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

Abstract

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, 3f cp, and 0.01B 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.

Resonance of Low-frequency Electromagnetic and Ion-sound Modes in the Solar Wind

“…The circularly polarized electromagnetic fluctuations with the frequency of the order of local proton cyclotron frequency are abundant in the solar wind (Boardsen et al 2015; Zhao Bowen et al 2020a;Liu et al 2023). These lowfrequency electromagnetic fluctuations may contribute to solar wind heating (Bowen et al 2022;Ofman et al 2022), while corresponding dissipation mechanisms are still investigated.…”

“…The parameters assumed in the Hall-MHD simulations (θ = 5°, δB/B 0 = 0.04, and f/f ci ≈ 1) are quite typical of circularly polarized low-frequency fluctuations observed in the solar wind (Boardsen et al 2015;Zhao et al 2018;Liu et al 2023), implying thereby that the revealed resonance can occur frequently. Several comments are in order about our simulations.…”

“…Parker Solar Probe (PSP) and Solar Orbiter measurements have recently shown that similar low-frequency fluctuations are abundant closer to the Sun (Bale et al 2019;Bowen et al 2020a;Khotyaintsev et al 2021). The occurrence of these fluctuations is around 30% within 0.3 au (Liu et al 2023). The analysis of particle distribution functions showed that the lowfrequency fluctuations can be both Alfvén ion-cyclotron and fast magnetosonic waves produced by local ion-driven instabilities (Verniero et al 2020;Klein et al 2021).…”

Radial Evolution of the Near-Sun Solar Wind: Parker Solar Probe Observations

“…A statistical study of the evolution of the solar wind will help us understand the origin of the solar wind and the physical mechanisms involved. Especially, considering the near-Sun solar wind may carry more coronal source characteristics, the accurate radial evolution of the near-Sun solar wind may be not consistent with the prediction of previous models at larger heliocentric distance (Halekas et al 2020;Abraham et al 2022;Mostafavi et al 2022;Šafránková et al 2023;Liu et al 2023). Therefore, in this Letter, we aim to present the radial evolution of solar wind parameters below 0.3 au by analyzing PSP observations from Encounters 1-15 statistically.…”

Electromagnetic Ion Beam Instability in the Solar Corona

“…Recently, based on in situ observations of Parker Solar Probe (PSP) in the near-Sun solar wind, Verniero et al (2020) have shown that the number density of proton beam component can be unexpectedly large relative to proton core component, and most of the coexistent ion-scale waves are excited by proton beams. Besides, significantly enhanced ionscale waves are observed in the near-Sun solar wind Liu et al 2023) and they are more likely excited by ion beams (Verniero et al 2020;Klein et al 2021;Liu et al 2021). Moreover, Bowen et al (2022) have found obvious cyclotron resonant heating in the near-Sun solar wind.…”