Electron Acceleration inside Jupiter’s Radiation Belt and the Origin of Synchrotron Radiation

Richard B. Horne1

R. M. Thorne2, S. A. Glauert1, J. D. Menietti3, Y. Y. Shprits2, and D. A. Gurnett3

1. British Antarctic Survey2. University of California, Los Angeles

3. University of Iowa

R.Horne@bas.ac.uk

REPW, Rarotonga, 8th August 2007

Application to Jupiter’s Radiation Belts

The Problem

[Bolton et al., Nature, 2002]

• How do you produce ~1 MeV electrons at L ~ 15?

• Synchrotron radiation indicates:– 50 MeV electrons at L=1.4

• Current theory– Betatron and Fermi acceleration by inward

transport

• Requires a source– > 1 MeV at 10 – 15 Rj

Inward Radial diffusion

• Flux at 90 at L=15.75 from Pioneer and Voyager

• Assume inward radial diffusion and at L=8.25 the flux should vary as the dotted line

• Some evidence for local plateau in flux

• Additional acceleration?

• Cannot be sure due to uncertainty over variability

• Radial diffusion also found to agree well with observations

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

Galileo Wave Data

Galileo Wave Data

Galileo Wave Data

• Bagenal [1994]

Density Model

Density Model – Latitude Variation

Galileo Data

• Survey of all Galileo data, 1996-2002

• Chorus wave power peaks outside orbit of Io

– Waves generated by flux interchange instabilities

• Wave power high where fpe/fce drops

– Energy diffusion more efficient

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 +-5 cyclotron resonances

– Bounce average over 10o latitude

• Energy diffusion peaks outside Io

– Wave acceleration

• Fokker-Planck equation

Wave Acceleration at Jupiter

• Evolution of electron flux

– 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

Conclusions

• Wave acceleration by whistler mode chorus is a viable process for producing ~ MeV electrons inside Jupiter’s radiation belt for t ~ 30 days

• Magnetic flux interchange instability provides energy to drive the waves

• Acceleration is most effective outside the moon Io

• Wave acceleration predicts pitch angle distributions peaked near 90o

• Trapped inside magnetic field• Energy dependent

• Wave acceleration is part of a multi-step process to produce synchrotron radiation from Jupiter

• And Saturn, Uranus, Neptune, exoplanets, ….???