Van Allen
Probes
Observational evidence of competing
source, loss, and transport processes
for relativistic electrons in Earth’s
outer radiation belt
Drew L. Turner1, V. Angelopoulos1, A. C.
Kellerman1, W. Li1, S. K. Morley2, M. G.
Henderson2, S. G. Claudepierre3, A. J. Boyd4, M.
Usanova5*, I. R. Mann5, J. V. Rodriguez6, K.-C.
Kim7
1UCLA; 2LANL; 3Aerospace
Thankful for funding from:
Corp.; 4UNH; 5U.Alberta;
6CU-CIRES; 7KASSI
*Now
at CU-LASP
EC FP7: MAARBLE
07 August 2014
Drew L. Turner – COSPAR
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Earth’s Outer Radiation Belt
Spacecraft available to study this system:!
(Note: Other missions incl.: LANL, Geotail, and
future missions (e.g., ERG, Resonance))!
Van Allen
Probes
The outer radiation belt consists primarily of quasi-stably trapped
relativistic electrons (100s of keV to MeV) from ~3 < L < ~7!
Various processes important
to the dynamics of Earth’s
radiation belts!
Acceleration!
Loss!
Precipitation!
Magnetopause!
Magnetopause losses!
Image from Van Allen Probes website!
07 August 2014
Drew L. Turner – COSPAR
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Extreme Outer Belt Variability
Van Allen
Probes
• 
The population of trapped 10s of keV to several MeV electrons in Earth’s outer radiation belt are
highly sensitive to changing solar wind and geomagnetic conditions
• 
Variations are particularly evident during geomagnetic storms; [e.g., Reeves et al., GRL, 2003;
Turner et al., JGR, 2013]:
– 
– 
– 
~50% of storms result in an outer belt enhancement,
~20% result in an outer belt depletion, and
~30% resulted in no significant change
• 
These results revealed the “delicate balance” between competing source, loss, and transport
processes in the outer belt during active conditions; we are only beginning to understand the true
nature of these processes and which dominate over others at different times!
• 
Importance of removing ambiguity in flux observations by converting them to phase space density
(PSD) in invariant coordinates (μ, K, L*)
Example storms from Reeves et al. [GRL, 2003]:
Note: Color shows log(flux)!
07 August 2014
Drew L. Turner – COSPAR
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Understanding Variability
Van Allen
Probes
Van Allen Probes Electron Fluxes!
•  Strong, energy-dependent
variations occur on a range
of time scales
•  E.g.: Sep-Oct 2012 period:
–  Flux dropouts (< 1 day)
–  Sudden, rapid enhancements (< 1
day)
–  Radial transport (few days)
–  Slow decay (> 10 days)
•  The 30 Sep. – 03 Oct. 2012
storm resulted in an
enhancement of < ~2 MeV
electrons, but a depletion of
electrons above that energy
•  Here we will:
–  Review the results of Turner et al.
[JGR, 2014a; 2014b], which revealed
the nature of competing source, loss,
and transport mechanisms during the
30 Sep. – 03 Oct. 2012 storm
07 August 2014
Drew L. Turner – COSPAR
Figure from Turner et al. [JGR, 2014a]!
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30 Sep. – 03 Oct. 2012 Storm
Van Allen
Probes
CME-driven storm; most likely 2 CMEs
hitting in only ~12 hrs!
2nd CME included clear magnetic cloud
with strong Bz-south portion!
Strong double-dip magnetic storm with fast recovery;
Dst-min ~ -130 nT; Kp of 6.7!
Strong AE activity during the double-dip main
phase; very little during recovery phase!
07 August 2014
Figure from Turner et al. [JGR, 2014b]!
Drew L. Turner – COSPAR
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Losses: Main Phase Dropout
Van Allen
Probes
•  Different dominant loss
mechanism depending on L-shell,
energy, and pitch-angle:
–  At L* > ~4, loss followed Pdyn
enhancements and was E- and αindependent
–  At L* < ~4, loss occurred at peak of
main phase and was highly E- and αdependent
THEMIS observations
of lower-energy
electrons reveal only
strong losses at L > 4.5;
consistent with RBSP
MagEIS (100s keV to
~1 MeV)!
