Suprathermal Ion Energy spectra and Anisotropies near the Heliospheric Current Sheet crossing observed by the Parker Solar Probe during Encounter 7

• Published in: arXiv.org
• Main Authors: Desai, M I, Mitchell, D G, McComas, D J, Drake, J F, Phan, T, Szalay, J R, Roelof, E C, Giacalone, J, Hill, M E, Christian, E R, Schwadron, N A, McNutt, R L, Wiedenbeck, M E, Joyce, C, Cohen, C M S, Davis, A J, Krimigis, S M, Leske, R A, Matthaeus, W H, Malandraki, O, Mewaldt, R A, Labrador, A, Stone, E C, Bale, S D, Verniero, J, Rahmati, A, Whittlesey, P, Livi, R, Larson, D, Pulupa, M, MacDowall, R J, Niehof, J T, Kasper, J C, Horbury, T S
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 01.11.2021
• Subjects: Acceleration; Coronal mass ejection; Current sheets; Electric fields; Energetic particles; Energy spectra; Heavy ions; Physics - Solar and Stellar Astrophysics; Physics - Space Physics; Pitch (inclination); Solar corona; Solar flares; Solar probes
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2111.00954

We present observations of >10-100 keV/nucleon suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances <0.1 au from the Sun. Our key findings are: 1) very few heavy ions are detected during the 1st full crossing, the heavy ion intensities are reduced during the 2nd partial crossing and peak just after the 2nd crossing; 2) ion arrival times exhibit no velocity dispersion; 3) He pitch-angle distributions track the magnetic field polarity reversal and show up to ~10:1 anti-sunward, field-aligned flows and beams closer to the HCS that become nearly isotropic further from the HCS; 4) the He spectrum steepens either side of the HCS and the He, O, and Fe spectra exhibit power-laws of the form ~E^4-6; and 5) maximum energies EX increase with the ion's charge-to-mass (Q/M) ratio as EX/EH proportional to [(QX/MX)]^alpha where alpha~0.65-0.76, assuming that the average Q-states are similar to those measured in gradual and impulsive solar energetic particle events at 1 au. The absence of velocity dispersion in combination with strong field-aligned anisotropies closer to the HCS appears to rule out solar flares and near-sun coronal mass ejection-driven shocks. These new observations present challenges not only for mechanisms that employ direct parallel electric fields and organize maximum energies according to E/Q, but also for local diffusive and magnetic reconnection-driven acceleration models. Re-evaluation of our current understanding of the production and transport of energetic ions is necessary to understand this near-solar, current-sheet-associated population of ST ions.