Transport of particles in strongly turbulent 3D magnetized plasmas

In this review, we examine particle transport in strongly turbulent three-dimensional magnetized plasmas, characterized by intense (large-amplitude) magnetic field (B) fluctuations δB (δB/B>1). Such environments naturally give rise to a network of coherent structures (CoSs), including current sheets, filaments, shocks, switchbacks, and significant magnetic perturbations, which critically influence particle dynamics at the kinetic level. Within this turbulent regime, two fundamental particle energization mechanisms emerge, stochastic acceleration and systematic acceleration. Systematic acceleration within open turbulent volumes promotes the development of power-law tails in energy distributions, a hallmark of many astrophysical plasmas. Our analysis distinguishes the roles of two electric fields: the perpendicular (or convective) fields (E⊥∼−V×B), which drive stochastic heating via interactions with randomly moving scatterers, and the parallel electric fields (E||∼J·(B/|B|), which enable systematic particle acceleration in regions of strong currents. Combined with accurate estimates of particle escape times in finite volumes, the interplay of these mechanisms leads to the formation of Kappa distributions. These distributions are frequently observed in strongly turbulent environments across laboratory, space, and astrophysical plasmas. The transport properties differ significantly between the two energization modes. Stochastic energization follows Gaussian statistics and can be effectively described by the Fokker–Planck equation. In contrast, systematic acceleration exhibits Lévy flight statistics, necessitating a fractional transport equation for an accurate description. Furthermore, the fractal spatial distribution of CoSs introduces deviations from traditional transport models, influencing, e.g., particle escape times. Systematic acceleration is most efficient during the early, high-energy phases of turbulence, while stochastic heating becomes dominant during the later stages, contributing to gradual particle energization. These mechanisms are ubiquitous across various plasma environments, including the edges of Tokamaks, the solar corona, the turbulent solar wind, the Earth's magnetotail, astrophysical jets, and supernova remnants, where Kappa distributions are routinely observed. This review underscores the gaps in our understanding of particle interactions with diverse CoSs beyond classical reconnection sites, emphasizing their critical role in accurately modeling particle dynamics in turbulent plasmas.