Regulation of Proton-$\alpha$ Differential Flow by Compressive Fluctuations and Ion-scale Instabilities in the Solar Wind

TL;DR

This paper investigates how large-scale compressive fluctuations in the solar wind influence proton-alpha differential flow and ion-scale instabilities, proposing a fluctuating-beam effect that maintains flow below instability thresholds and facilitates energy transfer.

Contribution

It introduces a model of the fluctuating-beam effect driven by compressive fluctuations, explaining how it regulates differential flow and excites ion-scale waves in the solar wind.

Findings

Measurements align with model predictions of the fluctuating-beam effect.

The effect maintains differential flow below instability thresholds.

It provides a new pathway for energy transfer across scales.

Abstract

Large-scale compressive slow-mode-like fluctuations can cause variations in the density, temperature, and magnetic-field magnitude in the solar wind. In addition, they also lead to fluctuations in the differential flow $U_{\rm p\alpha}$ between $\alpha$-particles and protons ($p$), which is a common source of free energy for the driving of ion-scale instabilities. If the amplitude of the compressive fluctuations is sufficiently large, the fluctuating $U_{\rm p\alpha}$ intermittently drives the plasma across the instability threshold, leading to the excitation of ion-scale instabilities and thus the growth of corresponding ion-scale waves. The unstable waves scatter particles and reduce the average value of $U_{\rm p\alpha}$. We propose that this "fluctuating-beam effect" maintains the average value of $U_{\rm p\alpha}$ well below the marginal instability threshold. We model the large-scale compressive fluctuations in the solar wind as long-wavelength slow-mode waves using a multi-fluid model. We numerically quantify the fluctuating-beam effect for the Alfv\'en/ion-cyclotron (A/IC) and fast-magnetosonic/whistler (FM/W) instabilities. We show that measurements of the proton-$\alpha$ differential flow and compressive fluctuations from the {\it Wind} spacecraft are consistent with our predictions for the fluctuating-beam effect. This effect creates a new channel for a direct cross-scale energy transfer from large-scale compressions to ion-scale fluctuations.