Field-induced axion emission via process $e^+ e^- \to a$ in plasma

The annihilation into axion $e^+ e^- \to a$ is investigated in a plasma and an external magnetic field. This process via a plasmon intermediate state has a resonant character at a particular energy of the emitted axion. The emissivity by $e^+ e^- \to a$ is compared with the axion cyclotron emissivity.