  Depending on the physical conditions involved the beam plasma systems may
reveal new unstable regimes triggered by the wave instabilities of different
nature. We show through linear theory and numerical simulations the existence
of an aperiodic electromagnetic instability which solely develops and control
the stability of two symmetric plasma populations counter-moving along the
regular magnetic field with a relative drift, $v_d$, small enough to not exceed
the particle thermal speed, $\alpha_e$. Emerging at highly oblique angles this
mode resembles properties of the aperiodic firehose instability driven by
temperature anisotropy. The high growth rates achieved with increasing the
relative drift or/and decreasing the plasma beta parameter lead to significant
saturation levels of the fluctuating magnetic field power, which explain the
relative fast relaxation of electrons. For $v_d>\alpha_e$ this instability can
coexist with the electrostatic two-stream instability, dominating the long-term
dynamics of the plasma as soon as $v_d$ has relaxed to values smaller than the
thermal speed.
