Neutrino processes in an external magnetic field in the density matrix formalism

In the matrix density formalism for a charged spinor particle located in a constant uniform magnetic field, we develop a technique for calculating the reaction rate and the four-momentum carried away from a plasma by a neutrino in one-vertex neutrino processes. Using this technique, we reproduce results for the luminosity in processes of neutrino synchrotron emission by electrons (positrons) and of electron-positron annihilation producing the neutrino pair.