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.