  In this work, we investigate how the complex structure found in solar wind
proton velocity distribution functions (VDFs), rather than the commonly assumed
two-component bi-Maxwellian structure, affects the onset and evolution of
parallel-propagating microinstabilities. We use the Arbitrary Linear Plasma
Solver (ALPS), a numerical dispersion solver, to find the real frequencies and
growth/damping rates of the Alfv\'en modes calculated for proton VDFs extracted
from Wind spacecraft observations of the solar wind. We compare this wave
behavior to that obtained by applying the same procedure to core-and-beam
bi-Maxwellian fits of the Wind proton VDFs. We find several significant
differences in the plasma waves obtained for the extracted data and
bi-Maxwellian fits, including a strong dependence of the growth/damping rate on
the shape of the VDF. By application of the quasilinear diffusion operator to
these VDFs, we pinpoint resonantly interacting regions in velocity space where
differences in VDF structure significantly affect the wave growth and damping
rates. This demonstration of the sensitive dependence of Alfv\'en mode behavior
on VDF structure may explain why the Alfv\'en ion-cyclotron instability
thresholds predicted by linear theory for bi-Maxwellian models of solar wind
proton background VDFs do not entirely constrain spacecraft observations of
solar wind proton VDFs, such as those made by the Wind spacecraft.

Solar and Stellar Astrophysics;Plasma Physics