Verniero, JL; Chandran, BDG; Larson, DE; Paulson, K; Alterman, BL; Badman, S; Bale, SD; ... Whittlesey, PL; + view all Verniero, JL; Chandran, BDG; Larson, DE; Paulson, K; Alterman, BL; Badman, S; Bale, SD; Bonnell, JW; Bowen, TA; de Wit, T Dudok; Kasper, JC; Klein, KG; Lichko, E; Livi, R; McManus, MD; Rahmati, A; Verscharen, D; Walters, J; Whittlesey, PL; - view fewer (2022) Strong Perpendicular Velocity-space Diffusion in Proton Beams Observed by Parker Solar Probe. The Astrophysical Journal , 924 (2) , Article 112. 10.3847/1538-4357/ac36d5.

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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 hr ion-scale wave storm and show results from a preliminary attempt at quantifying the occurrence of ion-strahl broadening through three-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.

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