Comparisons of Inner Radiation Belt Formation in Planetary Magnetospheres

1. Comparisons of Inner Radiation Belt Formation in Planetary Magnetospheres Richard B Horne British Antarctic Survey Cambridge R.Horne@bas.ac.uk Invited talk. AGU Chapman Conference on Universal Heliophysical Processes, Savannah, USA, 13th November 2008

2. Outline • The problem of electron acceleration • Earth’s radiation belts • Acceleration by radial diffusion • Cyclotron resonant acceleration • Application to Jupiter • Scaling • Rapid rotation • Application to Saturn • Is wave acceleration a new universal process?

3. Radiation Belts - The Problem • Basic science • How do you produce >1 MeV electrons? • Space weather • Hazard to humans and satellites • Climate link • Precipitation transmits solar variability to the atmosphere • Precipitation - chemistry – temperatures - winds

4. Why is Acceleration Needed? • Flux increases above pre-storm level before Dst recovered • Non adiabatic • Net acceleration • Timescale ~ 1-2 days Kim and Chan, [1997]

5. How do you Produce MeV electrons in the Radiation Belts? • Original theory: • Electrons originate from the solar wind • Diffuse inwards towards the planet and gain energy • (betatron and Fermi acceleration) • Lost by precipitation into the atmosphere

6. Cyclic motion 3 adiabatic invariants If conserved no net acceleration or loss Acceleration requires breaking 1 or more invariant Requires E, B fields at frequencies drift ~ 0.1-10 mHz bounce ~ Hz gyration ~ kHz Adiabatic Invariants

7. Breaks 3rd invariant Conservation of 1st + 2nd invariant Betatron and Fermi acceleration Toroidal cf poloidal waves Power Diffusion rates Inward Radial Diffusion • Fluctuations in E, B fields • ULF waves f ~ mHz • ~ Pc5 • Gradient in phase space density • Transport

8. Solar Wind velocity correlated with ULF (Pc 5) wave power [Mann et al., 2004] ULF waves (Pc5) correlated with 1.8 MeV electrons (GEO) ~ 2 day delay Source of ULF Waves • Fast solar wind drives Kelvin Helmholtz instabilities • SW pressure variations • Both drive ULF wave power inside magnetosphere

9. Peak in MeV electron phase space density observed near 5.5 Re Does not support radial diffusion from the outer magnetosphere Suggests “local” acceleration Electron Phase Space Density M = 2083 MeV/Gauss Chen et al., Nature Physics, [2007]

10. Diffusion into loss cone E > ~10 keV Whistler wave growth Diffusion at large pitch angles ~ MeV Acceleration Trapping Acceleration by Whistler Mode Waves

11. Whistler mode chorus waves Momentum diffusion more efficient for low fpe/fce Higher phase velocity Local Diffusion Coefficients Horne et al. GRL, [2003]

12. CRRES Survey of fpe/fce Meredith et al. [2002]

13. Concept Summers et al. [1998] • Injection of ~1 - 100 keV electrons • Temperature anisotropy excites chorus • Whistler mode chorus accelerates fraction of population to ~ MeV energies Horne et al. [2005]

14. New Wave Acceleration Concept Horne, Nature Physics [2007]

15. [Bolton et al., Nature, 2002] Jupiter - The Problem • Synchrotron radiation indicates intense radiation belt: • 50 MeV electrons at L=1.4 • Current theory • Acceleration by radial diffusion • Could gyro-resonant electron acceleration apply to Jupiter? • Differences • Dipole moment 20,000 times Earth • Io is the main source of plasma • Rapid rotation – flux interchange • Dust and rings - absorption

16. Electron Phase Space Density McIlwain and Fillius [1975] • Gradient in phase space density should drive inward radial diffusion for L < 10 • BUT – to get 50 MeV at L=1.4 still requires a source > 1 MeV at 10 – 15 Rj • How is the peak in phase space density produced at L>10?

17. Whistler Mode Waves at Jupiter

18. Scaling similar to Earth Energy transfer via whistler mode waves from low to high energy and large pitch angles Trapping inside magnetic field at high energy Resonant Diffusion R = 10 RJ

19. Diffusion Rates • Fokker-Planck equation • Diffusion rates • PADIE code [Glauert and Horne, 2005] • Model wave spectrum from Galileo 13:20-13:30 SCET • 30o angular spread of waves • Landau and n= +- 1,2,3,4,5 cyclotron resonances • Bounce average over 10o latitude • Energy diffusion peaks outside Io • Wave acceleration

20. Gyro-resonant Electron Acceleration at Jupiter • 2d Fokker-Planck • Initial flux from Divine and Garrett [1983] • Fixed boundary conditions at 0.3 and 100 MeV • Flux=0 inside loss cone and flat gradient at 90o • Timescale ~ 30 days for flux of 1 - 6 MeV electrons to increase by a factor of 10 • Timescale is comparable to transport timescale (20 - 50 days) for thermal plasma • Predict anisotropic pitch angle distribution

21. Production of Synchrotron Radiation • Suggest Gyro-resonant electron acceleration provides the missing step Horne et al., Nature Physics, [2008]

22. Saturn • Radiation belt intensity comparable to Earth’s • Weak synchrotron emission • Absorption by dust • Weak belts L < 2.3 Krimigis et al. [2005] • Radial diffusion for L<6 • Santo-Costa et al. [2003] • Rapid rotation • Flux interchange • Dipole moment 580 times Earth

23. Saturn • How is the peak in phase space density near L=6 produced? • Could resonant wave acceleration be important? Armstrong et al. [1983]

24. Saturn Hospodarsky et al. [2008]

25. Saturn 10 keV • E < 10 keV – weak precipitation • E > 30 keV • Trapped electrons • Acceleration to higher energies • Timescale ~ hours-days for >100 keV electrons • Formation of pancake Tp > Tz distributions • Gyro-resonant acceleration effective • Pitch angle diffusion • L=7 • fpe/fce = 10 • Energy diffusion 30 100 1 MeV 10 keV 30 100 1 MeV

26. New Wave Acceleration Concept Horne, Nature Physics [2007]

27. Conclusions • Universal processes for radiation belt formation • Radial diffusion – for transport and acceleration • Wave-particle interactions – for losses to the atmosphere • New Universal processes to add • Gyro-resonant wave-particle interactions – for acceleration • Plasma injection - to drive the waves • Leads to a new concept for radiation belt formation • Earth • Jupiter, Saturn – evidence – but requires more testing • Uranus, Neptune? • Exoplanets? • Solar context – resonant wave acceleration in solar flares