GalacticCosmic Rays
Trapped Electronsand Protons
The Radiation Belts and Killer ElectronsTerry Onsager, NOAA Space Environment Center
Solar EnergeticParticles
Radiation belt electron energies: ~ few hundred keV and aboveRing current electron energies: ~ few hundred keV and below
AF-Geospace, Courtesy of Greg Ginet, AFRL Magnetospheric Specification Model, Rice University
Ring Current and Radiation Belts
Radiation belt dynamics is controlled mainly by diffusion and magnetic drifts. Ring current dynamics is controlled mainly by electric and magnetic drifts.
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 (> 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
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
ElectronsIons
Energetic Plasma From the Tail Diverts Around Earth
Forming the Ring Current and Radiation Belts
Electrons move anti-clockwise around EarthIons move clockwise around Earth
Electrons are Trapped on the Magnetic Field as They Drift Around Earth
AFGL
Gyroperiod: ~ 10-3 sec (for 1 MeV electrons)Bounce between hemispheres: ~ 0.1 secDrift period around Earth: ~10 min
AF-Geospace, Courtesy of Greg Ginet, AFRL
- 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)
Question for Discussion:• Which electron flux profile would a geosynchronous
satellite (dashed circle) see as it orbited Earth?
Equatorial Plane View
midnight
noonnoon
midnight
noonnoon
midnight noonnoon
midnight noonnoon
2
1
3
4
Ele
ctro
n fl
ux
Location
1
10100
1000
Ele
ctro
ns
e/(
cms
sr) GOES8
> 2 MeV
60
100
Hp
nT
GOES8
110
1001000
Ele
ctro
ns
e/(
cms
sr) GOES10
> 2 MeV
40
80
120
Hp
nT
GOES10
midnight
noonnoon
2
Answer:
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
Electron Flux
(cm2 s sr)-1
Magnetic Field(nT)
Magnetic Field(nT)
- 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
Solar Wind is the Source of the Radiation Belt
I. Mann
Surface Waves Propagate into the Magnetosphere and Accelerate the Electrons
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
1E+02
1E+04
1E+06
1E+08
1E+10
0
50
100
150
200
250
Flu
en
ce
Su
nsp
ot
Nu
mb
er
1987 1989 1991 1993 1995 1997 1999 2001 2003 2005
Sunspot Number Electron Fluence
Monthly Mean Sunspot Number and Electron Fluence
Radiation Levels are Highest During Solar Minimum When Persistent High-Speed Solar
Wind Streams Occur
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.
2-day fluence*
Data days
ESD switches
Switch days
Green <108 2291 7 0.3%
Amber 108-109 952 102 10.7%
Red 109 332 105 31.6%
Total 3575 214 6.0%
Phantom commands are well correlated with 2-day fluence of >2 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
Satellite Anomaly Occurrence andSeasonal Variability of Electron Fluence
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
Future Challenge:Specify and Predict the Radiation in any Orbit
J. Goldstein, SWRI
M. Bodeau, Boeing
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
Satellite Impacts
Which spacecraft anomalies were likely to have been caused by radiation belt electrons?
Baker et al., 1998
1. Equator-S2. Polar3. Galaxy 44. Equator-S and Galaxy 45. POLAR and Galaxy 46. All of them7. None of them
Question for Discussion:
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.
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.
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