Fermi acceleration, sometimes referred to as ''diffusive shock acceleration'' (a subclass of Fermi acceleration
[On the Origin of the Cosmic Radiation, E. Fermi, Physical Review 75, pp. 1169-1174, 1949]), is the
acceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
that
charged particles
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from s ...
undergo when being repeatedly reflected, usually by a
magnetic mirror
A magnetic mirror, known as a magnetic trap (магнитный захват) in Russia and briefly as a pyrotron in the US, is a type of magnetic confinement device used in fusion power to trap high temperature plasma using magnetic fields. T ...
(see also
Centrifugal mechanism of acceleration
Centrifugal acceleration of astroparticles to relativistic energies might take place in rotating astrophysical objects (see also Fermi acceleration). It is strongly believed that active galactic nuclei and pulsars have rotating magnetospheres, t ...
). It receives its name from physicist
Enrico Fermi
Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...
who first proposed the mechanism. This is thought to be the primary mechanism by which particles gain non-thermal energies in astrophysical
shock wave
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
s. It plays a very important role in many astrophysical models, mainly of shocks including
solar flare
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s and
supernova remnant
A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...
s.
There are two types of Fermi acceleration: first-order Fermi acceleration (in shocks) and second-order Fermi acceleration (in the environment of moving magnetized gas clouds). In both cases the environment has to be collisionless in order for the mechanism to be effective. This is because Fermi acceleration only applies to particles with energies exceeding the thermal energies, and frequent collisions with surrounding particles will cause severe energy loss and as a result no acceleration will occur.
First order Fermi acceleration
Shock waves typically have moving magnetic inhomogeneities both preceding and following them. Consider the case of a charged particle traveling 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 energy. The resulting energy spectrum of many particles undergoing this process (assuming that they do not influence the structure of the shock) turns out to be a power law:
where the spectral index
depends, for non-relativistic shocks, only on the compression ratio of the shock.
The term "First order" comes from the fact that the energy gain per shock crossing is proportional to
, the velocity of the shock divided by the speed of light.
The injection problem
A mystery of first order Fermi processes is the ''injection problem''. In the environment of a shock, only particles with energies that exceed the thermal energy by much (a factor of a few at least) can cross the shock and 'enter the game' of acceleration. It is presently unclear what mechanism causes the particles to initially have energies sufficiently high to do so.
[
André Balogh; Rudolf A. Treumann]
"Physics of Collisionless Shocks: Space Plasma Shock Waves"
2013.
Section 7.4 "The Injection Problem".
p. 362.
Second order Fermi acceleration
Second order Fermi acceleration relates to the amount of energy gained during the motion of a charged particle in the 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
The Fermi glow consists of ultraviolet-glowing particles, mostly hydrogen, originating from the Solar System's bow shock, created when light from stars and the Sun enter the region between the heliopause and the interstellar medium
References
{{reflist, 30em
External links
David Darling's article on Fermi accelerationRieger, 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