presence of randomly moving "magnetic mirrors". So, if the magnetic mirror is moving towards the particle, the particle will end up with increased energy upon reflection. The opposite holds if the mirror is receding. This notion was used by Fermi (1949) to explain the mode of formation of cosmic rays. In this case the magnetic mirror is a moving interstellar magnetized cloud. In a random motion environment, Fermi argued, the probability of a head-on collision is greater than a head-tail collision, so particles would, on average, be accelerated. This random process is now called second-order Fermi acceleration, because the mean energy gain per bounce depends on the mirror velocity squared, . The resulting energy spectrum anticipated from this physical setup, however, is not universal as in the case of diffusive shock acceleration. See also Fermi-Ulam model Fermi glow References External links David Darling's article on Fermi acceleration Rieger, Bosch-Ramon and Duffy: Fermi acceleration in astrophysical jets. Astrophys.Space Sci. 309:119-125 (2007) Fusion power Physical quantities Dynamics (mechanics) Cosmic rays Acceleration