Revealing an unexpectedly low electron injection threshold via reinforced shock acceleration

Collisionless shock waves are ubiquitous in astrophysical plasmas, from supernova remnants and planetary atmospheres to coronal mass ejections and laboratory experiments. These shocks are known to be efficient particle accelerators, crucial for understanding the origin of cosmic rays, including ultra-relativistic particles. This study presents a novel model of reinforced shock acceleration for electrons, integrating in-situ data from NASA's Magnetospheric Multiscale (MMS) and Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction (ARTEMIS) missions. Focusing on Earth's planetary environment, our analysis reveals a suprathermal electron injection threshold, demonstrating how a multiscale framework involving foreshock transient phenomena, a suprathermal seed population, and wave-particle interactions can systematically accelerate suprathermal electrons to relativistic energies. By merging theoretical advancements in astrophysical plasmas and shock physics with these multi-spacecraft observations, we address the persistent electron injection problem and explore the broader applicability of our model to other planetary environments within our solar system and beyond, including stellar and interstellar contexts.