reacceleration

Reacceleration is the process by which charged particles that have already been energized by an initial acceleration gain additional energy from subsequent interactions with moving magnetic fields, shocks, or turbulent waves. It is distinct from the first energization that creates the original high-energy state. Reacceleration can occur in various astrophysical and laboratory contexts and is often described as diffusion of particle momentum due to stochastic, or second-order, Fermi processes.

In astrophysics, reacceleration plays a significant role in cosmic-ray propagation. As cosmic rays travel through the turbulent, magnetized interstellar medium, they scatter off irregular magnetic fields and waves, acquiring small but cumulative energy gains. This diffusive reacceleration tends to modify energy spectra and influences secondary-to-primary ratios, since particles spend more time in the Galaxy before escaping. Transport models use the momentum-diffusion coefficient and the spatial diffusion coefficient to quantify the effect.

Beyond the interstellar medium, reacceleration occurs in galaxy clusters, where turbulence in the hot intracluster medium can reenergize relativistic electrons, contributing to observed radio halos and relics. It is also relevant in the heliosphere and solar wind, where particles can be reaccelerated at collisionless shocks associated with coronal mass ejections and interplanetary shocks.

Reacceleration contrasts with first-order Fermi acceleration at shocks, which yields more rapid energy gains. Observationally, reacceleration leaves signatures in energy spectra, radiation across the electromagnetic spectrum, and cosmic-ray composition, and it is an important component of models of particle transport in plasmas.