Multi-spacecraft observations of the structure of the sheath of an interplanetary coronal mass ejection and related energetic ion enhancement

Abstract

Sheaths ahead of coronal mass ejections (CMEs) are large heliospheric structures that form with CME expansion and propagation. Turbulent and compressed sheaths contribute to the acceleration of particles in the corona and in interplanetary space, but the relation of their internal structures to particle energization is still relatively little studied. In particular, the role of sheaths in accelerating particles when the shock Mach number is low is a significant open problem. This work seeks to provide new insights on the internal structure of CME sheaths with regard to energetic particle enhancements. A good opportunity to achieve this aim was provided by observations of a sheath made by radially aligned spacecraft at 0.8 and $\sim$ 1 AU (Solar Orbiter, Wind, ACE and BepiColombo) on 19-21 April 2020. The sheath was preceded by a weak shock. Energetic ion enhancements occurred at different locations within the sheath structure at Solar Orbiter and L1. Magnetic fluctuation amplitudes at inertial-range scales increased in the sheath relative to the upstream wind. However, when normalised to the local mean field, fluctuation amplitudes did not increase significantly; magnetic compressibility of fluctuation also did not increase. Various substructures were embedded within the sheath at the different spacecraft, including multiple heliospheric current sheet (HCS) crossings and a small-scale flux rope. At L1, the ion flux enhancement was associated with the HCS crossings, while at Solar Orbiter, the enhancement occurred within the rope. Substructures that are swept from the upstream solar wind and compressed in the sheath can act as particularly effective acceleration sites. A possible acceleration mechanism is betatron acceleration associated with the small-scale flux rope and the warped HCS in the sheath.

14 pages, 12 figures; published in Astronomy & Astrophysics, Solar Orbiter First Results (Cruise Phase) special issue

References in corpus (5)

• Identification of Magnetic Flux Ropes from Parker Solar Probe Observations during the First Encounter
• Observational evidence for self-generation of small-scale magnetic flux ropes from intermittent solar wind turbulence
• The temporal and spatial scales of density structures released in the slow solar wind during solar activity maximum
• In situ multi-spacecraft and remote imaging observations of the first CME detected by Solar Orbiter and BepiColombo
• Radial Evolution of Magnetic Field Fluctuations in an Interplanetary Coronal Mass Ejection Sheath

Cited by in corpus (8)

• First year of energetic particle measurements in the inner heliosphere with Solar Orbiter's Energetic Particle Detector
• In situ multi-spacecraft and remote imaging observations of the first CME detected by Solar Orbiter and BepiColombo
• On the Mesoscale Structure of CMEs at Mercury's Orbit: BepiColombo and Parker Solar Probe Observations
• The solar cycle 25 multi-spacecraft solar energetic particle event catalog of the SERPENTINE project
• Small-scale flux ropes in ICME sheaths
• Structure and fluctuations of a slow ICME sheath observed at 0.5 au by the Parker Solar Probe
• A coronal mass ejection encountered by four spacecraft within 1 au from the Sun: Ensemble modelling of propagation and magnetic structure
• Modelling the interaction of Alfvénic fluctuations with coronal mass ejections in the low solar corona