# Fermi acceleration

Fermi acceleration is a mechanism, first suggested by Enrico Fermi in
1949, to explain the explain the origin of cosmic
rays. It involves charged particles being reflected by the moving interstellar
magnetic field and either gaining or losing energy, depending on whether
the "magnetic mirror" is approaching or receding. In a typical 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.
In 1977, theorists showed that Fermi acceleration by supernova
remnant 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 forth again and again, 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**.