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

TL;DR

This study shows that the detailed structure of solar wind proton velocity distribution functions significantly influences the behavior of Alfvén ion-cyclotron waves, affecting their growth and damping rates and explaining discrepancies with bi-Maxwellian models.

Contribution

It demonstrates the impact of realistic VDF structures on wave instabilities, using numerical dispersion analysis and quasilinear diffusion to reveal sensitive dependence on VDF shape.

Findings

VDF structure strongly affects wave growth/damping rates.

Differences between observed and bi-Maxwellian VDFs influence wave behavior.

Resonant regions in velocity space are key to wave-particle interactions.

Abstract

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.