source of the
  discrepancy, there are several possibilities
• The magnetic field
• The ionospheric conductivity model
• The ionosphere is more complex that many
  magnetospheric modelers care to admit.
• Modeling these effects properly will involve
  coupling an ionosphere model actively to a
  magnetosphere model. This kind of active coupling
  is beginning, for example with Hubas SAMI-2
  model.
• The input cross polar cap electric field model

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Interchange - preliminary results

• Using the T03 and the Tsyganenko and Mukai 2003
  (TM03) plasma-sheet model as inputs to the RCM
  during the March- 31, 2001 storm
• In these runs the plasma boundary condition was
  varied with universal time, as specified by TM03
  which uses solar wind input
• While the plasma boundary condition varied with
  universal time, it did not vary with local time
• Results indicate the occurrence of interchange
  during reductions in solar wind density that may
  have ionospheric signatures.
• This may be related to the interchange discussed
  Golovchanskaya and Maltsev (JGR., 108 (A3), doi
  10.1029/2002JA009505, 2003

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Plasmapause locations is controlled by E-field
(P)HV T03
EUV-derived plasmapause is denoted by blue
diamonds. Tail is formed by overall E.
EUV data from J. Goldstein
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Summary

• Two recently established observational features
  are displayed by RCM runs through the March 31,
  2001 storm
• SAPS events
• Main-phase ring current pressure peaks near local
  midnight
• SAPS occur in RCM through Southwood-Wolf
  mechanism. Essential ingredients are
• Ion pressure gradient equatorward of electrons on
  dusk side
• Low conductance equatorward of electrons
• Pressure maximum is near local midnight.
  Contributing factors
• Self-consistently calculated electric field
• Day-night conductance asymmetry
• Use of new Tsyganenko major-storm magnetic field
  model improves agreement between RCM and observed
  SAPS latitude.
• With local-midnight plasma-sheet density and
  temperature taken from Tsyganenko-Mukai (2003),
  RCM predicts Golovchanskaya-Maltsev interchange
  instability in plasma sheet after solar-wind
  density decrease.
• Overall improvement of Plasmasphere location

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SAPS/SAIDs in Old Runs of the Rice Convection
Model
1150 UT on Sept.19, 1976. MLT?19. Figure adapted
from Harel et al., (JGR, 86, 2242, 1981).

• This event was a substorm (not associated with a
  storm).
• The E-field peak was identified as a SAID
  (polarization jet) event.
• RCM got the E-field peak sort of right--but not
  intense enough and too broad.
• Region-2 currents flow in E-field peak and
  conductance is low there.

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The Rice Convection Model (RCM)

• Plasma sheet/ring current population is
  represented by 85 proton, 85 O and 27 electron
  isotropic fluids with energy invariants ?s and
  flux-tube contents ?s which are related to
  kinetic energy Ws, number density ns, and total
  particle pressure P through the flux tube
  volume
• Initially, the magnetosphere is empty. The
  boundary condition on the plasma fluxes is either
  constant or vary with time based on the
  statistical relationships of Tsyganenko and Mukai
  JGR, 2003. Boundary condition is uniform in
  local time.
• In addition, a plasmasphere is implemented with
  refilling time constants taken from Lambour et
  al. JGR, 24351, 1997.

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The Rice Convection Model (RCM) -2

• The RCM advects each fluid by solving
• where ? is the potential in an inertial
  frame, and L represents charge-exchange (ions)
  and precipitation (electrons) losses. ?i is found
  by solving the Vasyliunas equation using the
  GMRES iterative algorithm
• The loss terms for the ions are outflow through
  the dayside magnetopause and charge-exchange with
  geocorona. Electrons are lost by precipitation
  into the atmosphere at a fraction (0.3) of the
  strong-pitch-angle-scattering limit.
• ? has contributions from solar EUV conductances
  (IRI-90) and auroral zone conductances (computed
  consistently from the plasma sheet particle
  distribution).

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SAPS/SAIDs in Old Runs of the Rice Convection
Model
1150 UT on Sept.19, 1976. MLT?19. Figure adapted
from Harel et al., (JGR, 86, 2242, 1981).

• This event was a substorm (not associated with a
  storm).
• The E-field peak was identified as a SAID
  (polarization jet) event.
• RCM got the E-field peak sort of right--but not
  intense enough and too broad.
• Region-2 currents flow in E-field peak and
  conductance is low there.

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In the RCM, E-fields are determined by pressure
gradients

• Electric fields are 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

• .

Eshield
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Recent Results Location of Pmax

• In the simple classical picture, peak of Pmax
  is formed on the dusk side. If a simple E-field
  model is assumed, then positive ions (H and O)
  grad./curv. drift sunward and westward.
• In the RCM, at the inner edge of the ring
  current/plasma sheet, there is a twisting of
  equipotentials that complicates the electric
  fields and brings drifting ions back toward
  midnight.
• Sometimes, fluxes in certain energy ranges can
  peak postmidnight.
• What (in the model) determines distribution of P
  in the inner magnetosphere?
• Use 4 idealized test runs