  We study the stability of current filaments produced by the Weibel, or
current filamentation, instability in weakly magnetized counter-streaming
plasmas. It is shown that a resonance exists between the current-carrying ions
and a longitudinal drift-kink mode that strongly deforms and eventually breaks
the current filaments. Analytical estimates of the wavelength, growth rate and
saturation level of the resonant mode are derived and validated by
three-dimensional particle-in-cell simulations. Furthermore, self-consistent
simulations of counter-streaming plasmas indicate that this drift-kink mode is
dominant in the slow down of the flows and in the isotropization of the
magnetic field, playing an important role in the formation of collisionless
shocks.
