  Relative drifts between different species or particle populations are
characteristic to solar plasma outflows, e.g., in the fast streams of the solar
winds, coronal mass ejections and interplanetary shocks. This paper
characterizes the dispersion and stability of the low-beta alpha/proton drifts
in the absence of any intrinsic thermal anisotropies, which are usually invoked
in order to stimulate various instabilities. The dispersion relations derived
here describe the full spectrum of instabilities and their variations with the
angle of propagation and plasma parameters. The results unveil a potential
competition between instabilities of the electromagnetic proton cyclotron and
alpha cyclotron modes. For conditions specific to a low-beta solar wind, e.g.,
at low heliocentric distances in the outer corona, the instability operates on
the alpha cyclotron branch. The growth rates of the alpha cyclotron mode are
systematically stimulated by the (parallel) plasma beta and/or the alpha-proton
temperature ratio. One can therefore expect that this instability develops even
in the absence of temperature anisotropies, with potential to contribute to a
self-consistent regulation of the observed drift of alpha particles.