07 August
2014
Figures from Turner
et al. [JGR,
2014a; b]!
RBSP REPT (>2 MeV) observations
reveal strong E- and α-dependent loss at
L < 4 after second CME impact!
Drew L. Turner – COSPAR
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Sources: Acceleration during the
Main and Recovery Phases
Van Allen
Probes
•  Sudden enhancements of 10s to 100s of keV electrons, likely
due to energetic particle injections during substorm activity
•  Local acceleration of ~MeV electrons occurred during periods
of enhanced chorus waves and substorm activity; limited to
~3.5 < L* < ~5.5
Figures from Turner et al. [JGR 2014b]!
•  During recovery phase, peak smoothed out, filling in PSD at
higher L-shells likely via outward radial diffusion (i.e., transport)
07 August 2014
Drew L. Turner – COSPAR
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Event Summary and Additional
Evidence
Van Allen
Probes
•  Loss started due to magnetopause shadowing at higher L-shells, propagated to
lower L-shells via outward radial transport [e.g., Shprits et al., JGR 2006; Turner et
al., Nat. Phys. 2012]
•  Additional Evidence: 100s of keV to MeV protons dropped out too!
Figures from Turner et al. [JGR, 2014a; b]!
•  Loss of ultra-relativistic (> ~2 MeV)
electrons at lower L-shells occurred due
to scattering by EMIC waves
•  EMIC waves were likely very limited in Lshell near the plasmapause [e.g., Mann et
al., GRL, 2014]
•  Additional evidence: POES precipitation
•  Sources occurred due to:
–  Substorm injections of lower energy electrons (up to a few hundred keV); these dominated
over MP shadowing losses at L* < ~5
–  Local acceleration of relativistic electrons by whistler-mode chorus waves [e.g., Thorne et al.,
Nature 2013]; acceleration dominated over loss from EMIC waves at L* > ~4; did not
dominate over MP shadowing loss until solar wind Pdyn relaxed
•  Sources were occurring simultaneous to both loss mechanisms!
•  Additional Evidence: Injections from GOES and comparison to quasi-linear theory
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Drew L. Turner – COSPAR
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Conclusions
Van Allen
Probes
• 
30 Sep. – 03 Oct. 2012 storm: 15 spacecraft provided direct observations of competing wave-particle
interactions in the outer radiation belt
• 
E- and α- independent losses due to magnetopause shadowing and outward radial transport can occur
simultaneous and in concert with E- and α-dependent losses from interactions with EMIC waves [see also:
Bortnik et al., JGR 2006; Shprits et al., Nat. Phys. 2013]
• 
Substorm injections and local acceleration from interactions with chorus waves result in significant Edependent sources, which can overpower losses at specific L-shells
• 
The plasmapause is a critical boundary for outer radiation belt electrons
• 
Which source and loss mechanisms dominate for any given event is not straightforward: relies heavily
on variety of solar wind and magnetospheric conditions during individual events; we still have much to
understand here (systematic studies of multiple events using all data available are key)
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07 August 2014
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Drew L. Turner – COSPAR
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Outstanding Questions
• 
Van Allen
Probes
Many, many outstanding questions still remain, e.g. (to list just a few…):
–  What are key parameters and indicators to remotely infer activity levels of different wave
modes?
–  How important is IMF Bz for magnetopause shadowing to be effective [e.g., Blum et al.,
GRL 2013, Kim and Lee JGR 2014] ?
–  What is the role of Shabansky drift orbits in transport at higher L-shells [e.g., Ukhorskiy et
al., JGR 2011; 2014] ?
–  What is the nature and role of nonlinear wave-particle interactions [e.g., Bortnik et al., GRL
2008; Tao et al., GRL 2012; Artemyev et al, EGU 2014; Osmane et al., EGU 2014] ?
–  What is the role of energetic particle injections in non-storm enhancements: [e.g., Schiller
et al., GRL 2014; Su et al., GRL 2014] ?
–  What are the statistical characteristics of penetration depth in L* and energy thresholds
of energetic particle injections (both ions and electrons) and their correlation to chorus
and EMIC waves [e.g., Meredith et al., JGR 2001; JGR 2003]?
07 August 2014
Drew L. Turner – COSPAR
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