On the injection of electrons in oblique shocks
Аннотация
The mechanism by which superthermal electrons generate turbulence inside and behind an oblique shock is studied, and the implications for electron injection are considered. It is shown that, whereas in quasi-parallel shocks the streaming instability dominates, in superluminal shocks the waves are driven unstable by compressional anisotropies produced as a result of betatron acceleration of electrons traversing the shock. The injection process, in the diffusion approximation, is controlled by the structure of the shock and the corresponding diffusion coefficient, and appears to be markedly different for quasi-parallel than for superluminal shocks, owing to the differences between the field-aligned and cross-field transport properties. The requirements for efficient injection are examined and found to be less stringent in quasi-parallel shocks than in superluminal shocks. A naïve estimate suggests that efficient injection by this process may take place in superluminal shocks with Mach numbers in excess of ∼ 100, provided that the shocked electron plasma undergoes very effective collisionless heating. The implications of cross-field diffusion by field-line wandering for electron injection in perpendicular shocks are also discussed.