Kinetic Scale Slow Solar Wind Turbulence in the Inner Heliosphere: Co-existence of Kinetic Alfv\'en Waves and Alfv\'en Ion Cyclotron Waves

TL;DR

This study reveals the simultaneous presence of kinetic Alfvén waves and Alfvén ion cyclotron waves in the slow solar wind close to the Sun, providing new insights into plasma wave modes at kinetic scales in the inner heliosphere.

Contribution

It is the first to demonstrate the coexistence of KAWs and ACWs in the slow solar wind at 0.18 AU, contrasting with observations at 1 AU.

Findings

Coexistence of KAWs and ACWs in slow solar wind at 0.18 AU

Identification of two distinct wave populations based on magnetic helicity

Discrepancy with 1 AU observations explained by wave dissipation or wind properties

Abstract

The nature of the plasma wave modes around the ion kinetic scales in highly Alfv\'enic slow solar wind turbulence is investigated using data from the NASA's Parker Solar Probe taken in the inner heliosphere, at 0.18 Astronomical Unit (AU) from the sun. The joint distribution of the normalized reduced magnetic helicity ${\sigma}_m ({\theta}_{RB}, {\tau})$ is obtained, where ${\theta}_{RB}$ is the angle between the local mean magnetic field and the radial direction and ${\tau}$ is the temporal scale. Two populations around ion scales are identified: the first population has ${\sigma}_m ({\theta}_{RB}, {\tau}) < 0$ for frequencies (in the spacecraft frame) ranging from 2.1 to 26 Hz for $60^{\circ} < {\theta}_{RB} < 130^{\circ}$, corresponding to kinetic Alfv\'en waves (KAWs), and the second population has ${\sigma}_m ({\theta}_{RB}, {\tau}) > 0$ in the frequency range [1.4, 4.9] Hz for ${\theta}_{RB} > 150^{\circ}$, corresponding to Alfv\'en ion Cyclotron Waves (ACWs). This demonstrates for the first time the co-existence of KAWs and ACWs in the slow solar wind in the inner heliosphere, which contrasts with previous observations in the slow solar wind at 1 AU. This discrepancy between 0.18 and 1 AU could be explained, either by i) a dissipation of ACWs via cyclotron resonance during their outward journey, or by ii) the high Alfv\'enicity of the slow solar wind at 0.18 AU that may be favorable for the excitation of ACWs.