1. Extreme Events In The Earth’s Electron Radiation Belts
Sarah A. Glauert
Richard B. Horne, Nigel P. Meredith
British Antarctic Survey, Cambridge, UK
ESWW 14, Ostend, Belgium
29 November 2017
Image: NASA

2. Outline Background on the model Demonstration for an large storm
Extreme CME
Extreme period of fast solar wind
Comparison of results with extreme event analysis

3. EU-FP7 project April 2014 – March 2017
Aim: To model severe space weather events and mitigate their effects on satellites by developing better mitigation guidelines, forecasting, and by experimental testing of new materials and methodologies to reduce vulnerability
The research leading to these results funded in part by the European Union Seventh Framework Programme (FP7) under grant agreement No SPACESTORM

4. Electron Radiation Belts
High energy electrons trapped by Earth’s magnetic field
Two torus shaped regions:
Inner belt Between 1.2 and 2 Earth radii Fairly stable
Outer belt Between 3 and 7 Earth radii Highly dynamic
Slot region between the two belts
Flux is usually low but can be significant during strong storms
The Earth’s radiation belts are regions in space where high energy electrons are trapped by the Earth’s magnetic field. Generally, under quiet conditions two belts are formed. The inner belt lies between about 1.2 and 2 Earth radii at the equator and is relatively stable. The outer belt, between about 3.5 and 7 Earth radii at the equator, is very dynamic. Between these two belt lies the slot region which is often described as having much lower electron fluxes. This is true during quiet conditions but the flux here can be significant during more active periods.

5. BAS Radiation Belt Model
Diffusion equation for the drift averaged phase-space density
Includes:
Radial transport
Wave-particle interactions
Chorus, EMIC waves, plasmaspheric hiss, lightning-generated whistlers
Loss to atmosphere and magnetopause
Drivers
Kp sequence
Diffusion rates
Solar wind pressure and IMF Bz
Magnetopause location [Shue et al, 1998]
We model the behaviour of the radiation belts using the BAS-RBM. This is a state-of-the-art model that has been developed at BAS over the last 5 years or so. It is based around solving a diffusion equation for the electron phase-space density. The equation is shown on the right. If equations aren’t your thing then feel free to ignore it – and I promise there won’t be any more! I’ve just put it here to illustrate the processes we include in the model. We have a term to determine the radial transport of the electrons, terms that deal with the interactions between the electrons and various electromagnetic waves that are present in space and we also account for loss of electrons to the atmosphere and magnetopause. There are many types of wave present in space; we include the effects of plasmaspheric hiss, lightning-generated whistlers, whistler mode chorus waves and electro-magnetic ion cyclotron waves.
Glauert et al. [2014a, 2014b], Horne et al. [2013], Kersten et al. [2014]

6. Diffusion coefficients
Radial diffusion
Ozeke et al., [2014]
Chorus
Don’t have enough data for extreme conditions
Used extreme value analysis
Thanks to Ianto Cannon

7. Model boundaries 0o to 90o in pitch-angle (α) Between L*=1.5 and L*= 8
Minimum and maximum energy
Energy range varies with L*
Emin=100 keV, Emax = 30 MeV at L*=8
Need a boundary condition on each boundary
And an initial state for the whole grid

8. Boundary conditions α = 0o, α = 90o ∂f/∂α = 0 Emax(L) f = 0
Outer L* boundary
At L*= ∂f/∂L=0
Minimum energy
Emin = 100 keV at L*=8
Use POES data
Technique developed by Hayley Allison

9. 20 Nov. 2003 After Halloween storm Dst = -422 nT L* >800 keV Kp
>2 MeV
>4MeV
GOES flux
(cm-1s-1sr-1)
Bz Vsw (nT) (km s-1)
Psw Dst
(nPa) (nT) Kp
Baker et al.,
2004

10. An extreme CME What is extreme? Realistic extreme - July 2012
Magnetic field
Solar wind speed
Number density
Dst Kp
What is extreme?
Realistic extreme - July 2012
Very large CME missed Earth
Observed by STEREO-A
Baker et al. [2013]
Dst ~-470 nT Vsw > 2000 km/s
Kp from Peter Wintoft (Lund)
Storm phase dependent low energy boundary from POES data
How would the radiation belts have responded?
The other application of the model that I’m going to talk about is calculating the effects of an extreme CME. The first issue is defining the event. The Carrington event is the largest recorded event, but we don’t have enough information from the event to simulate it accurately. However, in July 2012 a very large CME left the sun. It missed the Earth but was observed by STEREO-A and those observations have been used to reproduce the solar wind and other parameters that would have been seen at Earth, had the CME come our way. Dan Baker estimated that Dst would be about -470 nT and the solar wind speed was over 2000 km/s. The panel shows the various parameters, the dotted lines are their best estimates after applying corrections. The question is, what would have happened had this CME hit the Earth?

11. July 2012 CME Dropout to L*=~3 L*<3 – rapid increase Recovery
GEO L*
Dropout to L*=~3
Magnetopause L*=~4.5
L*<3 – rapid increase
Recovery
Initially L*~3, later L*~4
L*<3 – slow decay

12. Range of results Baker et al. only has 1 day after storm
Have to decide what happens next
These choices affect peak fluxes
MEO more at risk than GEO

13. Very High Speed Solar Wind Stream
Super-posed epoch analysis
Meredith et al., [2011]
No density – No MP

14. Extreme case? Epoch-1 to epoch+7 days Upper quartile
2 MeV flux at GEO <103cm-1 sr-1 s-1 keV-1
Days from epoch
Days from epoch

15. Extreme case? Maximum values Epoch-1 to epoch+7 days Upper quartile
Peak flux ~ L* = 3
High flux at GEO
2 MeV flux >102 cm-2 sr-1 s-1 keV-1
Significant flux in the slot region
2 MeV flux >104 cm-2 sr-1 s-1 keV-1
Flux at GEO sensitive to MP location
Activity determines decay
Low activity => low wave power => slow decay
Another HSS may enhance the belts
Epoch-1 to epoch+7 days
Upper quartile
Not extreme
Days from epoch

16. Comparison with Meredith et al.
1 in 100 year event [Meredith et al., 2017]
HSS Maximum flux
CME maximum flux
Energy
2.05 MeV
2 MeV
L* = 4.5
5.8x102
1.4x x102
5.2x x102
L* = 6.0
1.6x102
5.3x x103
788 keV
800 keV
9.3x103
8.1x x104
7.9x x104
3.0x103
1.0x x103
~1.1x103
Flux units are cm-1 sr-1 s-1 keV-1
CME
Activity following storm has greatest influence on flux at GEO
HSS
Magnetopause location has greatest influence

17. Flux in MEO
Good agreement between maximum model flux and extreme value analysis
800 keV flux similar for both HSS and CME
CME associated with higher 2 MeV flux
HSS
CME

18. Flux near GEO CME Large dropout HSS
Large variation in maximum flux depending on magnetopause location
At GEO, HSS may lead to higher fluxes than a CME
HSS
CME

19. Conclusions Extreme CME Initially extreme fluxes at low L*, not at GEO
Outer belt (L*>~3 )flux drop-out
Slot region (L*<~3) flux increases rapidly by several orders of magnitude
Peak flux at GEO depends on activity following the CME
Extreme HSS
Flux at GEO can be orders of magnitude higher than extreme CME
Peak flux is around L* = 3 (L*=4 for CME)
Slot region fluxes are again raised by several orders of magnitude
Harder spectrum than CME
Timescale for decay depends on geomagnetic activity
Results are consistent with an extreme value analysis [Meredith et al., 2017]

20. Thank you

22. Low energy boundary Don’t have POES data for the extreme event
POES data for November 2003 event
Divide 2003 event into 4 phases:
Pre-storm 00:00 16/ :00 20/11
Drop-out 10:30 20/ :30 20/11
Recovery 12:00 23/ :00 26/11
Post storm 00:00 28/ :00 28/11
Calculate average flux for each phase
Identify corresponding phases in July 2012 event
Use average flux for the phase as boundary condition