RadiationBeltsin
the SolarSystem
EliasRoussos
MaxPlanckInstitutefor SolarSystemResearch
Göttingen,Germany
Radiation Belts
• Component of planetary
magnetospheres
• Stable, magnetic trapping of
energetic charged particle
radiation
• Energies: >~10 keV
(electrons and ions)
• High fluxes
• Can affect their environment,
spacecraft and/or astronauts
Radiation Belts in the Solar System
Spacecraft measurements
Mauk et al. (2010)
Spacecraft measurements
• >5 MeV ion and electron fluxes
at Jupiter: 1-2 orders of
magnitude higher than at other
planets
• Earth the strongest proton
radiation belts below 1 MeV
• Saturn has weak proton and
electron belts
• Why are Jupiter’s MeV belts the
strongest?
Mauk et al. (2010)
The role of the magnetic field
• Closed magnetic field
configuration
• Large magnetic moment
Credit: Anna
Kotova, MPS
• Strong magnetic field ensures
energetic particle trapping
stability
• Jupiter is by far the strongest
magnetic trap in the solar
system
Balancing sources and losses
The “leaky bucket model”
Energetic particle sources
• Direct or acceleration of low
energy particles
• Direct:
• Galactic Cosmic Rays
Credit: NASA
• Solar Cosmic Rays (…)
• Acceleration:
• Transport from weak to strong
magnetic fields
• Solar wind & internal plasma
sources
• Injections, waves (…)
P. Kollmann, PhD Thesis (2013)
Energetic particle sources
• Weak solar wind source at
Jupiter but there is a strong
internal plasma source: Io
• Circulation of Iogenic plasma
Credit: NASA/JPL
produces high fluxes of
energetic particles
• Waves and transport into strong
field regions further accelerates
this population
• Saturn, Uranus, Neptune also
Credit: LASP/Univ. of Colorado
have internal sources: why are
they so different?
Energetic particle sinks
• Waves/diffusion direct particles
onto the planets
• Moons and rings absorb particle
radiation
Credit: NASA/JPL/SSI
• Gas & dust clouds lower the
particle energies (charge
exchange, energy loss)
• Synchrotron radiation lowers
particle energies
• Most intense losses at Saturn.
Why?
Credit: MPS
Energetic particle sinks:
Saturn vs other planets
• Rotational and magnetic axis
aligned
• All trapped particles at Saturn
cross the orbital plane of moons
and rings
Credit: Anna
Kotova, MPS
• Main rings are a heavy loss
region
• Dipole geometry and rings
unique to Saturn
• Losses at other planets much
less severe
Credit: LASP/Univ. of Colorado
Balancing sources and losses
The “leaky bucket model”: does the bucket “fill-up”?
(ie. are any of those belts saturated?)
The Kennel-Petschek Limit
• Waves change the mirror points
• Whistler (electrons) and ioncyclotron waves (ions)
• Particles lost if mirror points are
in the atmosphere (”loss-cone
scattering”)
• Rapid wave growth with
increasing trapped fluxes
• Intense loss-cone scattering
(“strong diffusion limit”) if fluxes
above the Kennel-Petschek
limit (KP-limit)
The Kennel-Petschek Limit
• Electron fluxes at all planets
excluding Saturn saturated up to
about 1 MeV
• Difference in absolute fluxes
due to the different KP-limits
• Only Jupiter close to the
electron KP-limit above 1 MeV
• Effect of heavy losses at Saturn
clearly visible
• Similar situation (for Jupiter,
Earth and Saturn) for ions
Mauk et al. 2010
The Galileo C22 orbit
The extreme fluxes of orbit C22 have severely limited lifetimes!
Garrett et al. 2012
Summary
• Flux levels in planetary radiation belts depend on:
• The strength and the geometry of the intrinsic magnetic field
• The balance between sources and sinks
• The field and plasma dependent trapping limit
• Jupiter combines:
• Strong intrinsic field and particle acceleration sources
• A field geometry that reduces the effects of particle sinks
• The highest flux trapping limit
• Saturn’s belts are severely weakened by the moons & rings