Effects of non-WKB Alfven waves on a multicomponent solar wind with differential ion flow

TL;DR

This study models the solar wind incorporating non-WKB Alfvén waves, revealing their significant impact on wind acceleration, ion fluxes, and alpha abundance, with effects depending on wave frequency regimes.

Contribution

It introduces a self-consistent multicomponent solar wind model including dissipationless, finite-wavelength Alfvén waves, highlighting their non-WKB effects on wind dynamics and ion behavior.

Findings

Non-WKB effects significantly influence solar wind acceleration.

WKB approximation is valid only for higher wave frequencies.

Different wave frequency regimes affect ion acceleration and energy transfer.

Abstract

We present multicomponent solar wind models self-consistently incorporating the contribution from dissipationless, monochromatic, finite-wavelength (non-WKB), hydromagnetic, toroidal Alfv\'en waves, which are coupled to the flow only through the ponderomotive forces. We find that the non-WKB effects are significant, for the fast and slow solar wind solutions alike. Compared with their non-WKB counterparts the WKB ones are more effective in accelerating the solar wind inside the Alfv\'en point, producing significantly enhanced ion fluxes and considerably reduced alpha abundance in the inner corona. Only when $\omega \gtrsim 3.5\times 10^{-3}$ ($1.5\times 10^{-3}$) $\rm rad s^{-1}$ can the fast (slow) wind models be adequately approximated by the WKB one. Moreover, while the Alfv\'en waves tend to reduce the magnitude of the proton-alpha speed difference $|U_{\alpha p}|$ in general, different mechanisms operate in two different regimes separated by an $\omega_c\sim {several}\times 10^{-5}$ $\rm rad s^{-1}$. When $\omega > \omega_c$, the fluctuations are wave-like and tend to accelerate both ion species, thereby losing most of their energy by doing work on ion flows; whereas when $\omega < \omega_c$, a quasi-static behavior results: the fluctuations may act to accelerate the slower flowing ion species but decelerate the faster moving one in a large portion of the computational domain, and only a minor fraction of the wave energy flux injected at the base is lost. The consequences of $\omega_c$ on the velocity fluctuation spectra of protons and alpha particles, which are likely to be obtained by future missions like Solar Orbiter and Solar Probe, are discussed.