Plasma wakefield acceleration for ultrahigh-energy cosmic rays

Chen, P.*; Tajima, Toshiki; Takahashi, Yoshiyuki*

A cosmic acceleration mechanism is introduced which is based on the wakefields excited by the Alfven shocks in a relativistically flowing plasma,where the energy gain per distance of a test particle is Lorentz invariant. We show that there exists a threshold condition for transparency below which the accelerating particle is collision-free and suffers little energy loss in the plasma medium. The stochastic encounters of the random accelerating-decelerating phases results in a power-law energy spectrum: 1/E. The environment suitable for such plasma wakefield acceleration can be cosmically abundant. As an example, we discuss the possible production of super-GZK ultra high energy cosmic rays ( UHECR ) through this mechanism in the atmosphere of gamma ray bursts. We show that the acceleration gradient can be as high as 10eV/cm. The estimated event rate in our model agrees with that from UHECR observations.