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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.
Astrophysical journal/The Astrophysical journal, 03/2022, Volume: 927, Issue: 1Journal Article
Abstract We present observations of ≳10–100 keV nucleon −1 suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances of <0.1 au from the Sun. Our key findings are as follows: (1) very few heavy ions are detected during the first full crossing, the heavy-ion intensities are reduced during the second partial crossing and peak just after the second 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 farther 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 – E 6 ; and (5) maximum energies E X increase with the ion’s charge-to-mass ( Q / M ) ratio as E X / E H ∝ ( Q X / M X ) δ , where δ ∼ 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. Reevaluation 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.
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