A magnetoplasma in which energetic particles are injected is considered. The laws which govern the magnetic induction, the magnetic field and the kinetic energy density of the particles are given. It is shown that whereas the beta of the plasma can reach very high values, the perpendicular kinetic energy density cannot exceed a certain critical value Wc which is equal to (11.6/L)6 keV.cm-3 in the magnetosphere and which is of the order of 1 keV.cm-3 in the tail.