Abstract
We present a new particle acceleration mechanism called ultra‐relativistic acceleration (URA). URA comprises electron energization due to a special form of nonlinear phase trapping by a coherent whistler‐mode wave for electrons with an initial Lorentz factor γ0 satisfying γ0 > ΩEQ/ω; ω is the wave frequency and ΩEQ is the electron cyclotron frequency at the equator of an assumed dipole magnetic field. Radiation belt electrons that encounter a combination of relativistic turning acceleration (RTA) followed by multiple URA interactions can undergo significant energy increase. Under ideal conditions, at Earth (L = 4) several‐hundred‐keV electrons can be energized to several MeV within a few seconds, while at Jupiter (L = 8), several‐hundred‐keV electrons can be energized by tens of MeV in a few tens of seconds. URA can play a prominent role in generating the several‐MeV electrons observed in Earth's outer zone and the tens‐of‐MeV electrons observed in Jupiter's magnetosphere. More generally, we expect URA to be an effective electron energization mechanism in cosmic plasma environments that contain a magnetic mirror geometry and electromagnetic whistler‐mode emissions.
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