New insight into short wavelength solar wind fluctuations from Vlasov theory

TL;DR

This paper uses Vlasov kinetic theory to analyze short wavelength plasma modes in the solar wind, revealing that kinetic Alfvén waves are more likely than whistler waves to carry turbulence energy to electron scales under realistic conditions.

Contribution

It provides new insights into the linear properties of plasma modes at sub-ion scales in the solar wind, highlighting the relevance of KAW over whistler modes for energy cascade.

Findings

KAW can extend to electron scales depending on anisotropy.

Whistler modes are more damped than KAWs, making them less likely to carry energy.

Shear Alfvén mode's behavior varies with propagation angle and anisotropy.

Abstract

The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is being investigated extensively nowadays. Although recent observations gave evidence of the dominance of Kinetic Alfv\'en Waves (KAW) at sub-ion scales with $\omega<{\omega_{ci}}$, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., $\omega>\omega_{ci}$) is more relevant. Here, we propose to study key properties of the short wavelength plasma modes under realistic SW conditions, typically $\beta_i\gtrsim \beta_e\sim 1$ and for high oblique angles of propagation $80^\circ\leq \Theta_{\bf kB}<90^\circ$ as observed from the Cluster data. The linear properties of the plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation ($\Theta_{\bf kB} <80^\circ$). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfv\'en mode extends at scales $k\rho_i\gtrsim1$ following either a whistler mode ($\omega>\omega_{ci}$) or a KAW mode (with $\omega<\omega_{ci}$) depending on the anisotropy $k_\parallel/ k_\perp$. This contrasts with the well-accepted idea that the whistler branch develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter a more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW.