  Solar wind stream interaction regions (SIRs) are often characterized by
energetic ion enhancements. The mechanisms accelerating these particles, as
well as the locations where the acceleration occurs, remain debated. Here, we
report the findings of a simulation of a SIR event observed by Parker Solar
Probe at ~0.56 au and the Solar Terrestrial Relations Observatory-Ahead at
~0.95 au in 2019 September when both spacecraft were approximately radially
aligned with the Sun. The simulation reproduces the solar wind configuration
and the energetic particle enhancements observed by both spacecraft. Our
results show that the energetic particles are produced at the compression waves
associated with the SIR and that the suprathermal tail of the solar wind is a
good candidate to provide the seed population for particle acceleration. The
simulation confirms that the acceleration process does not require shock waves
and can already commence within Earth's orbit, with an energy dependence on the
precise location where particles are accelerated. The three-dimensional
configuration of the solar wind streams strongly modulates the energetic
particle distributions, illustrating the necessity of advanced models to
understand these particle events.
