  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.
