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

Determining the mechanism responsible for the 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 \(\theta\), the absolute value of ellipticity \(\epsilon\), the wave frequency \(f\) normalized by the proton cyclotron frequency \(f_{\mathrm{cp}}\), and the wave amplitude \(\delta B\) normalized by the local background magnetic field \(B_0\) slightly vary with the radial distance. The median values of \(\theta\), \(|\epsilon|\), \(f\), and \(\delta B\) are about \(9^\circ\), \(0.73\), \(3f_{\mathrm{cp}}\), and \(0.01B_0\), respectively. Furthermore, this study proposes that the wave mode nature of the observed left-handed and right-handed polarized waves corresponds to the Alfvén ion cyclotron mode wave and the fast-magnetosonic whistler mode wave, respectively.