Once Upon a Time in the Electron Radiation Belts

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Once Upon a Time in the Electron Radiation Belts

• R. Friedel, G. Reeves, J. Koller, Y Chen, S.
  Zaharia, V. Jordanova
• ISR-1, Los Alamos National Laboratory, USA
• Paul OBrien (presenting)
• The Aerospace Corporation, Chantilly, USA

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Contents

• Rationale
• Brief History, Current Status
• Once upon a time in the electron radiation
  belts
• Limitations of modeling
• Wave particle interactions
• Pitch Angle Diffusion in realistic fields
• Radial Diffusion
• Loss processes
• Lets not get depressed the path forward
• Data Assimilation prospects
• The DREAM Code coupling Plan
• Summary/Conclusion

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Rationale (1)

• The energetic electron radiation belt is of
  increasing scientific and operational interest.
• Complex natural system.
• Affects a large amount of commercial and military
  space assets.
• Space Situational Awareness.
• Propagating environment for high altitude nuclear
  explosions.
• A large body of recent theoretical work
  (incomplete list).
• Radial diffusion revisited (Chan Rice,
  Elkington, LASP).
• Detailed work on wave-particle interactions (R.
  Thorne UCLA, R. Horne BAS, D. Summers St.
  Johns, J. Albert AFRL).

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Rationale (2)

• Increasingly detailed modeling work (incomplete
  list).
• Diffusve models (BoscherBourdarie ONERA,
  Shprits UCLA, Koller LANL, ).
• In ring current simulations MiyoshiJordanova
  LANL, Fok GSFC, .).
• Coupled Energy/Pitch Angle diffusion (Albert
  AFRL, Shprits UCLA, Koller LANL, ).
• Increasingly higher fidelity data work.
• Data inter-calibration and use of phase space
  coordinates _at_ constant adiabatic invariants (Chen
  LANL, Bourdarie Onera, Green LASP, ).
• Relatively many data sources LANL GEO/GPS, GOES,
  Polar, Cluster, HEO, Sampex, NOAA-POES, .

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Brief History (1)From Friedel et al. review
Initially observed as dropout followed by a
delayed increase of relativistic electrons at
geosynchronous orbit during recovery phase of
storm. Up to 3 orders of magnitude increase of
2 MeV electrons (blue line) Initially a zoo of
proposed mechansims (See review, Friedel et.al,
2002) external source, recirculation, internal
source, MeV electrons from Jupiter
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Brief History (2)Results form Reeves et al.
Difficulty in understanding dynamics of system
Wide range of responses for similar geomagnetic
storms Increase / Decrease / Shift of peak / No
change - are all possible responses Many
processes operate simultaneously that cannot be
seperated observationally Response thought to be
result of a delicate balance of loss, transport
and internal energization processes.
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Current Status (1) Internal/External
SourceResults from Yue Chen et al.
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Current Status (2) The wave picture(or no
talk is complete without this graphic Summers et
al)
Plasmasheet Source of seed population
(convectionimpulsive injection) Magnetopause Po
ssible loss mechanism for intersecting distorted
drift paths outward diffusion Waves Drifting
electrons encounter several possible wave
regions Hiss (loss) inside plasmasphere/plumes, Ch
orus (energization) outside plasmasphere, and
EMIC (strong loss) at edge of plasmasphere /
plumes.
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Current Status (3) Chorus internal source?
Evidence from Meredith et al.
CRRES data October 9th 1990 Storm

• Recovery phase associated with
• prolonged substorm activity.
• enhanced levels of whistler mode chorus.
• gradual acceleration of electrons to relativistic
  energies.

