The Radiation Belts and Killer Electrons
Terry Onsager, NOAA Space Environment Center
Solar Energetic
Particles
Galactic
Cosmic Rays
Trapped Electrons
and Protons
Ring Current and Radiation Belts
AF-Geospace, Courtesy of Greg Ginet, AFRL
Magnetospheric Specification Model, Rice University
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.
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
Energetic Plasma From the Tail Diverts Around Earth
Forming the Ring Current and Radiation Belts
Ions
Electrons
Electrons move anti-clockwise around Earth
Ions move clockwise around Earth
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
-
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)
AF-Geospace, Courtesy of Greg Ginet, AFRL
Question for Discussion:
• Which electron flux profile would a geosynchronous
satellite (dashed circle) see as it orbited Earth?
noon
noon
Electron flux
1
midnight
2
midnight
noon
3
4
noon
noon
midnight
noon
noon
midnight
noon
Location
Equatorial Plane View
Answer:
2
midnight
Electrons
e/(cm s sr)
noon
Electron 1000
100
Flux
(cm2 s sr)-1 10
1
nT
GOES8
100
60
Electrons
e/(cm s sr)
Hp
Magnetic
Field
(nT)
Electron 1000
100
Flux
(cm2 s sr)-1 10
GOES10
> 2 MeV
1
120
nT
Hp
Magnetic
Field
(nT)
GOES8
> 2 MeV
80
40
GOES10
noon
When studying and
monitoring space
weather, multiple
satellite locations
and models are
needed to obtain a
complete picture of
the radiation belt
properties.
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
Surface Waves Propagate into the
Magnetosphere and Accelerate the Electrons
I. Mann
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
Radiation Levels are Highest During Solar
Minimum When Persistent High-Speed Solar
Wind Streams Occur
Monthly Mean Sunspot Number and Electron Fluence
1E+10
250
200
Fluence
150
1E+06
100
1E+04
50
1E+02
0
1987
1989
1991
1993
1995
Sunspot Number
1997
1999
Electron Fluence
2001
2003
2005
Sunspot Number
1E+08
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.
Satellite Anomaly Occurrence and
Seasonal Variability of Electron Fluence
2-day
fluence*
8
Green
<10
Amber
10 -10
Red
10
8
9
Total
9
Data
days
ESD
switches
Switch
days
2291
7
0.3%
952
102
10.7%
332
105
31.6%
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
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
M. Bodeau, Boeing
J. Goldstein, SWRI
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
Question for
Discussion:
Which spacecraft anomalies
were likely to have been caused
by radiation belt electrons?
1.
2.
3.
4.
5.
6.
7.
Equator-S
Polar
Galaxy 4
Equator-S and Galaxy 4
POLAR and Galaxy 4
All of them
None of them
Baker et al., 1998
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.