The behavior of charged test particles in a turbulent magnetic field is a problem of long-standing importance in space and astrophysics, and one for which it has been unexpectedly dif- ficult to achieve closure (Jokipii 1966; Giacalone & Jokipii 1999; Mace, Matthaeus, & Bieber 2000). The diffusion tensor, for particle transport parallel and perpendicular to the ordered magnetic field, is essential for describing solar energetic par- ticles (Droege 2000), the modulation of Galactic cosmic rays (Burger & Hattingh 1998), diffusive shock acceleration, and the lifetime of cosmic rays in the Galaxy (Jokipii & Parker 1969). Perpendicular diffusion continues to pose a number of enigmatic problems in heliospheric studies, in which obser- vations provide strong constraints. It is also troubling that direct numerical simulations have failed to verify known theoretical formulations of perpendicular diffusion. Some problems, such as channeling or dropouts (Mazur et al. 2000) or the occurrence of subdiffusive transport (Kota & Jokipii 2000; Qin, Matthaeus, & Bieber 2002a), may require significant theoretical refor- mulations. In the latter case, the onset of collisionless parallel diffusion, usually the stronger of the two effects, can thwart the occurrence of perpendicular diffusion. However, perpen- dicular transport can be diffusive, when the three-dimensional random magnetic field possesses adequate complexity in the directions transverse to the mean field. In this Letter, we de- velop a nonlinear theory of perpendicular diffusion that is ap- plicable to this incompletely explored regime.

One of the most striking observations made by Parker Solar Probe during its first solar encounter is the omnipresence of rapid polarity reversals in a magnetic field that is otherwise mostly radial. These so-called switchbacks strongly affect the dynamics of the magnetic field. We concentrate here on their macroscopic properties. First, we find that these structures are self-similar, and have neither a characteristic magnitude, nor a characteristic duration. Their waiting time statistics show evidence of aggregation. The associated long memory resides in their occurrence rate, and is not inherent to the background fluctuations. Interestingly, the spectral properties of inertial range turbulence differ inside and outside of switchback structures; in the latter the 1/f range extends to higher frequencies. These results suggest that outside of these structures we are in the presence of lower-amplitude fluctuations with a shorter turbulent inertial range. We conjecture that these correspond to a pristine solar wind.

An overview is given of important properties of spatial and temporal intermittency, including evidence of its appearance in fluids, magnetofluids and plasmas, and its implications for understanding of heliospheric plasmas. Spatial intermittency is generally associated with formation of sharp gradients and coherent structures. The basic physics of structure generation is ideal, but when dissipation is present it is usually concentrated in regions of strong gradients. This essential feature of spatial intermittency in fluids has been shown recently to carry over to the realm of kinetic plasma, where the dissipation function is not known from first principles. Spatial structures produced in intermittent plasma influence dissipation, heating, and transport and acceleration of charged particles. Temporal intermittency can give rise to very long time correlations or a delayed approach to steady-state conditions, and has been associated with inverse cascade or quasi-inverse cascade systems, with possible implications for heliospheric prediction.

We examine statistics of rapid spatial variations of the magnetic field in simulations of magnetohydrodynamic (MHD) turbulence, by analyzing intermittency properties, and by using classical methods for identifying discontinuities. The methods identify similar structures, and give very similar event distribution functions. When the results are scaled to the correlation length, the average waiting times agree with typically reported waiting times between solar wind discontinuities. Thus discontinuities may be related to flux tube boundaries and intermittent structures that appear spontaneously in MHD turbulence.

Kinetic plasma turbulence cascade spans multiple scales ranging from macroscopic fluid flow to sub-electron scales. Mechanisms that dissipate large scale energy, terminate the inertial range cascade, and convert kinetic energy into heat are hotly debated. Here, we revisit these puzzles using fully kinetic simulation. By performing scale-dependent spatial filtering on the Vlasov equation, we extract information at prescribed scales and introduce several energy transfer functions. This approach allows highly inhomogeneous energy cascade to be quantified as it proceeds down to kinetic scales. The pressure work, $-(\mathit{P}·\nabla )·\mathit{u}$ , can trigger a channel of the energy conversion between fluid flow and random motions, which contains a collision-free generalization of the viscous dissipation in collisional fluid. Both the energy transfer and the pressure work are strongly correlated with velocity gradients.