Observational quantification of three-dimensional anisotropies and scalings of space plasma turbulence at kinetic scales

A statistical survey of spectral anisotropy of space plasma turbulence is performed using five years measurements from MMS in the magnetosheath. By measuring the five-point second-order structure functions of the magnetic field, we have for the first time quantified the three-dimensional anisotropies and scalings at sub-ion-scales ($<$ 100 km). In the local reference frame $(\hat L_{\perp}, \hat l_{\perp}, \hat l_{\parallel})$ defined with respect to local mean magnetic field $\boldsymbol {B}_0$ (Chen et al. 2012), the "statistical eddies" are found to be mostly elongated along $\boldsymbol {B}_0$ and shortened in the direction perpendicular to both $\boldsymbol {B}_0$ and local field fluctuations. From several $d_i$ (ion inertial length) toward $\sim$ 0.05 $d_i$, the ratio between eddies' parallel and perpendicular lengths features a trend of rise then fall, whereas the anisotropy in the perpendicular plane appears scale-invariant. Specifically, the anisotropy relations for the total magnetic field at 0.1-1.0 $d_i$ are obtained as $l_{\parallel} \simeq 2.44 \cdot l_{\perp}^{0.71}$, and $L_{\perp} \simeq 1.58 \cdot l_{\perp}^{1.08}$, respectively. Our results provide new observational evidence to compare with phenomenological models and numerical simulations, which may help to better understand the nature of kinetic scale turbulence.Comment: Accepte

THE ROLE OF ALPHA PARTICLES IN THE EVOLUTION OF THE SOLAR-WIND TURBULENCE TOWARD SHORT SPATIAL SCALES

We present a numerical study of the kinetic dynamics of protons and alpha particles during the evolution of the solar-wind turbulent cascade, in which the energy injected in large-scale slab-type Alfvenic fluctuations is transferred toward short spatial scale lengths, across the proton skin depth. We make use of a hybrid Vlasov-Maxwell code that integrates numerically the Vlasov equation for both the ion species, while the electrons are considered as a fluid. The system evolution is investigated in terms of different values of the electron to proton and alpha particle to proton temperature ratios. The numerical results show that the previously studied kinetic dynamics of protons is not strongly affected by the presence of alpha particles, at least when they are present in low concentration. Our simulations not only provide a physical explanation for the generation of beams of accelerated particles along the direction of the ambient magnetic field for both protons and alpha particles, but also show that this mechanism is more efficient for protons than for alpha particles, in agreement with recent solar-wind data analyses

SHORT-WAVELENGTH ELECTROSTATIC FLUCTUATIONS IN THE SOLAR WIND

Hybrid Vlasov-Maxwell simulations have been used recently to investigate the dynamics of the solar-wind plasma in the tail at short wavelengths of the energy cascade. These simulations have shown that a significant level of electrostatic activity is detected at wavelengths smaller than the proton inertial scale in the longitudinal direction with respect to the ambient magnetic field. In this paper, we describe the results of a new series of hybrid Vlasov-Maxwell simulations that allow us to investigate in more detail the generation process of these electrostatic fluctuations in terms of the electron-to-proton temperature ratio Te /Tp . This analysis gives evidence for the first time that even in the case of cold electrons, Te Tp (the appropriate condition for solar-wind plasmas), the resonant interaction of protons with large-scale left-hand polarized ion-cyclotron waves is responsible for the excitation of short-scale electrostatic fluctuations with an acoustic dispersion relation. Moreover, through our numerical results we propose a physical mechanism to explain the generation of longitudinal proton-beam distributions in typical conditions of the solar-wind environment

A SHELL MODEL TURBULENT DYNAMO

Turbulent dynamo phenomena, observed almost everywhere in astrophysical objects and also in the laboratory in the recent VKS2 experiment, are investigated using a shell model technique to describe magnetohydrodynamic turbulence. Detailed numerical simulations at very high Rossby numbers (α2 dynamo) show that as the magnetic Reynolds number increases, the dynamo action starts working and different regimes are observed. The model, which displays different large-scale coherent behaviors corresponding to different regimes, is able to reproduce the magnetic field reversals observed both in a geomagnetic dynamo and in the VKS2 experiment. While rough quantitative estimates of typical times associated with the reversal phenomenon are consistent with paleomagnetic data, the analysis of the transition from oscillating intermittent through reversal and finally to stationary behavior shows that the nature of the reversals we observe is typical of α2 dynamos and completely different from VKS2 reversals. Finally, the model shows that coherent behaviors can also be naturally generated inside the many-mode dynamical chaotic model, which reproduces the complexity of fluid turbulence, as described by the shell technique

Turbulence-driven ion beams in space plasmas

The description of the local turbulent energy transfer and the high-resolution ion distributions measured by the Magnetospheric Multiscale mission together provide a formidable tool to explore the cross-scale connection between the fluid-scale energy cascade and plasma processes at subion scales. When the small-scale energy transfer is dominated by Alfv´enic, correlated velocity, and magnetic field fluctuations, beams of accelerated particles are more likely observed. Both space observations and numerical simulations suggest the nonlinear wave-particle interaction as one possible mechanism for the energy dissipation in space plasmas

A comparative analysis of the causes of air pollution in three cities of the Danube region: implications for the implementation of the air quality directives

The causes of air pollution in three cities of the Danube region (Budapest, Sofia and Zagreb) were studied using datasets of measurements and modelling tools. The contributions from different activity sectors, including natural sources and their geographical origin were quantified. It was observed that most of the pollutants are emitted locally. However, the medium to long range transport may be also considerable. On the basis of the output of the source identification, a series of measures were proposed to deal wtih the pollution problem at local, national and international levels.JRC.H.2-Air and Climat

Wave-polarization Analysis of the Alfvénic Slow Solar Wind at Kinetic Scales

This paper reports the first polarization measurement in the Alfvénic slow solar wind. The normalized magnetic helicity is used as a diagnostic parameter for studying the polarization status of the high-frequency magnetic fluctuations, along with an attempt to identify various wave modes in the solar wind turbulence. Clear evidence for the existence of ion cyclotron waves (ICWs) and kinetic Alfvén waves (KAWs) is also found in the Alfvénic lowspeed plasma, robustly supporting the idea that the Alfvénic content of the solar wind fluctuations at fluid scales is the key parameter driving wave generation at kinetic scales. By separating the contributions to helicity from the two modes, it is possible to address the thermodynamical properties of ICWs and KAWs and provide the first direct estimate of their magnetic compressibility. In particular, while ICWs are mainly associated with higher levels of anisotropy and appear to be bounded by the threshold of proton–cyclotron kinetic instability, KAWs (which end up being more compressive than ICWs) are found at lower anisotropies and seem to be limited by the mirror mode instability threshold, extending as well to near the parallel fire hose unstable region. These result are relevant to theories of turbulence and dissipation in the solar wind