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R. Friedel, Y. Chen, G. Reeves, T. Cayton ISR-1, Los Alamos National Laboratory, USA Yuri Shprits University of California, Los Angeles, USA. Pitch angle evolution of energetic electrons at geosynchronous orbit during disturbed times. Contents. Rationale

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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

Pitch angle evolution of energetic electrons at geosynchronous orbit during disturbed times
contents
Contents
  • Rationale
  • Geosynchronous pitch angle distributions
    • Instrumentation and Data
    • Drift shell splitting example and explanation
  • Mapping to constant L* = 6.5
    • Assumptions
    • Demonstration of method
    • Quiet time test of method (10-13 December 2002)
  • Application to small relativistic electron event
    • August 2-5, 2002
    • Theoretical predictions
  • Summary/Conclusion
rationale
Rationale
  • 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)
  • These “geometric” effects may mask the changes that may be due to in-situ acceleration or pitch angle scattering processes.
  • 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.
geosynchronous instrumentation and data
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.
mapping to l 6 5 assumptions
Mapping to L*=6.5Assumptions

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:

  • Phase space density gradients near GEO are flat or small (SCATHA [Fennell] and GEO/Polar [Chen] observations).
  • Over this small range of L* we can approximate our µ mapping using a dipolar approximation.
  • The change in the mapping of K to pitch angle is over this range of L* is negligible.
mapping to l 6 5 demonstration of method near midnight

At satellite, different

pitch angles map to

different L*.

Example of mapping

satellite near midnight

Mapping to L*=6.5Demonstration of method near midnight
mapping to l 6 5 map at constant k or pitch angle
Mapping to L*=6.5Map 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.

pitch angle mapping small rel electron event august 2 5 2002

Particle Loss – field aligned electrons lost

Coincident with cold dense plasma

Signature at MPA -> conditions for

EMIC waves

Continued Recovery

Original PA -> night-side butterfly

Mapped PA -> 90o peaked at times

Butterfly not completely gone ->

possible PSD gradient

1st Recovery

Original PA -> slight night-side butterfly

Mapped PA -> 90o peaked

Average of all

available

LANL GEO data

500 – 1500 KeV

Original

Pitch Angle

Distribution

750 KeV

Orig. Normalized

Pitch Angle

Distribution

750 KeV

Map. Normalized

Pitch Angle

Distribution

750 KeV

LANL MPA

Plasma Data

Ion Spectra

MPA inferred

Mag Field Dir

0o = To earth

90o = North

Red = T01s model

L* at 90o to 10o PA

MLT in Red

T01 storm model

Pitch angle MappingSmall rel. electron Event August 2-5, 2002
evolution of pitch angle distributions modeling effects of waves ucla
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

lanl geo pitch angle observations at all energies 50 kev 1 5 mev
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

summary conclusion
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.