  Context: Compensated-current systems are established in response to hot ion
beams in terrestrial foreshock regions, around supernova remnants, and in other
space and astrophysical plasmas. Aims: We study a non-resonant reactive
instability of Alfv\'en waves (AWs) propagating quasi-parallel to the
background magnetic field $\mathbf{B}_{0}$ in such systems. Methods: The
instability is investigated analytically in the framework of kinetic theory
applied to the hydrogen plasmas penetrated by hot proton beams. Results: The
instability arises at parallel wavenumbers $k_{z}$ that are sufficiently large
to demagnetize the beam ions, $k_{z}V_{Tb}/\omega_{Bi}\gtrsim $ $1$ (here
$V_{Tb}$ is the beam thermal speed along $\mathbf{B}_{0}$ and $\omega _{Bi}$ is
the ion-cyclotron frequency). The Alfv\'en mode is then made unstable by the
imbalance of perturbed currents carried by the magnetized background electrons
and partially demagnetized beam ions. The destabilizing effects of the beam
temperature and the temperature dependence of the instability threshold and
growth rate are demonstrated for the first time. The beam temperature, density,
and bulk speed are all destabilizing and can be combined in a single
destabilizing factor $\alpha_{b}$ triggering the instability at {$\alpha _{b}>$
$\alpha _{b}^{\mathrm{thr}}$}, where the threshold varies in a narrow range
$2.43\leq $ $\alpha _{b}^{\mathrm{thr}}\leq $ $4.87$. New analytical
expressions for the instability growth rate and its boundary in the parameter
space are obtained and can be directly compared with observations. Two
applications to terrestrial foreshocks and foreshocks around supernova remnants
are shortly discussed. In particular, our results suggest that the ions
reflected by the shocks around supernova remnants can drive stronger
instability than the cosmic rays.
