Turbulence is ubiquitous in space plasmas. It is one of the most important subjects in heliospheric physics, as it plays a fundamental role in the solar wind - local interstellar medium interaction and in controlling energetic particle transport and acceleration processes. Understanding the properties of turbulence in various regions of the heliosphere with vastly different conditions can lead to answers to many unsolved questions opened up by observations of the magnetic field, plasma, pickup ions, energetic particles, radio and UV emissions, and so on. Several space missions have helped us gain preliminary knowledge on turbulence in the outer heliosphere and the very local interstellar medium. Among the past few missions, the Voyagers have paved the way for such investigations. This paper summarizes the open challenges and voices our support for the development of future missions dedicated to the study of turbulence throughout the heliosphere and beyond.

Kelvin-Helmholtz instability is a widespread phenomenon in space plasmas, such as at the planetary magnetospheres. During its nonlinear phase, the generation of Kelvin-Helmholtz vortices takes place. The identification of such coherent structures is not straightforward in observational data contrary to numerical simulations where both temporal evolution and spatial behavior can be observed. Recently, a comparison between a hybrid Vlasov-Maxwell simulation and Magnetospheric Multi-Scale satellites observation of a Kelvin-Helmholtz event has shown the presence of kinetic features that can uniquely characterize the boundaries of Kelvin-Helmholtz vortices. Indeed, a strong total current density has been observed in correspondence of the edges of each vortex associated with a weakly distorted distribution function from the equilibrium distribution; while the opposite occurs inside the vortex region. Moreover, a new tool has been proposed to distinguish the different phases of the Kelvin-Helmholtz instability and to identify the trajectory of the spacecraft across the vortex itself. Such a tool takes into consideration the mixing degree between the magnetospheric-like and magnetosheath-like particles population in the Earth environment. The clear identification of a vortex in in situ data is an important achievement since it can provide a better understanding of the role that Kelvin-Helmholtz instability plays in weakly collisional space plasmas in the contest of energy dissipation.

The majority of space plasmas are in a turbulent state, displaying fluctuations and non-linear behaviour at a broad range of scales. As well as being of fundamental interest, this turbulence may have important effects, such as heating of the solar wind and corona, acceleration of energetic particles, and interaction with magnetic reconnection and shocks. Measurements also suggest the presence of plasma instabilities which may generate quasi-linear waves, such as, e.g., Alfven-Ion-Cyclotron waves at ion scales and whistler waves at electron scales. Many aspects of the turbulence and instabilities are not well understood, in particular, the energy injection mechanism to the cascade, the non-linear turbulent cascade and dissipation mechanisms, non-linear instability saturation mechanisms, and the interaction between instabilities and turbulence. This session will address these questions though discussion of observational, theoretical, numerical, and laboratory work to understand these processes. This session is relevant to many currently operating missions (e.g., Wind, Cluster, MMS, STEREO, THEMIS, Van Allen Probes, DSCOVR) and in particular for Solar Orbiter and Solar Probe Plus.

Observed turbulence in space and astrophysics is expected to involve cascade and subsequent dissipation and heating. Contrary to standard collisional fluid turbulence, the weakly collisional magnetized plasma cascade may involve several channels of energy conversion, interchange, and spatial transport, leading eventually to the production of internal energy. This paper describes these channels of transfer and conversion, collectively amounting to a complex generalization of the Kolmogorov cascade. Channels may be described using compressible magnetohydrodynamic (MHD) and multispecies Vlasov-Maxwell formulations. Key steps are conservative transport of energy in space, parallel incompressible and compressible cascades in scale, electromagnetic work on particles driving macroscopic and microscopic flows, and pressure-strain interactions, both compressive and shear-like, that produce internal energy. A significant contrast with the collisional case is that the steps leading to the disappearance of large-scale energy in favor of internal energy are formally reversible. This property motivates a discussion of entropy, reversibility, and the relationship between dissipation with collisions and in the Vlasov system without collisions. Where feasible, examples are given from MHD and Particle in Cell simulations and from MMS observations.

The Kelvin-Helmholtz instability (KHI) is a ubiquitous physical process in ordinary fluids and plasmas, frequently observed also in space environments. In this paper, kinetic effects at proton scales in the nonlinear and turbulent stage of the KHI have been studied in magnetized collisionless plasmas by means of hybrid Vlasov-Maxwell simulations. The main goal of this work is to point out the back-reaction on particles triggered by the evolution of such instability, as energy reaches kinetic scales along the turbulent cascade. Interestingly, turbulence is inhibited when KHI develops over an initial state that is not an exact equilibrium state. On the other hand, when an initial equilibrium condition is considered, energy can be efficiently transferred toward short scales, reaches the typical proton wavelengths, and drives the dynamics of particles. As a consequence of the interaction of particles with the turbulent fluctuating fields, the proton velocity distribution deviates significantly from the local thermodynamic equilibrium, the degree of deviation increasing with the level of turbulence in the system and being located near regions of strong magnetic stresses. These numerical results support recent space observations from the Magnetospheric MultiScale mission of ion kinetic effects driven by the turbulent dynamics at Earth's magnetosheath and by the KHI in Earth's magnetosphere.

