Strong perpendicular velocity-space in proton beams observed by Parker Solar Probe

Abstract

The SWEAP instrument suite on Parker Solar Probe (PSP) has detected numerous proton beams associated with coherent, circularly polarized, ion-scale waves observed by PSP's FIELDS instrument suite. Measurements during PSP Encounters 4-8 revealed pronounced complex shapes in the proton velocity distribution functions (VDFs), in which the tip of the beam undergoes strong perpendicular diffusion, resulting in VDF level contours that resemble a `hammerhead.' We refer to these proton beams, with their attendant `hammerhead' features, as the ion strahl. We present an example of these observations occurring simultaneously with a 7-hour ion-scale wave storm and show results from a preliminary attempt at quantifying the occurrence of ion-strahl broadening through 3-component ion-VDF fitting. We also provide a possible explanation of the ion perpendicular scattering based on quasilinear theory and the resonant scattering of beam ions by parallel-propagating, right circularly polarized, fast-magnetosonic/whistler waves.

13 pages, 8 figures, Under Review in the Astrophysical Journal

References in corpus (8)

• 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
• Predicted Impacts of Proton Temperature Anisotropy on Solar Wind Turbulence
• The Dispersion Relations and Instability Thresholds of Oblique Plasma Modes in the Presence of an Ion Beam
• Inferred Linear Stability of Parker Solar Probe Observations using One- and Two-Component Proton Distributions
• The Electromagnetic Signature of Outward Propagating Ion-Scale Waves
• Deceleration of Alpha Particles in the Solar Wind by Instabilities and the Rotational Force: Implications for Heating, Azimuthal Flow, and the Parker Spiral Magnetic Field

Cited by in corpus (13)

• Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum
• The In Situ Signature of Cyclotron Resonant Heating
• The Radial Distribution of Ion-scale Waves in the Inner Heliosphere
• Modeling ion beams, kinetic instabilities, and waves observed by the Parker Solar Probe near perihelia
• Cross Helicity and the Helium Abundance as an in situ Metric of Solar Wind Acceleration
• Fully Kinetic Simulations of Proton-Beam-Driven Instabilities from Parker Solar Probe Observations
• Modeling hot, anisotropic ion beams in the solar wind motivated by the Parker Solar Probe observations near perihelia
• The Evolution of Heavy Ion Abundances with Solar Activity
• Decoding the formation of hammerhead ion populations observed by Parker Solar Probe
• Characterizing the Impact of Alfvén Wave Forcing in Interplanetary Space on the Distribution of near-Earth Solar Wind Speeds
• Evolution of an Alfvén Wave-Driven Proton Beam in the Expanding Solar Wind
• On the Regulation of the Solar Wind Helium Abundance by the Hydrogen Compressibility
• Hybrid simulations of the proton beam instabilities in the young solar wind. The formation of hammerhead-like distributions