  We perform a statistical assessment of solar wind stability at 1 AU against
ion sources of free energy using Nyquist's instability criterion. In contrast
to typically employed threshold models which consider a single free-energy
source, this method includes the effects of proton and He$^{2+}$ temperature
anisotropy with respect to the background magnetic field as well as relative
drifts between the proton core, proton beam, and He$^{2+}$ components on
stability. Of 309 randomly selected spectra from the Wind spacecraft, $53.7\%$
are unstable when the ion components are modeled as drifting bi-Maxwellians;
only $4.5\%$ of the spectra are unstable to long-wavelength instabilities. A
majority of the instabilities occur for spectra where a proton beam is
resolved. Nearly all observed instabilities have growth rates $\gamma$ slower
than instrumental and ion-kinetic-scale timescales. Unstable spectra are
associated with relatively-large He$^{2+}$ drift speeds and/or a departure of
the core proton temperature from isotropy; other parametric dependencies of
unstable spectra are also identified.
