Transcript Pitch angle evolution of energetic electrons at

UNCLASSIFIED
Pitch angle evolution of
energetic electrons at
geosynchronous orbit during
disturbed times
R. Friedel, Y. Chen, G. Reeves, T. Cayton
ISR-1, Los Alamos National Laboratory, USA
Yuri Shprits
University of California, Los Angeles, USA
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Operated by the Los Alamos National Security, LLC for the DOE/NNSA
REPW 2007, Rarotonga, Augustl 2007
UNCLASSIFIED
Contents
•
•
Rationale
Geosynchronous pitch angle distributions
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–
•
Mapping to constant L* = 6.5
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•
Assumptions
Demonstration of method
Quiet time test of method (10-13 December 2002)
Application to small relativistic electron event
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•
Instrumentation and Data
Drift shell splitting example and explanation
August 2-5, 2002
Theoretical predictions
Summary/Conclusion
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Slide 2
REPW 2007, Rarotonga, Augustl 2007
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Rationale
1. Energetic electron pitch angle distributions show clear
local time variations due to the asymmetry of the Earth’s
field: Drift shell splitting
L* = f (pitch angle)
2. These “geometric” effects may mask the changes that
may be due to in-situ acceleration or pitch angle
scattering processes.
3. We “remove” here the geometric effects by mapping the
observed pitch angle distributions to a fixed 3rd (L*,Φ)
adiabatic invariant preserving the 1st (μ) and 2nd (K,J)
adiabatic invariants.
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Slide 3
REPW 2007, Rarotonga, Augustl 2007
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Geosynchronous
Instrumentation and Data
• Data is presented from the relativistic electron channels
of the LANL SOPA instrument – 50 keV – 1.5 MeV.
• As there is no magnetometer on the LANL GEO
spacecraft, the magnetic field direction is inferred using
the MPA plasma measurements by deducing the
symmetry axis of the pressure tensor (Thomsen et al,
1996).
• GEO spacecraft have a 10 sec spin period. SOPA data
sampling is at 0.16 seconds. Data is collected in 32
azimuthal bins averaged over 10 minutes.
• Pitch angle resolved GEO data is available for LANL-97a,
1991-080 and 1990-095 for most of Jul 2002 – Dec 2003.
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Slide 4
REPW 2007, Rarotonga, Augustl 2007
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Geosynchronous Orbit
Drift shell splitting example and explanation
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Slide 5
REPW 2007, Rarotonga, Augustl 2007
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Mapping to L*=6.5
Assumptions
Drift shell splitting at geo orbit leads to observations over
L* = 6 – 7. We map observations to a fixed L* =6.5 at constant
µ (1st) and K (2nd) invariant using the following assumptions:
1. Phase space density gradients near GEO are
flat or small (SCATHA [Fennell] and
GEO/Polar [Chen] observations).
2. Over this small range of L* we can
approximate our µ mapping using a
dipolar approximation.
3
 6. 5 
  E6.5
Esat = 
 Lsat 
3. The change in the mapping of K to pitch angle
is over this range of L* is negligible.
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Slide 6
REPW 2007, Rarotonga, Augustl 2007
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Mapping to L*=6.5
Demonstration of method near midnight
3
 6. 5 
  E6.5
Esat = 
 Lsat 
At satellite, different
pitch angles map to
different L*.
Example of mapping
satellite near midnight
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Slide 7
REPW 2007, Rarotonga, Augustl 2007
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Mapping to L*=6.5
Map at constant K or pitch angle?
We tested the change in
the 2nd invariant K to
pitch angle mapping at a
variety of local times for
L*=6 and L*=7, the
maximum mapping
needed in this study.
Changes in pitch angle at
constant K are < 3 Deg,
which below our 10 deg
pitch angle resolution.
Near geosynchronous orbit we thus are safe
to map at constant pitch angle.
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Slide 8
REPW 2007, Rarotonga, Augustl 2007
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Pitch Angle Mapping
Quiet time test of Method - Olson Pfitzer Model
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Slide 9
REPW 2007, Rarotonga, Augustl 2007
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Particle Loss – field aligned electrons lost Continued Recovery
Pitch angle Mapping
Coincident with cold dense plasma
Original PA -> night-side butterfly
Smallatrel.
August
2002
o peaked at times
Signature
MPA electron
-> conditionsEvent
for
Mapped 2-5,
PA -> 90
EMIC waves
Butterfly not completely gone ->
1st Recovery
Average of all
possible
PSDbutterfly
gradient
Original PA -> slight
night-side
available
Mapped PA -> 90o peaked
LANL GEO data
Original
500 – 1500 KeV
Pitch Angle
Distribution
Orig. Normalized
750 KeV
Pitch Angle
Distribution
Map. Normalized
750 KeV
Pitch Angle
LANL
MPA
Distribution
Plasma
Data
750 KeV
MPA
inferred
Ion
Spectra
Mag
L* at Field
90o toDir
10o PA
o
0
earth
MLT=inToRed
o North
90
T01 =storm
model
Red = T01s model
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Slide 10
REPW 2007, Rarotonga, Augustl 2007
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Evolution of Pitch Angle Distributions –
Modeling effects of Waves - UCLA
Addition of EMIC
waves leads to
rapid loss of
electrons at
energies down to
~0.5 MeV
Higher pitch angles
are affected for
higher energies
< 60o ~1 MeV
< 30o ~400 keV
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Slide 11
REPW 2007, Rarotonga, Augustl 2007
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LANL GEO Pitch Angle observations at all
energies 50 keV – 1.5 MeV
Losses to <60o
for ~1MeV
Losses become
less severe as
energy
decreases
Observations are
roughly
consistent
with EMIC
theory and
modeling
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Slide 12
REPW 2007, Rarotonga, Augustl 2007
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Summary / Conclusion
• Pitch angle mapping seems to work and can reveal the
“real” PA distribution dynamics
• We show that method works in principle – however many
assumptions probably violated during very active periods
• For the week relativistic electron event of August 2-5,
2002:
– the pitch angle distribution seems to show evidence of
acceleration processes (-> peaked at 90o)
– The loss period at the end of the event is clearly associated with
cold dense plasma and losses are due to precipitation -> field
aligned electrons vanish
– Association with EMIC waves: Detailed evolution modeling of PA
distributions shows roughly consistent behavior with data.
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Slide 13