A cosmic acceleration mechanism was introduced by Chen, Tajima, and Takahashi in 2002, which is based on the wakefields excited by the Alfvén shocks in a relativistically flowing plasma. 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 result in a power-law energy spectrum: f(ϵ) ∝ 1/ϵ2. As an example, we discuss the possible production in the atmosphere of gamma ray bursts of ultrahigh-energy cosmic rays (UHECR) exceeding the Greisen-Zatsepin-Kuzmin (GZK) cutoff. The estimated event rate in our model agrees with that from UHECR observations.
