Galactic

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The Radiation Belts and Killer Electrons
Terry Onsager, NOAA Space Environment Center
Solar Energetic Particles
Trapped Electrons and Protons
Galactic Cosmic Rays
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Ring Current and Radiation Belts
Radiation belt electron energies few hundred
keV and above Ring current electron energies
few hundred keV and below
Radiation belt dynamics is controlled mainly by
diffusion and magnetic drifts. Ring current
dynamics is controlled mainly by electric and
magnetic drifts.
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The Radiation Belts and Killer Electrons
What are they, and why do we care?

• Persistent region of high-energy electron
  radiation trapped within the magnetosphere
• Shape of the radiation belt is controlled by the
  magnetic field
• - Intensity of the radiation is controlled by
  acceleration and loss processes inside the
  magnetosphere the energy source for all these
  processes is the solar wind
• Highest energy electrons (gt 500 keV) penetrate
  deep into satellite components and create
  internal discharges
• Lower energy electrons charge the satellite
  surface and create electrical discharges
• - High energy electrons represent a radiation
  hazard for astronauts

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How is Knowledge of the Radiation Belts Valuable?

• Long-term measurements of the radiation levels
  are critical for designing (and not
  over-designing) satellite components
• Real-time measurements give situational awareness
  If an anomaly occurs in a satellite, what is
  the probable cause?
• Prediction of the radiation levels are useful for
  planning satellite operations
• - Long-term measurements, real-time measurements,
  and predictions are all valuable for assessing
  and avoiding radiation exposure to astronauts
• - Long-term measurements allow us to study the
  radiation belts, improve our understanding, and
  deliver higher accuracy predictions and
  specifications

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Energetic Plasma From the Tail Diverts Around
Earth Forming the Ring Current and Radiation
Belts
Electrons move anti-clockwise around Earth Ions
move clockwise around Earth
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Electrons are Trapped on the Magnetic Field as
They Drift Around Earth
Gyroperiod 10-3 sec (for 1 MeV
electrons) Bounce between hemispheres 0.1
sec Drift period around Earth 10 min
AFGL
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• Electrons drift around Earth on surfaces of
  roughly constant magnetic field magnitude
• - Magnetic field is distorted compressed on the
  sunward side and stretched out on the night side
  which also gives a day/night distortion to the
  radiation belt.

Peak flux is near L 4 5 (where L is roughly
the distance from the center of the Earth to the
location where the magnetic field line crosses
the equator)
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Question for Discussion

• Which electron flux profile would a
  geosynchronous satellite (dashed circle) see as
  it orbited Earth?

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2
Electron flux
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4
Equatorial Plane View
Location
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Answer
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When studying and monitoring space weather,
multiple satellite locations and models are
needed to obtain a complete picture of the
radiation belt properties.
Electron Flux (cm2 s sr)-1
Magnetic Field (nT)
Electron Flux (cm2 s sr)-1
Magnetic Field (nT)
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Solar Wind is the Source of the Radiation Belt
- Radiation belt electrons are trapped in the
magnetosphere, but accelerated by the solar wind
energy - Flowing solar wind causes ripples on
the surface of the magnetosphere that pump up the
electrons energy - Radiation levels increase
with increases in solar wind speed
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Surface Waves Propagate into the Magnetosphere
and Accelerate the Electrons
I. Mann
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Extreme Changes in the Radiation Levels are
Driven by the Solar Wind

• High-speed solar wind often recurs predictably
  due to the rotation of the sun
• Models can predict the radiation levels fairly
  well using the solar wind speed as input

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Radiation Levels are Highest During Solar Minimum
When Persistent High-Speed Solar Wind Streams
Occur
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Low-energy electrons stick to the spacecraft
surface. High-energy electrons penetrate the
satellite and can get embedded in insulating
materials. Electrons can slowly drift out of the
material, and therefore long periods (days) of
high electron fluxes are associated with
deep-dielectric anomalies.
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Satellite Anomaly Occurrence and Seasonal
Variability of Electron Fluence
Phantom commands are well correlated with 2-day
fluence of gt2 MeV electrons Solar cycle, solar
rotation and seasonal effects are also observed
peak fluxes observed during high-speed streams
and near the equinoxes.
G. Wrenn
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Models can be used to predict the intensity of
the radiation belts
Chris Smithtro, USAF NOAA/SEC
Input Vsw (ACE) GOES electrons - 1-,
2-, and 3-day predictions Pred. Vsw
(Wang-Sheeley) e-- up to 8-day predictions
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Future Challenge Specify and Predict the
Radiation in any Orbit
M. Bodeau, Boeing
J. Goldstein, SWRI
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Satellite Impacts
26 Mar 1996 - Anik E1- Solar panel failed, ESD.
Half of the transponders turned off. 11 Jan 1997
- Telstar 401 - Electrostatic discharge total
loss 11 Apr 1997 - Tempo 2 - Solar flare zapped
three transponders, DC power loss 4 Oct 1997 -
Insat 2D - Short circuit, Electrostatic
discharge, loss of power, total loss Dec 1998 -
TOMS - Single Event Upset disrupts spacecraft's
computer operations 15 Jul 2000 - ASCA (Astro-D)
- Satellite started spinning during solar
activity, total loss. 27 Sep 2001 - Solar Flare
Activity Postpones Kodiak Star Launch 21 Nov 2001
- Stardust Blinded By Solar Flare 21 Apr 2002 -
Nozomi - Hit by solar storm, loss of most
communications, mission loss. 25 Oct 2003 -
ADEOS-2 impacted by solar activity total loss
28 Oct 2003 - Mars Odyssey Probe MARIE
instrument destroyed due to solar activity Nov
2004 - Double Star redundant attitude Computer
failed
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Question for Discussion
Which spacecraft anomalies were likely to have
been caused by radiation belt electrons?

• 1. Equator-S
• 2. Polar
• Galaxy 4
• Equator-S and Galaxy 4
• 5. POLAR and Galaxy 4
• 6. All of them
• 7. None of them

Baker et al., 1998
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Which spacecraft anomalies were likely to have
been caused by radiation belt electrons?
The Equator-S and Galaxy 4 failures both occurred
after a long period of enhanced electrons. The
POLAR failure occurred shortly after the electron
flux rose, and coincident with enhanced energetic
protons. However, the cause of spacecraft
anomalies is often hard to pin down.
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Summary

• Radiation belt is a persistent and highly dynamic
  region of electron radiation within the
  magnetosphere
• Radiation fills much of the inner magnetosphere
  and impacts nearly all satellite orbits
• - Highest energy electrons cause internal
  discharges and lower energy electrons cause
  surface discharges
• Highest energy electrons are a radiation hazard
  for astronauts
• Solar wind is the energy source for the electrons
  the most intense radiation levels occur during
  solar minimum when the solar wind speed can be
  persistently high
• - A 3-D specification of the radiation belt is
  needed to help with the planning and operation of
  satellites in many different orbits.