Inner Magnetospheric Electric Fields

1
Inner Magnetospheric Electric Fields

• R. A. Wolf
• Rice University
• (Unpublished results provided by Stanislav
  Sazykin, Mei-Ching Fok, Trevor Garner, and Jerry
  Goldstein)

2
Introduction and Outline

• Introduction/Outline
• Neutral Wind Effects
• What Laws Govern Magnetospheric Generators of
  Inner Magnetospheric E?
• Shielding of the Inner MagnetosphereBasic
  Physics
• OvershieldingTheory and Observations
• UndershieldingTheory and Observations
• Asymmetry of Main-Phase Ring Current
• Large Eastward Electric Fields in Post-Dusk
  Equatorial Ionosphere
• Subauroral Polarization Streams and SAID Events
• Summary

3
Inner Magnetospheric Electric Fields

• Limitations
• By inner magnetosphere, I mean the region
  earthward and equatorward of the electron plasma
  sheet.
• Maps to the subauroral ionosphere
• This talk emphasizes potential electric fields,
  not induction.
• Consider time scales gt a few minutes. No waves.
• I assume that E??? in ionosphere, for these time
  scales.
• Also assume that F is constant along field lines.
  (Were talking about the subauroral ionosphere
  here.)
• Im going to offer only a very quick summary of
  neutral-wind effects.
• Why are inner magnetospheric potential electric
  fields important?
• They control plasmasphere and ring-current
  dynamics. We wont understand ring current
  injection until we understand the associated
  electric fields.
• They affect the radiation belts somewhat (though
  induction electric fields, and waves are probably
  more important).
• They drive much of the dynamics of the low- and
  mid-latitude ionosphere
• They seem to be the most unknown element in
  modern ionospheric modeling of the subauroral
  ionosphere.
• Disruptions of the mid- and low-latitude
  ionosphere seem to be the most important aspects
  of space weather at present, particularly for the
  military.

4
Quick Summary of Neutral-Wind-Driven
Inner-Magnetospheric E Fields

• Fields driven by winds resulting from solar
  heating of the day side. These drive the Solar
  Quiet (Sq) currents and are pretty well
  understood, quantitatively.
• Dynamic processes are only partly understood
• Disturbance dynamo winds are equatorward and
  westward winds driven by heating of the auroral
  zone in storms. They propagate to equatorial
  region in 1 day. There still isnt convincing
  agreement between the best models and
  observations.
• Tidal winds propagate up from lower atmosphere.
• Traveling ionospheric disturbances are gravity
  waves that propagate equatorward, driven by
  auroral zone heating.
• Neutral-wind effects probably dominate
  inner-magnetospheric E fields in times of
  geomagnetic quiet and they are significant in
  active times.
• See tutorial reviews by Richmond (in
  Solar-Terrestrial Physics, ed. Carovillano and
  Forbes, Reidel, 1983) and (in Handbook of
  Atmospheric Electrodynamics, Vol. II., Ed. H.
  Volland, CRC Press, 1995)

5
Magnetospherically Driven Inner-Magnetospheric
Electric Fields

• The large-scale electric field in the
  magnetosphere is that of magnetospheric
  convection.
• Simplest approximation

Corotation
Uniform dawn-dusk field

• Next simplest approximationStern-Volland and
  corotation

• Neither of these simple models captures the
  complexity of the real system. This talk will
  concern effects that are not captured by these
  simple models.

6
What Determines the Inner Magnetospheric E?
Governing Equations

• Vasyliunas equation in MHD form (comes from
  neglecting inertial term in momentum equation and
  assuming ??J0)

(1)

• where in and is mean northern and southern
  ionosphere, we have assumed the magnetic field
  strength is the same at either end of the field
  line,

• and the right side of (1) can be evaluated
  anywhere on a field line.
• The form (1) assumes isotropic pressure, but it
  can be generalized.

7
Equations

• Ohms law for ionosphere

(2)
Field-line-integrated current (includes both
hemispheres)
Field-line integrals of products of Hall and
Pedersen conductivities and neutral winds
Field-line-integrated Conductivity (both
hemispheres)

• Conservation of ionospheric current

(3)

• Substituting (2) and (3) in (1), and neglecting
  winds, gives the Fundamental equation of
  ionosphere-magnetosphere coupling

• In the plasma sheet and storm-time ring current,
  the right side of this equation is typically
  important
• The ring current strongly affects the electric
  field in storm times, as Liemohn and Ridley have
  recently demonstrated. To calculate the electric
  field properly, ring-current effects must be
  taken into account.
• Conductance non-uniformities are also important,
  particularly the day-night effect.

