The Radial Distribution of Ion-scale Waves in the Inner Heliosphere

Determining the mechanism responsible for plasma heating and particle acceleration is a fundamental problem in the study of the heliosphere. Due to efficient wave–particle interactions of ion-scale waves with charged particles, these waves are widely believed to be a major contributor to ion energization, and their contribution considerably depends on the wave occurrence rate. By analyzing the radial distribution of quasi-monochromatic ion-scale waves observed by the Parker Solar Probe, this work shows that the wave occurrence rate is significantly enhanced in the near-Sun solar wind, specifically 21%–29% below 0.3 au, in comparison to 6%–14% beyond 0.3 au. The radial decrease of the wave occurrence rate is not only induced by the sampling effect of a single spacecraft detection, but also by the physics relating to the wave excitation, such as the enhanced ion beam instability in the near-Sun solar wind. This work also shows that the wave normal angle θ , the absolute value of ellipticity ϵ , the wave frequency f normalized by the proton cyclotron frequency f cp , and the wave amplitude δ B normalized by the local background magnetic field B 0 slightly vary with the radial distance. The median values of θ , ∣ ϵ ∣, f , and δ B are about 9°, 0.73, 3 f cp , and 0.01 B 0 , respectively. Furthermore, this study proposes that the wave mode natures of the observed left-handed and right-handed polarized waves correspond to the Alfvén ion cyclotron mode wave and the fast magnetosonic whistler mode wave, respectively.