Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsm53b1286s&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #SM53B-1286
Physics
2720 Energetic Particles: Trapped, 2774 Radiation Belts, 7807 Charged Particle Motion And Acceleration, 7845 Particle Acceleration
Scientific paper
The main mechanism for the formation of an electron radiation belt in a planetary magnetosphere is considered to be radial (cross-L) diffusion toward the planet from an external boundary. In this process the first and second adiabatic invariants are conserved and the third is violated. Typical electron energies in the Earth's outer radiation belt are E=400keV-10MeV ,while in Jupiter's inner magnetosphere electron energies are in the range E=1- 100MeV. Radial diffusion alone cannot explain the observed relativistic electron populations at Earth and Jupiter. For some time it has been thought that local acceleration due to electron cyclotron resonance with plasma waves(associated with violation of the first adiabatic invariant)could be important in energizing radiation belt electrons. We introduce a new particle acceleration mechanism called ultra-relativistic acceleration (URA). URA consists of electron energization due to a special form of nonlinear phase trapping by a coherent whistler-mode wave for electrons with an initial Lorentz factor exceeding a critical value; the critical value is the inverse of the wave frequency scaled by the cyclotron frequency at the magnetic equator of an assumed dipole field. Electrons that encounter multiple URA interactions together with relativistic turning acceleration(RTA) can undergo significant energization,e.g., for Earth(L=4) several-hundred keV electrons can be accelerated to a few MeV in about a second,while at Jupiter(L=8) several-hundred keV electrons can be accelerated to tens of MeV in seconds. We expect URA (and RTA) to be effective in the wider context of space and cosmic plasmas. Necessary conditions for significant energization by the URA and RTA mechanisms include a magnetic mirror geometry,a sufficient supply of seed electrons,and multiple whistler-mode wave packets of sufficient duration.
Omura Yuji
Summers Donald
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