  Large-scale compressive slow-mode-like fluctuations can cause variations in
the density, temperature, and magnetic-field magnitude in the solar wind. In
addition, they also lead to fluctuations in the differential flow $U_{\rm
p\alpha}$ between $\alpha$-particles and protons ($p$), which is a common
source of free energy for the driving of ion-scale instabilities. If the
amplitude of the compressive fluctuations is sufficiently large, the
fluctuating $U_{\rm p\alpha}$ intermittently drives the plasma across the
instability threshold, leading to the excitation of ion-scale instabilities and
thus the growth of corresponding ion-scale waves. The unstable waves scatter
particles and reduce the average value of $U_{\rm p\alpha}$. We propose that
this "fluctuating-beam effect" maintains the average value of $U_{\rm p\alpha}$
well below the marginal instability threshold. We model the large-scale
compressive fluctuations in the solar wind as long-wavelength slow-mode waves
using a multi-fluid model. We numerically quantify the fluctuating-beam effect
for the Alfv\'en/ion-cyclotron (A/IC) and fast-magnetosonic/whistler (FM/W)
instabilities. We show that measurements of the proton-$\alpha$ differential
flow and compressive fluctuations from the {\it Wind} spacecraft are consistent
with our predictions for the fluctuating-beam effect. This effect creates a new
channel for a direct cross-scale energy transfer from large-scale compressions
to ion-scale fluctuations.
