Bow shock injection and acceleration: a theoretical overview

Acceleration of charged particles at shocks by the Fermi mechanism is widely believed to be responsible for nonthermal particle production and associated emission in a variety of astrophysical objects. The mechanism is of a bootstrap type in nature which means that it requires a seed population of suprathermal particles for its initialization. These particles not only form a pool for further acceleration but more importantly, also generate wave turbulence that scatters and thermalizes plasma, inflowing into the shock, thus maintaining their own population. The problem of identification of the origin of seed particles and description of their dynamics (injection problem), while being technically a part of the diffusive shock (or first order Fermi) acceleration theory, clearly roots in or is almost equivalent to the classical problem of collisionless shock dissipation as it was formulated more than forty years ago. Furthermore, injection is believed to play a crucial role in formation of particle spectra and even the shock structure in such powerful astrophysical shocks as the supernova remnant shocks which are thought to accelerate galactic cosmic rays. However, an excellent laboratory for the study of injection mechanisms and further acceleration to moderate energies is the Earth's bow shock. A wealth of observational results along with the ever improving computer simulations has inspired some theoretical suggestions for possible injection scenarios of both ions and electrons. These will be discussed in this talk including injection selectivity with respect to different ion species and shock parameters such as its obliquity and Mach number.