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

TL;DR

This study uses multi-spacecraft observations to analyze the internal structure of a CME sheath and its role in energetic ion enhancements, revealing substructures like flux ropes and HCS crossings as potential acceleration sites.

Contribution

It provides new insights into the internal structure of CME sheaths and their relation to particle energization, especially at low shock Mach numbers, using multi-spacecraft data.

Findings

Energetic ion enhancements linked to sheath substructures.

Magnetic fluctuations increased but normalized fluctuations remained stable.

Small-scale flux ropes and HCS crossings serve as acceleration sites.

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.