A Self-consistent Simulation of Proton Acceleration and Transport Near a High-speed Solar Wind Stream

TL;DR

This study uses a self-consistent simulation to analyze proton acceleration and transport in solar wind stream interaction regions, revealing that particles are accelerated at compression waves without shocks, influenced by the solar wind's 3D structure.

Contribution

It introduces a novel simulation approach that reproduces observed energetic particle enhancements and clarifies acceleration mechanisms in SIRs without shock waves.

Findings

Particles are accelerated at compression waves, not shocks.

Suprathermal solar wind tails serve as seed populations.

3D solar wind structure modulates energetic particle distributions.

Abstract

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.