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

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?

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

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]

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

Adiabatic Invariants

• 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

Inward Radial Diffusion

• Breaks 3rd invariant

• Conservation of 1st + 2nd invariant – Betatron and Fermi acceleration

• Toroidal cf poloidal waves– Power– Diffusion rates

• Fluctuations in E, B fields– ULF waves f ~ mHz– ~ Pc5

• Gradient in phase space density– Transport

Source of ULF Waves

• Fast solar wind drives Kelvin Helmholtz instabilities

• SW pressure variations

• Both drive ULF wave power inside magnetosphere

• 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

Electron Phase Space Density

• Peak in MeV electron phase space density observed near 5.5 Re

• Does not support radial diffusion from the outer magnetosphere

• Suggests “local” acceleration

Chen et al., Nature Physics, [2007]

M = 2083 MeV/Gauss

Acceleration by Whistler Mode Waves

• Diffusion into loss cone E > ~10 keV– Whistler wave growth

• Diffusion at large pitch angles ~ MeV– Acceleration– Trapping

Local Diffusion Coefficients

• Whistler mode chorus waves

• Momentum diffusion more efficient for low fpe/fce

– Higher phase velocity

Horne et al. GRL, [2003]

CRRES Survey of fpe/fce

Meredith et al. [2002]

Concept

• Injection of ~1 - 100 keV electrons

• Temperature anisotropy excites chorus

• Whistler mode chorus accelerates fraction of population to ~ MeV energies

Summers et al. [1998]

Horne et al. [2005]

New Wave Acceleration Concept

Horne, Nature Physics [2007]

Jupiter - The Problem

[Bolton et al., Nature, 2002]

• 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

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?

Whistler Mode Waves at Jupiter

Resonant Diffusion

• 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

R = 10 RJ

Diffusion Rates

• 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

• Fokker-Planck equation

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

Production of Synchrotron Radiation• Suggest Gyro-resonant electron acceleration provides the missing step

Horne et al., Nature Physics, [2008]

Saturn

• Radiation belt intensity comparable to Earth’s

• Weak synchrotron emission

– Absorption by dust

– Weak belts L < 2.3

• Radial diffusion for L<6

• Santo-Costa et al. [2003]

• Rapid rotation

– Flux interchange

• Dipole moment 580 times Earth

Krimigis et al. [2005]

Saturn

• How is the peak in phase space density near L=6 produced?

• Could resonant wave acceleration be important?

Armstrong et al. [1983]

Saturn

Hospodarsky et al. [2008]

Saturn

• 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

10 keV

30

100 1 MeV

1 MeV100

30

10 keV

New Wave Acceleration Concept

Horne, Nature Physics [2007]

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