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

Abstract

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.

Accepted for publication by The Astrophysical Journal (ApJ)

References in corpus (11)

• The Solar Probe Cup on Parker Solar Probe
• Ion Scale Electromagnetic Waves in the Inner Heliosphere
• Parker Solar Probe observations of proton beams simultaneous with ion-scale waves
• The In Situ Signature of Cyclotron Resonant Heating
• Inferred Linear Stability of Parker Solar Probe Observations using One- and Two-Component Proton Distributions
• Anisotropy of Solar-Wind Turbulence in the Inner Heliosphere at Kinetic Scales: PSP Observations
• Strong perpendicular velocity-space in proton beams observed by Parker Solar Probe
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• The Electromagnetic Signature of Outward Propagating Ion-Scale Waves
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• Electromagnetic Proton Beam Instabilities in the Inner Heliosphere: Energy Transfer Rate, Radial Distribution, and Effective Excitation