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 effificient 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 signifificantly enhanced in the

near-Sun solar wind, specififically 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 ò, the wave frequency f

normalized by the proton cyclotron frequency fcp, and the wave amplitude δB normalized by the local background

magnetic fifield B0 slightly vary with the radial distance. The median values of θ, ò, f, and δB are about 9°, 0.73,

3fcp, and 0.01B0, 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.