Suprathermal Ion Energy Spectra and Anisotropies near the Heliospheric Current Sheet Crossing Observed by the Parker Solar Probe during Encounter 7

Academic 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 −4E 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.
  • Authors

  • Desai, MI
  • Mitchell, DG
  • McComas, DJ
  • Drake, JF
  • Phan, T
  • Szalay, JR
  • Roelof, EC
  • Giacalone, J
  • Hill, ME
  • Christian, ER
  • Schwadron, Nathan
  • Jr, McNutt RL
  • Wiedenbeck, ME
  • Joyce, Colin
  • Cohen, CMS
  • Davis, AJ
  • Krimigis, SM
  • Leske, RA
  • Matthaeus, WH
  • Malandraki, O
  • Mewaldt, RA
  • Labrador, A
  • Stone, EC
  • Bale, SD
  • Verniero, J
  • Rahmati, A
  • Whittlesey, P
  • Livi, R
  • Larson, D
  • Pulupa, M
  • MacDowall, RJ
  • Niehof, Jonathan
  • Kasper, JC
  • Horbury, TS
  • Status

    Publication Date

  • March 1, 2022
  • Has Subject Area


  • astro-ph.SR
  • Digital Object Identifier (doi)

    Start Page

  • 62
  • End Page

  • 62
  • Volume

  • 927
  • Issue

  • 1