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

TL;DR

This paper reports the observation of strong perpendicular velocity-space features in proton beams by Parker Solar Probe, revealing complex velocity distribution functions and proposing a scattering mechanism involving ion-scale waves.

Contribution

It presents new observations of proton beam features with 'hammerhead' shapes and analyzes their perpendicular diffusion, linking them to ion-scale wave interactions.

Findings

Proton beams exhibit pronounced perpendicular diffusion in velocity space.

'Hammerhead' shaped velocity distribution functions are observed.

Ion-scale waves are associated with proton beam broadening.

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.