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In this work, we investigate how the complex structure found in solar windproton velocity distribution functions (VDFs), rather than the commonly assumedtwo-component bi-Maxwellian structure, affects the onset and evolution ofparallel-propagating microinstabilities. We use the Arbitrary Linear PlasmaSolver (ALPS), a numerical dispersion solver, to find the real frequencies andgrowth/damping rates of the Alfv\'en modes calculated for proton VDFs extractedfrom Wind spacecraft observations of the solar wind. We compare this wavebehavior to that obtained by applying the same procedure to core-and-beambi-Maxwellian fits of the Wind proton VDFs. We find several significantdifferences in the plasma waves obtained for the extracted data andbi-Maxwellian fits, including a strong dependence of the growth/damping rate onthe shape of the VDF. By application of the quasilinear diffusion operator tothese VDFs, we pinpoint resonantly interacting regions in velocity space wheredifferences in VDF structure significantly affect the wave growth and dampingrates. This demonstration of the sensitive dependence of Alfv\'en mode behavioron VDF structure may explain why the Alfv\'en ion-cyclotron instabilitythresholds predicted by linear theory for bi-Maxwellian models of solar windproton background VDFs do not entirely constrain spacecraft observations ofsolar wind proton VDFs, such as those made by the Wind spacecraft.

The Effects of Non-Equilibrium Velocity Distributions on Alfv\'en Ion-Cyclotron Waves in the Solar Wind