We present a study of signatures of energy dissipation at kinetic scales in plasma turbulence based on observations by the Magnetospheric Multiscale mission (MMS) in the Earth's magnetosheath. Using ...several intervals, and taking advantage of the high-resolution instrumentation on board MMS, we compute and discuss several statistical measures of coherent structures and heating associated with electrons, at previously unattainable scales in space and time. We use the multi-spacecraft Partial Variance of Increments (PVI) technique to study the intermittent structure of the magnetic field. Furthermore, we examine a measure of dissipation and its behavior with respect to the PVI as well as the current density. Additionally, we analyze the evolution of the anisotropic electron temperature and non-Maxwellian features of the particle distribution function. From these diagnostics emerges strong statistical evidence that electrons are preferentially heated in subproton-scale regions of strong electric current density, and this heating is preferentially in the parallel direction relative to the local magnetic field. Accordingly, the conversion of magnetic energy into electron kinetic energy occurs more strongly in regions of stronger current density, a finding consistent with several kinetic plasma simulation studies and hinted at by prior studies using lower resolution Cluster observations.
VARIANCE ANISOTROPY IN KINETIC PLASMAS Parashar, Tulasi N.; Oughton, Sean; Matthaeus, William H. ...
The Astrophysical journal,
06/2016, Letnik:
824, Številka:
1
Journal Article
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ABSTRACT Solar wind fluctuations admit well-documented anisotropies of the variance matrix, or polarization, related to the mean magnetic field direction. Typically, one finds a ratio of ...perpendicular variance to parallel variance of the order of 9:1 for the magnetic field. Here we study the question of whether a kinetic plasma spontaneously generates and sustains parallel variances when initiated with only perpendicular variance. We find that parallel variance grows and saturates at about 5% of the perpendicular variance in a few nonlinear times irrespective of the Reynolds number. For sufficiently large systems (Reynolds numbers) the variance approaches values consistent with the solar wind observations.
Using data from the Magnetospheric Multiscale (MMS) and Cluster missions obtained in the solar wind, we examine second-order and fourth-order structure functions at varying spatial lags normalized to ...ion inertial scales. The analysis includes direct two-spacecraft results and single-spacecraft results employing the familiar Taylor frozen-in flow approximation. Several familiar statistical results, including the spectral distribution of energy, and the sale-dependent kurtosis, are extended down to unprecedented spatial scales of ∼6 km, approaching electron scales. The Taylor approximation is also confirmed at those small scales, although small deviations are present in the kinetic range. The kurtosis is seen to attain very high values at sub-proton scales, supporting the previously reported suggestion that monofractal behavior may be due to high-frequency plasma waves at kinetic scales.
The quadrupolar out‐of‐plane Hall magnetic field generated during collisionless reconnection propagates away from the x line as a kinetic Alfvén wave (KAW). While it has been shown that this KAW ...carries substantial Poynting flux and propagates super‐Alfvenically, how this KAW damps as it propagates away from the x line is not well understood. In this study, this damping is examined using kinetic particle‐in‐cell simulations of antiparallel symmetric magnetic reconnection in a one‐dimensional current sheet equilibrium. In the reconnection simulations, the KAW wave vector has a typical magnitude comparable to an inverse fluid Larmor radius (effectively an inverse ion Larmor radius) and a direction of 85–89° relative to the local magnetic field. We find that the damping of the reconnection KAW is consistent with linear Landau damping results from a numerical Vlasov dispersion solver. This knowledge allows us to generalize our damping predictions to regions in the magnetotail and solar corona where the magnetic geometry can be approximated as a current sheet. For the magnetotail, the KAW from reconnection will not damp away before propagating the approximately 20 Earth radii associated with global magnetotail distances. For the solar corona, on the other hand, these KAWs will completely damp before reaching the distances comparable to the flare loop length.
Key Points
The quadrupolar out‐of‐plane magnetic field associated with reconnection propagates away from the x line as a kinetic Alfvén wave (KAW)
The attenuation of this KAW is consistent with linear Landau damping theory
For magnetotail plasma conditions, this KAW can propagate tens of Earth radii with little damping
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.
Reconnection and turbulence are two of the most commonly observed dynamical processes in plasmas, but their relationship is still not fully understood. Using 2.5D kinetic particle-in-cell simulations ...of both strong turbulence and reconnection, we compare the cross-scale transfer of energy in the two systems by analyzing the generalization of the von Kármán Howarth equations for Hall magnetohydrodynamics, a formulation that subsumes the third-order law for steady energy transfer rates. Even though the large scale features are quite different, the finding is that the decomposition of the energy transfer is structurally very similar in the two cases. In the reconnection case, the time evolution of the energy transfer also exhibits a correlation with the reconnection rate. These results provide explicit evidence that reconnection dynamics fundamentally involves turbulence-like energy transfer.
We study the effects of plasma β (the ratio of plasma pressure to magnetic pressure) on the evolution of kinetic plasma turbulence using fully kinetic particle-in-cell simulations of decaying ...turbulence. We find that the plasma β systematically affects spectra, measures of intermittency, decay rates of turbulence fluctuations, and partitioning over different channels of energy exchange. More specifically, an increase in plasma β leads to greater total heating, with proton heating preferentially more than electrons. Implications for achieving magnetosheath like temperature ratios are discussed.
The Parker Solar Probe (PSP) primary mission extends seven years and consists of 24 orbits of the Sun with descending perihelia culminating in a closest approach of ∼9.8 R . In the course of these ...orbits PSP will pass through widely varying conditions, including anticipated large variations of turbulence properties, such as energy density, correlation scales, and cross helicities. Here we employ global magnetohydrodynamic simulations with self-consistent turbulence transport and heating to preview conditions that will likely be encountered by PSP by assuming suitable boundary conditions at the coronal base. The code evolves large-scale parameters-such as the velocity, magnetic field, and temperature-as well as the turbulent energy density, cross helicity, and correlation scale. These computed quantities provide the basis for evaluating additional useful parameters that are derivable from the primary model outputs. Here we illustrate one such possibility in which computed turbulence and large-scale parameters are used to evaluate the accuracy of the Taylor "frozen-in" hypothesis along the PSP trajectory. Apart from the immediate purpose of anticipating turbulence conditions that PSP will encounter, as experience is gained in comparisons of observations with simulated data, this approach will be increasingly useful for planning and interpretation of subsequent observations.
In this work, we implement and examine a new flow reconstruction methodology using cubic-splines for inter-polations in the gas kinetic method (GKM). We compare this version of GKM with the existing ...WENO based interpolation method. The comparisons are made in terms of accuracy and computational speed. We find that at low to intermediate range of Mach number (Mt
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