  The transition between non resonant (Weibel-type) and resonant (whistler)
instabilities is investigated numerically in plasma configurations with an
ambient magnetic field of increasing amplitudes. The Vlasov-Maxwell system is
solved in a configuration where the fields have three components but depend
only on one coordinate and on time. The nonlinear evolution of these
instabilities is shown to lead to the excitation of electromagnetic and
electrostatic modes at the first few harmonics of the plasma frequency and, in
the case of a large ambient magnetic field, to a long-wavelength, spatial
modulation of the amplitude of the magnetic field generated by the whistler
instability.
