Continuous Suprathermal Ion Acceleration Flanking Two Heliospheric Current Sheet Crossings During Parker Solar Probes Perihelion 7

Close to its 7th perihelion, Parker Solar Probe encountered the heliospheric current sheet (HCS) in a particularly clear and perpendicular crossing at about 20 solar radii. The Integrated Science Investigation of the Sun-Energetic Particle Instrument-Lo (ISOIS/EPI-Lo) observed significant non-dispersive intensities of ~100-1500 keV protons, 4He, O, and Fe, flanking the HCS, which was clearly delineated in the FIELDS and SWEAP instrument data. These intensities were observed from about 0800 January 17 through 1100 on January 18, and then reappeared much more weakly flanking a second HCS crossing, with enhanced ion intensities between 1900 on January 18 through 0000 on January 20. We focus more on the first HCS crossing, for which the ions were accelerated to higher energies, and their signatures were better defined in the EPI-Lo data. The ion pitch angle distributions, while not entirely conclusive, suggest a conic distribution about the radial magnetic field on either side of the HCS. Inside the HCS, the ion intensities were much lower, and the anisotropy more distinctly beam-like. The observations are consistent with continuous perpendicular heating of the ions closer to the sun on open magnetic field lines, over many hours (and large distances in apparent source surface longitudes). The association with the HCS crossing is very clear, but given the apparently broad source surface connections, we argue that the source is not the HCS itself. Based on the shape of the ion spectra, we argue for an acceleration mechanism mediated by electric fields parallel to the magnetic field at lower altitudes.