Kinetic simulations based on the Eulerian Hybrid Vlasov-Maxwell (HVM) formalism permit the examination of plasma turbulence with a useful resolution of the proton velocity distribution function. The HVM model is employed here to study the balance of energy, focusing on channels of conversion that lead to proton kinetic effects, including growth of internal energy and temperature anisotropies. We show that this Eulerian simulation approach, which is almost noise-free, is able to provide an accurate energy balance for protons. The results demonstrate explicitly that the recovered temperature growth is directly related to the role of the pressure-strain interaction. Furthermore, analysis of local spatial correlations indicates that the pressure-strain interaction is qualitatively associated with strong-current, high-vorticity structures although other local terms-such as the heat flux-weaken the correlation. These numerical capabilities based on the Eulerian approach will enable a deeper study of transfer and conversion channels in weakly collisional Vlasov plasmas.

Turbulence at kinetic scales is an unresolved and ubiquitous phenomenon that characterizes both space and laboratory plasmas. Recently, new theories, in situ spacecraft observations and numerical simulations suggest a novel scenario for turbulence, characterized by a so-called phase-space cascade-the formation of fine structures, both in physical and velocity-space. This new concept is here extended by directly taking into account the role of inter-particle collisions, modeled through the nonlinear Landau operator or the simplified Dougherty operator. The characteristic times, associated with inter-particle correlations, are derived in the above cases. The implications of introducing collisions on the phase-space cascade are finally discussed.

In weakly collisional space plasmas, the turbulent cascade provides most of the energy that is dissipated at small scales by various kinetic processes. Understanding the characteristics of such dissipative mechanisms requires the accurate knowledge of the fluctuations that make energy available for conversion at small scales, as different dissipation processes are triggered by fluctuations of a different nature. The scaling properties of different energy channels are estimated here using a proxy of the local energy transfer, based on the third-order moment scaling law for magnetohydrodynamic turbulence. In particular, the sign-singularity analysis was used to explore the scaling properties of the alternating positive-negative energy fluxes, thus providing information on the structure and topology of such fluxes for each of the different type of fluctuations. The results show the highly complex geometrical nature of the flux, and that the local contributions associated with energy and cross-helicity non-linear transfer have similar scaling properties. Consequently, the fractal properties of current and vorticity structures are similar to those of the Alfvenic fluctuations.

A detailed comparison between the Landau and the Dougherty collision operators has been performed by means of Eulerian simulations, in the case of relaxation toward equilibrium of a spatially homogeneous field-free plasma in three-dimensional velocity space. Even though the form of the two collisional operators is evidently different, we found that the collisional evolution of the relevant moments of the particle distribution function (temperature and entropy) are similar in the two cases, once an 'ad hoc' time rescaling procedure has been performed. The Dougherty operator is a nonlinear differential operator of the Fokker-Planck type and requires a significantly lighter computational effort with respect to the complete Landau integral; this makes self-consistent simulations of plasmas in presence of collisions affordable, even in the multi-dimensional phase space geometry.

The problem of collisions in a plasma is a wide subject with a huge historical literature. In fact, the description of realistic plasmas is a tough problem to attack, both from the theoretical and the numerical point of view. In this paper, a Eulerian time-splitting algorithm for the study of the propagation of electrostatic waves in collisional plasmas is presented. Collisions are modeled through one-dimensional operators of the Fokker-Planck type, both in linear and nonlinear forms. The accuracy of the numerical code is discussed by comparing the numerical results to the analytical predictions obtained in some limit cases when trying to evaluate the effects of collisions in the phenomenon of wave plasma echo and collisional dissipation of Bernstein-Greene-Kruskal waves. Particular attention is devoted to the study of the nonlinear Dougherty collisional operator, recently used to describe the collisional dissipation of electron plasma waves in a pure electron plasma column [M. W. Anderson and T. M. O'Neil, Phys. Plasmas 14, 112110 (2007)]. Finally, for the study of collisional plasmas, a recipe to set the simulation parameters in order to prevent the filamentation problem can be provided, by exploiting the property of velocity diffusion operators to smooth out small velocity scales. © 2013 AIP Publishing LLC.