Observations of Turbulence and Particle Transport at Interplanetary Shocks: Transition of Transport Regimes

TL;DR

This study uses Cluster spacecraft data to observe how energetic proton transport transitions from normal diffusion to superdiffusion near interplanetary shocks, influenced by turbulence dynamics and wave-particle interactions.

Contribution

It provides the first observational evidence of a transport regime transition at interplanetary shocks linked to turbulence properties and wave excitation.

Findings

Proton fluxes shift from exponential to power-law decay with distance from shock.

Transition correlates with increased ratio of proton isotropization time to turbulence correlation time.

Energetic particles excite Alfvén-like waves, affecting turbulence structure.

Abstract

The transport of energetic particles is intimately related to the properties of plasma turbulence, a ubiquitous dynamic process that transfers energy across a broad range of spatial and temporal scales. However, the mechanisms governing the interactions between plasma turbulence and energetic particles remain incompletely understood. Here we present comprehensive observations from the upstream region of a quasi-perpendicular interplanetary (IP) shock on 2004 January 22, using data from four Cluster spacecraft to investigate the interplay between turbulence dynamics and energetic particle transport. Our observations reveal a transition in energetic proton fluxes from exponential to power-law decay with increasing distance from the IP shock. This result provides possible observational evidence of a shift in transport behavior from normal diffusion to superdiffusion. This transition correlates with an increase in the time ratio from $\tau_s/\tau_{c}<1$ to $\tau_s/\tau_{c}\gg1$, where $\tau_s$ is the proton isotropization time, and $\tau_{c}$ is the turbulence correlation time. Additionally, the frequency-wavenumber distributions of magnetic energy in the power-law decay zone indicate that energetic particles excite linear Alfv\'en-like harmonic waves through gyroresonance, thereby modulating the original turbulence structure. These findings provide valuable insights for future studies on the propagation and acceleration of energetic particles in turbulent astrophysical and space plasma systems.