Production of Ultrarelativistic Positrons by an Oblique Magnetosonic Shock Wave in an Electron-Positron-Ion Plasma

H. Hasegawa and Y. Ohsawa

Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan

Positron acceleration in an oblique magnetosonic shock wave in a plasma consisting of electrons, ions, and positrons is studied by means of a one-dimensional, relativistic, electromagnetic, particle simulation code with full particle dynamics. In the simulations, high-energy positrons with their Lorentz factors around 600 have been observed.
The electric field parallel to the magnetic field plays an essential role in the acceleration. It prevents some positrons from passing through the shock wave, i.e., they are reflected in the shock front. In many cases, these reflected particles may go away from the shock wave to the upstream region. When the shock propagation velocity and angle (the angle between the wave normal and the external magnetic field) satisfy a certain relation, however, these positrons cannot quickly escape from the wave. They stay around the shock front and suffer acceleration for long periods of time. These positrons move roughly parallel to the external magnetic field, and their Lorentz factors increase almost linearly with time.
A theory for the positron acceleration is developed. It explains these properties found in the simulations.