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

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.