a mechanism first suggested by the Italian - American psysicist Enrico fermi in 1949 , to explain the origin of COSMIC RAYS it involves charged particles being reflected by moving intersteller magnetic field and either gaining or losing energy , depending on whether the " magnetic mirror " is approaching or receding . In a typicial environment , fermi argued , the probability of a head-on collision is greater then a head-tail collision , so particles would , on average be accelerated . this ramdon ( act of kindness ) process is now called second -order fermi acceleraration, because the mean energy gain per bounce depends on the mirror verlosity squared , in 1977 theorists showed that fermi accerlation by SUPERNOVA REMANT shocks is particularly efficient , because the motions are not random a charged particle ahead of the shock front can pass through the shock and then be scattered by magnetic inhomogeneities behind the shock , the particle gains energy from this bounce and flies back across the shock , where it can be scattered by magnetic inhomogeneities ahead of the shock , this enables the particle to bounce back and fourth agian and agian , gaining energy each time , because the mean energy gain depends only linearly on the shock velocity , this process is now called first-order-Fermi acceleration.
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