Energetic ion enhancements in sheaths driven by interplanetary coronal mass ejections

Abstract

We analyze here an energetic proton enhancement in a sheath ahead of a slow interplanetry coronal mass ejection (ICME) detected by Parker Solar Probe on June 30, 2021 at the heliospheric distance of 0.76 AU. The shock was likely quasi-parallel and had a high Mach number. However, the proton fluxes were not enhanced at the shock but about an hour later. The fluxes stayed elevated with a sporadic behaviour throughout the sheath. We suggest that some mechanism internal to the sheath was responsible for the energization. The observations show enhanced levels of magnetic field fluctuations in the sheath and frequent presence of highly reduced magnetic helicity structures ($\sigma _{m}$ σ m ) at various time scales, representing either small-scale flux ropes or Alfvénic fluctuations that could have contributed to the energization. The correlation between the energetic proton fluxes and normalized fluctuation amplitudes/occurrence of high $\sigma _{m}$ σ m structures was generally weak or negligible. The most striking feature of the sheath was a strong enhancement of density (up to 50 cm−3) that implies the importance of compressive acceleration in the sheath. A statistical analysis of ion enhancements of 73 sheaths detected by ACE at ${\sim} 1$ ∼ 1 AU reveals that this sheath was peculiar as in ICME-driven sheaths preceded by strong shocks the ion fluxes typically peak at the shock and strongly decline through the sheath.

Parker Solar Probe Energetic Particle Observations of the September 5, 2022 SEP Event

“…Within this narrow region, there is an isotropic population of SEPs that may be trapped between the shock and the magnetic field increase marking the end of the sheath region. This has some similarities to an unusual event observed by Parker at 0.76 AU on 30 June 2021 [14], which also had a strong particle enhancement in the sheath region suggestive of particle energization via processes internal to the sheath. However, in that event, the particle enhancement was limited to lower energies, < 0.5 MeV, and the sheath region was quite turbulent and lasted for 7.2 hours.…”

Properties of an Interplanetary Shock Observed at 0.07 and 0.7 au by Parker Solar Probe and Solar Orbiter

“…Interestingly, some of the fluxes (e.g., the ones for ∼0.33 and ∼0.7 MeV, shown as green and yellow lines, respectively) have their peaks at times that are not coincident with the shock crossing time at SolO, but instead are downstream of the shock. This behavior, consistent with the statistical study presented in Lario et al (2003), has implications for the production and propagation of shock-accelerated particles (Perri et al 2022), and may well be due to additional acceleration mechanisms downstream of the shock (Zank et al 2015;Zhao et al 2018;Kilpua et al 2023).…”