8
Cartoon of I-M Coupling Equation
Blob of plasma in a dipole magnetic field
9
Evolution of Magnetospheric Particle Population

• For simplicity, assume that the particle
  distribution is isotropic
• Amounts to assuming strong elastic pitch-angle
  scattering
• Chaotic ion motion in the plasma sheet works in
  this direction..
• Not really valid in inner magnetosphere.
• There is a version of the RCM (called
  Comprehensive Ring Current Model, CRCM),
  developed in collaboration with Mei-Ching Fok,
  that conserves the first and second adiabatic
  invariants, and uses different version of
  Vasyliunas equation.
• Isotropic energy invariant ? is conserved

• where WKkinetic energy

10
Evolution of Magnetospheric Particle Population

• Equation for evolution of the particle
  distribution

• where hs number of particles per unit magnetic
  flux with certain chemical species and certain
  range in l. S and L are sources and losses.
• hs is proportional to the distribution function.

• Bounce-averaged drift equation

• Reference Wolf, in Solar-Terrestrial Physics,
  ed. Carovillano and Forbes, 1983.

11
Magnetospheric Effects Shielding

• Top diagram shows equilibrium condition no
  convection, with plasma-sheet edge aligned with
  contours of constant V.
• Particles gradient/curvature drift along contours
  of constant V
• Effect of applying cross-tail E (bottom) is to
  move edge sunward
• Causes a partial westward ring across night side
• Dusk side of edge charges , dawn side -.
• Charging occurs near eq. plane and in ionosphere
• Currents flow up from dawnside ionosphere near
  inner edge, down to dusk side.
• Those are the region-2 currents.
• They tend to shield the near-Earth region from
  the dawn-dusk E. Dusk-dawn polarization E opposes
  convection in the inner magnetosphere.

12
How Good is the Shielding in Steady State, for
Typical Conditions?

• Answer Its not clear.
• Shielding is often pretty good in Rice Convection
  Model simulations, but it is also often marginal.
• Observational uncertainty Its hard to
  distinguish steady-state magnetospheric
  penetration field from neutral-wind effects.
• Theoretical uncertainties
• Because of the pressure balance inconsistency
  (Erickson and Wolf, 1980), its hard to know what
  value to place on pV5/3 at the tailward boundary
  of the RCM calculation (middle plasma sheet). I
  dont think we will eliminate this uncertainty
  until we solve the substorm problem.
• In RCM, efficiency of shielding is sensitive to
  plasma-sheet temperature, with higher temperature
  giving weaker shielding. Reason

• The Alfvén layer lies further from Earth for more
  energetic particles.
• If the layer lies too far from Earth for most
  plasma-sheet particles, the partial rings arent
  strong enough to shield.

13
Overshielding The Idea

• If the shielding layer is configured to shield
  the inner magnetosphere from a strong convection
  field, and that convection field suddenly
  decreases, due to a northward turning of the IMF,
  the result will be a backwards E field (dusk to
  dawn) in the inner magnetosphere, until the
  shielding layer readjusts.
• Originally seen in Jicamarca data by Kelley et
  al. (GRL, 6, 301, 1977)
• Observations were interpreted in terms of
  overshielding picture

14
Overshielding Pattern Detail

• RCM (Spiro et al., Ann. Geophys., 6, 39, 1988)
  indicated that the eastward penetration field was
  not spread uniformly across the night side but
  was concentrated in the midnight-dawn sector,
  particularly at low L. Senior and Blanc model
  (JGR, 89, 261, 1984) also showed that feature.
• Agreed with earlier observations of equatorial E
  in response to northward turning of IMF (Fejer,
  in Solar-Wind Magnetosphere Coupling, 1986)