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Current Status (4) Losses? From Green et al.,
Ukhorisky et al., Chen et al, Shprits et al.,
Magnetopause Green concluded it not to be a
major loss source. Work by Ukhorisky shows
distorted drift paths near dusk during storms
that can intersect magnetopause. Chen computes
last closed drift shell from T01s model and shows
that this boundary is often near GEO, down to
L4.5. This plus outward diffusion due to
negative gradients (Chen) can lead to significant
losses (Shprits). Waves EMIC for strong
diffusion losses proposed (Summers). Recent data
(Friedel) and pitch angle diffusion simulations
(Shprits) support this mechanism.
Whistler/lightning induced losses play a role
(Rogers). Microburst / Precipitation
bands Observations from Sampex (OBrien).
Co-located with Chorus region. With some
assumptions, could explain all relativistic
electron losses on their own.
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Limitations of modeling (1)Wave particle
interactions HISS, CHORUS, EMIC
Theoretical work (Horne/Thorne/Albert) is mainly
based on a quasi-linear approximation. To
estimate pitch angle and energy diffusion
coefficients, a range of input parameters are
needed Background plasma density, ion
composition, magnetic field strength, wave
strength frequency / k distribution. For
bounce/drift averaged quantities, these need to
be known globally. -gt Many approximations, many
degrees of freedom. PLUS all the
approximations of quasi-linear theory. Non-linear
effects are not taken into account at all,
however it is known that non-linear effects can
produce macroscopic changes in pitch angle in one
interaction. Q Which of these parameters is
most important? -gt Little gain in detailed
modelling if unconstrained parameters are biggest
unknown. -gt Pick most important parameter -gt
estimated by Data Assimilation.
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Limitations of modeling (2) Pitch angle
diffusion in realistic fields drift shell
splitting
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Limitations of modeling (2)Pitch angle diffusion
in realistic fields drift shell splitting
Current diffusion modeling is performed in a
drift averaged sense with all wave processes
acting over an assumed fraction of the drift
orbit. REAL particle distributions have a
geometric background variation of the pitch angle
shape due to the asymmetric magnetic field. The
REAL pitch angle distribution at any point is
always a convolution of both radial and pitch
angle gradients. In the magnetosphere, pitch
angle diffusion acts on the LOCAL slope of the
pitch angle distribution. Redistribution by
diffusion depends on diffusion coefficients AND
local gradients. Q How large an error is made
by ignoring these effects?
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Limitations of modeling (3)Radial diffusion
This is the oldest and best established part of
the electron radiation belt dynamics. A large
part of the dynamics (80) can be recovered
using diffusion alone. However, radial
diffusion coefficients have been derived based on
simple assumptions in the past (dipole fields,
fluctuations of electric and magnetic field)
SchultzLanzerotti) Other derivations are
based on data (which mixes radial, pitch angle
and energy diffusion). Depending on the study
different parameterizations of radial diffusion
have been proposed both as a function of L and
activity (Kp). Q Which one is the best? How can
one tell? Is a broad Kp dependence sufficient? -gt
estimate activity dependence with Data
Assimilation?
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Limitations of modeling (4)Loss processes
This is still the part of electron radiation belt
dynamics that is least understood. Many
mechanisms have been proposed and with the
appropriate assumptions, may individually be
blamed for all the losses. In reality, a
combination of all the mechanisms may act
together, with varying combinations depending on
energy, activity level and location. Observationa
lly these mechanisms are difficult to constrain.
Q Is there a strategy that can differentiate
between these processes? And if not, can a simple
parameterization of losses in terms of energy and
location dependent lifetimes be sufficient? -gt
estimate these lifetimes with Data Assimilation?
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The Path Forward (1)Data Assimilation prospects
Once the best reduced set of controlling
parameters has been found, they can be added to
the model state and can be estimated using an
Ensemble Kalman Filter.
Results from an identical twin test case show how
this works in principle In Red is the actually
used variation of the radial diffusion parameter
Blue the recovered variation when treated as a
free parameter and estimated using an Ensemble
Kalman filter.
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The Path Forward (2)LANL DREAM effort - Four
integrated lines of modeling


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The path forward (3) The full DREAM coupling plan
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Summary / Conclusion (1)

• Fascinating problem Modeling Relativistic
  electrons, although only passive riders on
  magnetospheric activity, requires a full
  self-consistent treatment of plasmasphere, ring
  current, magnetic and electric fields, wave
  generation and wave-particle interaction.
• The times of data only and modeling only work are
  over.
• To improve our modeling capability in
  representing a real electron radiation belt,
  further detailed modeling may be less effective
  compared to better specification of some of the
  controlling parameters.
• A reduced parametric specification may actually
  be better than modeling the full physics with
  wrong assumptions.

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Summary / Conclusion (2)

• The upcoming RBSP mission is designed to provide
  the best data possible for the electron radiation
  belt problem. Still
• Only two point measurements
• Limited and varying time resolution at each L.
• Between now and RBSP launch, what can be done to
  fill the remaining gaps in needed inputs for
  successful modeling?
• Focus on ground based data a much
  under-utilized resource
• Whistler data is plentiful and on-going and can
  give equatorial plasma density and plasmapause
  position.
• ULF wave data from ground magnetometers can infer
  mass loading of field lines.
• DMSP data has also been used to infer plasmapause
  position (Anderson).