Kinetic Scale Slow Solar Wind Turbulence in the Inner Heliosphere: Coexistence of Kinetic Alfvén Waves and Alfvén Ion Cyclotron Waves

Abstract

The nature of the plasma wave modes around the ion kinetic scales in highly Alfvénic slow solar wind turbulence is investigated using data from the NASA’s Parker Solar Probe taken in the inner heliosphere, at 0.18 au from the Sun. The joint distribution of the normalized reduced magnetic helicity σ m ( θ RB , τ ) is obtained, where θ RB is the angle between the local mean magnetic field and the radial direction and τ is the temporal scale. Two populations around ion scales are identified: the first population has σ m ( θ RB , τ ) < 0 for frequencies (in the spacecraft frame) ranging from 2.1 to 26 Hz for 60° <  θ RB  < 130°, corresponding to kinetic Alfvén waves (KAWs), and the second population has σ m ( θ RB , τ ) > 0 in the frequency range [1.4, 4.9] Hz for θ RB  > 150°, corresponding to Alfvén ion cyclotron waves (ACWs). This demonstrates for the first time the coexistence 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 Alfvénicity of the slow solar wind at 0.18 au that may be favorable for the excitation of ACWs.