Equatorial equipotentials. Corotation not
displayed.
15
Physical Reason for Overshielding Eastward Field
Being Concentrated Midnight-Dawn
Ionospheric equipotentials for overshielding,
with no distortion. Heavy arrows are E, and
light arrows are Hall currents. In
overshielding, low latitudes have antisunward E?B
drift, corresponding to sunward Hall currents.
Hall currents are stronger on day side than on
night side.
Hall currents remove charge from terminators,
causing them to charge negative and distorting
the equipotentials as shown. Dawnside
equipotentials are pushed to lower latitudes,
while duskside contours are squeezed against the
duskside auroral zone. Penetration to low
latitudes is concentrated on dawn side.
Equatorial view of the effect. Dawnside
equipotentials are Pushed to the Earth, while
duskside equipotentials get pushed toward the
plasma sheet. Gives characteristic V-shaped
equipotentials in inner magnetospehre
16
Recent Evidence of Overshielding From IMAGE
Shoulder
Shoulder
Data from IMAGE EUV imager. Sun is to lower
right, 704 UT, May 24, 2000, after northward
turning of IMF.
MSM simulation for same time.
From Goldstein et al., accepted for GRL, 2002
17
Undershielding

• Undershielding is temporary penetration of
  dawn-dusk electric field in times of increasing
  convection.
• Pattern at low L is much the same as for
  overshielding, but the field is reversed
  westward penetration electric field
  post-midnight.

• The main physical reason is the same as the one
  given for the concentration of eastward E field
  in that sector in overshielding

Equatorial ionospheric E from Fejer and
Scherliess (JGR, 102, 24047, 1997).
From Sazykin Ph.D thesis, Utah State, 2000.
18
Ring Current Injection

• The injection of the ring current in the main
  phase of a major magnetic storm involves massive
  undershielding.
• A westward electric field on the night side
  injects the ring current deep into the
  magnetosphere.
• The new ring current forms a partial ring early
  in the main phase, but forms a complete ring
  eventually.
• According to the conventional wisdom, the partial
  ring is centered near local dusk, because that is
  where the ion Alfvén layer comes closest to Earth
  if the convection field is dawn-dusk.

19
Ring Current Injection Conflicting Cartoons

• The magnetic field decrease observed in the early
  main phase at low latitudes on the Earths
  surface has a clear dawn-dusk asymmetry the
  depression is much greater on the dusk side.
• Conventional wisdom associates this with an
  asymmetric ring current, centered near dusk.

• Picture of region-2 currents and shielding, which
  had the plasma sheet coming closest to Earth near
  local midnight.
• Suggests partial ring centered near midnight.
• The dawn-dusk asymmetry in the ground magnetic
  signature was interpreted in terms of the sum of
  region-1 and region-2 currents being down on the
  day side, up on the night side, with connection
  through antisunward Hall currents in the auroral
  zone (Wolf et al., JGR, 86, 2242, 1981 Crooker
  and Siscoe, JGR, 86, 11201, 1981 Chen et al.,
  JGR, 87, 6137,1982)

20
IMAGE Observations of Ring Current Injection
CRCM Model, 8 UT, 32 keV
IMAGE ENA, 27-39 keV

• Observations and CRCM model fluxes for 12 August
  2000, at the peak of the main phase of a storm.
  Ring current peaks between midnight and dawn in
  both observation and model. From Fok et al.
  (submitted to Space Sci. Rev., 2002)

21
CRCM Equipotentials
Note the extreme twisting of the CRCM
equipotentials, with the westward electric field
centered near dawn. From Fok et al. (submitted to
Space Sci. Rev., 2002)

• In the spirit of full disclosure
• Most RCM runs show pressure peaks near local
  midnight in ring-current injection. The degree of
  potential twisting and the location of the
  pressure peak vary among different RCM storm runs
  and we havent figured out what controls them.
  Some possibilities
• strength of convection
• plasma sheet ion temperature
• background conductance (including sunspot number
  and dipole tilt)
• auroral conductance enhancement

22
Effect of Strong Penetration on Equatorial
Ionosphere

• Basu et al. (GRL, 28, 3577, 2001) showed dropout
  of the equatorial ionosphere at 840 km altitude
  in main phase of the Bastille Day storm. They
  interpreted that as the result of upward drift of
  the F-layer above that altitude.
• This uplift was accompanied by strong
  scintillations.
• Eastward electric field causes a downward drag
  that acts like increased gravity and encourages
  the Rayleigh-Taylor instability that causes
  spread F.
• Note Massive rearrangements of the low-latitude
  ionosphere imply massive changes in conductance.
  Proper modeling of something like this requires
  active coupling of ionosphere and magnetosphere
  modelshavent done that yet.

