Particle injection from the thermal pool into diffusive shock acceleration

Determine how particles are injected from the thermal pool into diffusive shock acceleration at collisionless shocks, including the microphysical processes that allow downstream thermal particles to become capable of diffusing upstream and undergoing repeated Fermi cycles.

Background

The paper identifies injection from the downstream thermal population as a critical stage of diffusive shock acceleration (DSA). Kinetic simulations indicate that a fraction of thermal particles may be reflected at the shock, pre-accelerated through processes such as shock-drift acceleration and interactions with self-generated turbulence, and eventually injected into the diffusive acceleration process.

Despite theoretical and numerical work, the microscopic injection mechanism remains unresolved. This uncertainty affects the phenomenological injection prescriptions used in semi-analytical models and limits the reliability of predictions for the acceleration efficiency of supernova-remnant shocks.

References

A critical aspect of this problem is the injection of particles from the thermal pool into the acceleration process. Despite decades of theoretical and numerical efforts, the question of how particles get injected into DSA from the thermal pool remains one of the major open issues in the theory of collisionless shocks.

— The Potential Contribution of Radiative Supernova Remnants to the Local Cosmic Ray Spectrum  (2610.08474 - Cristofari et al., 6 Oct 2026) in Section 2, subsection “Injection of particles from the thermal pool in DSA”

Therefore, making the connection between injection efficiencies that are measured in kinetic simulations to macroscopic models of DSA in astrophysical environments remains an open challenge.

— The Potential Contribution of Radiative Supernova Remnants to the Local Cosmic Ray Spectrum  (2610.08474 - Cristofari et al., 6 Oct 2026) in Section 2, subsection “Injection of particles from the thermal pool in DSA”