Fermi acceleration
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Fermi acceleration, also called diffusive shock acceleration, is the acceleration that charged particles undergo when reflected by a magnetic mirror. This is one of the primary means by which particles are heated in shock waves. It is thought to be important in many astrophysical shocks, including solar flares and supernovae.
Shock waves typically have moving magnetic inhomogeneities both preceding and following them. Consider the case of a charged particle travelling through the shock wave (from upstream to downstream). If it encounters a moving change in the magnetic field, this can reflect it back through the shock (downstream to upstream) at increased velocity. If a similar process occurs upstream, the particle will again gain energy. These multiple reflections greatly increase its velocity (and hence, en masse, the temperature).
Fermi Acceleration relates also to the amount of energy gained or lost by a bouncing particle depending on the relative movement of the "magnetic mirror". So, if the 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 formatoin of cosmic rays. in this case the magnetic mirror is a moving interstellar magnetic field. 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.
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