23
RCM Simulation of Bastille Day Storm

• Source Stanislav Sazykin
• Movie shows potential in equatorial plane
• Top panel shows polar-cap potential (blue) and
  Dst (white)
• This simulation was done with IRI model of
  sunlight-driven ionosphere, with auroral
  enhancement
• Notice
• Skewing of equipotentials
• Strong westward flow at low L from dusk to past
  midnight (Subauroral Polarization Stream,
  discussed shortly).
• Outward flow on dusk side near Earth

Movie is file 0700_Veq_iri_short.avi
24
RCM for Bastille Day Storm Plots of Ionospheric
E

• At low latitude, strong westward E in
  post-midnight sector, eastward E across night
  side.
• Snapshot shows an example of local time
  dependence of ionospheric electric fields, for
  latitudes of 15, 40, and 55 at 2220 UT on 15
  July, late in the main phase.

25
Bastille Day with SUPIM Conductance Model

• Plot shows E fields for 1240 UT on July 15.
• This conductance model has sharper conductance
  jump at terminator. Note eastward E concentrated
  post-dusk.
• Overall conclusion
• A huge storm like this can result in strong
  eastward electric fields on the dusk side.
• The local-time distribution of this eastward
  field depends strongly on the details of the
  ionospheric model.
• To treat this properly, really need a model
  with an active ionosphere, because conductance is
  clearly affected by the dramatic
  magnetosphere-caused layer motions. The
  conductance acts back on the electric field.

26
SubAuroral Polarization Streams (SAPS)

• RCM simulations of active conditions frequently
  show strong flows in the subauroral region
• ExampleTrevor Garners RCM simulation of June,
  1991 storm
• Rapid flow is just earthward and equatorward of
  the electron plasma sheet
• That flow occurs in the dusk-midnight sector,
  sometimes extending past midnight.
• Note that there are stronger electric fields in
  the inner magnetosphere than in the tail the
  opposite of shielding.
• This feature has been observed in strong storms
  by CRRES (RowlandWygant, JGR, 103, 14959, 1998),
  Burke et al. (JGR, 103, 29399, 1998) (see bottom
  panel).
• Also observed very convincingly from Millstone
  Hill (Foster and Burke, subm. EOS). They coined
  the name.

(Garner, Ph.D. Thesis, Rice Univ., 2000)
27
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. That is what causes SAPS
  in the RCM.

Electrons
Ions
28
Physical Interpretation of SAPS

• The obvious interpretation of SAPS is that they
  are the strong E field region between inner edge
  of region 2 and the equatorward edge of the
  auroral electrons.
• This gap between these two regions becomes larger
  in great storms, because precipitation erodes the
  electron inner edge.
• A peak in sunward flow velocity just equatorward
  of the duskside auroral is a usual feature of RCM
  simulations for active conditions.

29
Physical Interpretation of SAPS

• This is essentially the same interpretation as
  Southwood and Wolf (JGR, 83, 5227, 1978) for
  SubAuroral Ion Drift events (SAIDs).
• SAID events (also called polarization jets) were
  discovered by Galperin (Kosm.Issled.,11,273,1973)
  and by Spiro et al. (JGR, 83, 4255, 1978).
• SAID events are narrow (1º wide) and occur
  frequently.
• Banks and Yasuhara (GRL, 5, 1047, 1978) suggested
  a mechanism involving conductivity reduction due
  to the very fast drift. This was essentially an
  ionospheric instability.
• Both types of mechanisms probably operate. Foster
  has suggested that the broad fast-flow region
  (SAPS) and the superposed narrow features (SAID)
  are different phenomea. That is an appealing
  interpretation SAPS are essentially a
  magnetospheric phenomenon and SAIDS are due to
  ionospheric instability.

30
Summary

• Rice Convection Model and similar models that
  self-consistently calculate inner-magnetospheric
  electric fields and display features that agree
  with observed features, some of which only
  recently have been clearly identified.
• Overshielding equatorial radar, new IMAGE
  plasmapause measurements
• Undershielding equatorial radar, IMAGE
  ring-current injections
• SAPS and SAIDs magnetospheric and ionospheric E
  fields.
• Models are less good at getting quantitatively
  accurate electric fields at any given time.
• Many details are not worked out what controls
  degree of skewing of undershielded penetration
  field in large storms, relationship between SAPS
  and SAIDs. We cant reliably calculate how good
  shielding should be in steady state.
• We really need a coupled global-magnetosphere/ring
  -current/thermosphere/ionosphere model. Such big
  coupled models should result from the big
  Michigan and Boston University modeling efforts.