1. Richard B. Horne British Antarctic Survey Cambridge UK
The Predictability of the Earth’s Radiation Belts
Richard B. Horne
British Antarctic Survey
Cambridge UK
Invited talk, VarsITI Closing Symposium, Sofia, Bulgaria, 10th June 2019

2. Outline Why do we need to predict the radiation belts ?
The proton belt – an example
Variability of the electron belts
Physical processes that control the electron radiation belts
Example forecasting system - SaRIF

3. Earth’s Radiation Belts
Electrons and ions trapped inside the magnetic field
Only one proton belt
Two electron belts
Energies > 1 MeV
Peaks near 1.6 and 4.5 Re
Outer electron belt highly variable
Hazardous for spacecraft and humans

4. Solar Energetic Particle Events – Proton Belt
Maybe 1 – 3 per solar cycle
Protons become trapped in the outer region of the proton belt
Last for months – then released in a magnetic storm
O’Hare et al., (2019):
10Be and 36Cl in ice cores in ~660 BC indicate an event 10 times bigger than we have ever recorded directly (1956 event)

5. Proton Radiation Belt
Before March 1991
After March 1991 SEP event

6. Proton Belt - Degradation of Satellite Solar Arrays
Cover-glass thickness 150 um
Launch before event – lose 3.5% of power before reaching orbit
Launch after event – lose 8% of power before reaching orbit
Electric propulsion used commercially since 2015
~ 200 days to reach orbit
Lozinski et al. SW (2019)

7. Variability of the Electron Belts - 1. Day to Day
Geostationary orbit
How do you produce high energy ( >1 MeV) electrons?
What causes the variations?
No electrons greater than ~800 keV in the inner belt

8. Variability of the Electron Belts – 2. CME/Shock Compression
Horne and Pitchford, (2015)
New Electron Belt Formed in 2 Minutes
Slot region ‘filled-in’
Decay timescale ~ year
Geostationary orbit
8,000 km Slot region orbit

9. Variability of the Electron Belts – 3. Major Geomagnetic Storms
Baker et al. Nature (2004)
GEO orbit (approx.)
Haloween storms of 23rd Oct to 6th Nov 2003
47 satellites reported malfunctions
1 total loss
10 satellites – loss of service for more than 1 day
Electrons at much lower L
Decay time ~ year

10. Variability of the Electron Belts - 4. Rapid Loss
Rapid loss – or drop out
Baker et al. [2016]

11. Electron Radiation Belt Dynamics
b). Plasmaspheric hiss
13:49:24
13:49:29
13:49:34
0.0
2.0
1.0
0.5
1.5
Frequency (kHz) UT
10-15
10-14
10-13
10-12
SC1 Rumba
V2m-2Hz-1
Magnetopause
inward motion – causes loss
rapid motion – acceleration
Magnetic field fluctuations driven by ULF waves
Hiss waves
Loss
13:01:12
13:01:17
13:01:22 UT
0.0
0.5
2.0
1.0
1.5
10-13
10-12
10-11
10-10
10-8
10-9
V2m-2Hz-1
SC1 Rumba
a). EMIC waves
13:00
14:00
15:00 UT
0.0
1.0
1.5
2.0
0.5
Frequency (Hz)
10-2
100
102
nT2Hz-1
CRRES - FMI
Chorus waves Acceleration and loss
EMIC waves
Loss > 2 MeV
Electron drift path
Plus other waves and transport processes
Activity, location and energy dependent
Nonlinear, timescale –us to days

12. Time Dependent Model of the Electron Belts
Pitch angle diffusion
Energy diffusion
Solve the Fokker-Planck Equation
Model includes:
Wave-particle interactions
Radial transport
Loss to the atmosphere
Loss to the magnetopause
Radial diffusion
Loss to atmosphere
Loss to the magnetopause

13. Satellite Risk Prediction and Radiation Forecast (SaRIF)
Solar wind data from L1
Pressure and IMF Bz
Forecast magnetopause
Forecast Kp
Plasma wave properties scaled by Kp
Calculate pitch angle and energy diffusion coefficients
ULF wave power scaled by Kp
Calculate radial diffusion coefficients
Electron flux and energy spectrum scaled by Kp
Calculate spectrum at outer boundary
Calculate flux at low energy boundary
Solve Fokker-Planck Equation and forecast entire radiation belt flux

14. Example - 30 year Reconstruction of the Radiation Belts
Glauert et al., SW, (2018)
Coronal holes - Fast solar wind
GEO
Slot
Inner belt

15. Satellite Risk Prediction and Radiation Forecast (SaRIF) – Geostationary Orbit
Available as SaRIF via the ESA Space Weather Web portal

16. Satellite Risk Prediction and Radiation Forecast (SaRIF) – Geostationary Orbit
Available as SaRIF via the ESA Space Weather Web portal

17. Predictability - Timescales
Sun to L1 – usually 1-2 days, but the fastest is 14.5 hours
L1 to magnetopause - ~ 30 minutes, but can be much faster
Magnetopause to radiation belts – from tens of minutes to a few hours
Inside the magnetosphere:
Plasma waves can cause acceleration to MeV energies and loss – from milliseconds to days
Transport across the magnetic field - from hours to days
Magnetopause compression can cause rapid loss ~ tens of minutes – and or acceleration
Substorm cycle – few hours – but can repeat for days - storms
If the solar wind is ‘quiet’ we may be able to forecast up to 24 hours, maybe more – but rapid changes in the solar wind can reduce or remove our ability to predict
Essential to know solar wind pressure and IMF Bz

18. Summary
Electron radiation belts can vary on timescales from few minutes to days
To predict – need forecast of solar wind at L1 – pressure, IMF Bz
We must also develop more understanding of:
Non-linear wave-particle interactions on timescales from milliseconds to days
Diffusive and non-diffusive transport
Shock driven compression of the magnetopause
Magnetic field disruptions during storms
Changes in the source electrons
These are major challenges
But they are needed to protect satellites from Space Weather