Pitch angle evolution of energetic electrons at geosynchronous orbit

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

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

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 3 ^rd (L*,Φ)

adiabatic invariant preserving the 1 ^st (μ) and 2 ^nd (K,J)

adiabatic invariants.

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.

Geosynchronous Orbit

Drift shell splitting example and explanation

Mapping to L*=6.5 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. The change in the mapping of K to pitch angle is over this range of L* is negligible.

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 (1 ^st ) and K (2 ^nd ) invariant using the following assumptions:

6.5

5

.

6 E

= L E

sat

sat

  

 





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

6.5

5 3

.

6 E

= L E

sat

sat  

 





Mapping to L*=6.5

Map at constant K or pitch angle?

We tested the change in the 2 ^nd 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

Quiet time test of Method - Olson Pfitzer Model

Pitch angle Mapping

Small rel. electron Event August 2-5, 2002

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 0

= To earth 90 ^o = North

L* at 90

to 10

PA MLT in Red

T01 storm model 1

Recovery

Original PA -> slight night-side butterfly Mapped PA -> 90

peaked

Original PA -> night-side butterfly Mapped PA -> 90

peaked at times Butterfly not completely gone ->

possible PSD gradient Coincident with cold dense plasma

Signature at MPA -> conditions for

EMIC waves

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

< 60 ^o ~1 MeV

< 30 ^o ~400 keV

LANL GEO Pitch Angle observations at all energies 50 keV – 1.5 MeV

Losses to <60 ^o for ~1MeV

Losses become less severe as energy

decreases

Observations are roughly

consistent

with EMIC

theory and

modeling

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 90

)

– 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.