  The process of neutrino production of electron positron pairs in a magnetic
field of arbitrary strength, where electrons and positrons can be created in
the states corresponding to excited Landau levels, is analysed. The mean value
of the neutrino energy loss due to the process $\nu \to \nu e^- e^+$ is
calculated. The result can be applied for calculating the efficiency of the
electron-positron plasma production by neutrinos in the conditions of the Kerr
black hole accretion disc considered by experts as the most possible source of
a short cosmological gamma burst. The presented research can be also useful for
further development of the calculation technic for an analysis of quantum
processes in external active medium, and in part in the conditions of
moderately strong magnetic field, when taking account of the ground Landau
level appears to be insufficient.
