Cosmic Ray Acceleration in Space Plasmas

4.1. Acceleration particles in space plasmas as universal phenomenon in the Universe

Understanding the generation of CR (or acceleration of energetic charged particles) is one of the most fundamental goals of Astrophysics. As we note in Section 1.1 of Dorman (M2004), the basis of any mechanisms of charged particles acceleration in space plasma up to very high energies observed in CR is the interaction of individual particles with huge moving ensembles of particles through frozen in magnetic fields and induced electric fields. These moving ensembles (mass ejections, clouds, shock waves, magneto-hydrodynamic waves, etc.) have a huge kinetic energy many orders higher than energy of an individual particle. Therefore the energy of moving ensembles will lead to the gain of the energy of individual particles, and their energy become many orders higher than the energy of background plasma particles, so these individual particles became CR particles. The acceleration of an individual particle is possible only if the gain of energy per unit of time is bigger than the loss of energy. The loss of energy depends upon the mass mac, charge Ze, and total energy E (or kinetic energy Ek) of the accelerated particle, as well as upon properties of background plasma, magnetic field intensity, and electromagnetic radiation. The energy loss is especially important at small kinetic energies Ek of an accelerated particle (mostly ionization losses), and it become smaller than the energy gain only at Ek > E^ , where Eki is the minimal energy of ejection to the accelerated process. The value of Eki which depends upon the mass mac and effective charge Z*e of accelerated particles (where Z* < Z), and properties of the background plasma will be determine the chemical and isotopic contents of accelerated particles. In the middle energy region the energy loss per unit of time becomes much smaller than the energy gain, and the energy spectrum of accelerated particles will be formatted mainly by the rate of energy gain and probability of accelerated particles escaping from the acceleration volume. In the very high, super-relativistic energy region, the loses of energy again becomes important (for CR electrons, - synchrotron radiation in the magnetic fields and interactions with photons; for CR protons and nuclei, - interactions with photons). These energy loses and CR escaping from the acceleration volume lead to sufficient deformation of the power energy spectrum ^ E— with gradual increase of the power index y with the particle energy increasing and to a sharp cutting of CR spectrum from the high energy side.

First we shall consider the statistical mechanism of charged particle acceleration when the energy of a particle increases and decreases in collisions with magnetic clouds, but increases are bigger and more often than decreases. This mechanism originally was proposed more than 50 years ago by Fermi (1949). According to Fermi (1949) the frequency of collisions with increasing energy is higher than the frequency of collisions with decreasing energy. This gives a gradual increase of particle energy with the time up to the moment when the acceleration mechanism finishes affecting particle's energy (e.g., the particle escaping from the acceleration volume). We shall show that there is also another cause of particle energy increasing: the energy increasing and decreasing in collisions are not equal, but systematically increasing energy is little bigger than decreasing. As result, we show that the statistical mechanism of acceleration is about two times more effective than was considered originally by Fermi (1949). We shall consider the description of this mechanism and its development including the problem of ejection and changing of effective parameters of the mechanism during particle acceleration in Sections 4.2-4.8.

Statistical acceleration by plasma turbulence and by electromagnetic radiation will be considered in Sections 4.9-4.10. We consider statistical acceleration of particles by the Alfven mechanism of magnetic pumping and scattering in Section 4.11. The problem of the formation of accelerated particle flux escaped from CR source we consider in Section 4.12 (in general this flux is proportional to the CR intensity inside the source and to the probability of particles run away from the source, which depends from particle energy and other parameters).

The induction acceleration mechanisms, mostly by rotating magnetic stars we consider in Section 4.13, and particle acceleration by moving magnetic piston or magnetic cloud as result of single interaction and reflection we shortly consider in Section 4.14. Mechanisms of particle acceleration by shock waves and other moving magneto-hydrodynamic discontinuities during a single interaction are considered in Section 4.15. We consider the acceleration of particles in the case of magnetic collapse and the cumulative acceleration mechanism near the zero lines of magnetic field in Sections 4.16-4.17. The problem of tearing instability in neutral sheet region and triggering mechanisms of formatting fractals, percolation, and particle acceleration we consider in Section 4.18. Particle acceleration in sheer space plasma flows we consider in Section 4.19. Additional regular particle acceleration in space plasmas with two or more types of scatters moving with different velocities is considered in Section 4.20.

Very important universal shock-wave diffusion (regular) acceleration of charged particles, which is intensively developed during about last 30 years, we consider in details in Sections 4.21-4.31.

Cosmic Ray Acceleration in Space Plasmas

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