Session NM8: Mini-conference: Turbulence and Particles in Astrophysical and Space Plasmas

Measuring turbulence and transport processes in the hot intergalactic plasma in galaxy clusters

Clusters of galaxies are filled with hot, weakly-collisional and high-$\beta $ plasma. The large sizes of clusters and their relative simplicity make them unique laboratories to study such plasmas. While the global characteristics of the intergalactic plasma are now routinely measured, microphysics is still poorly understood. In my talk, I will present indirect measurements of turbulence in the intracluster plasma based on the statistical analysis of X-ray surface brightness fluctuations imprinted in the X-ray images of galaxy clusters. I will discuss the role of this turbulence in the global cooling-heating balance of clusters. In the second half of my talk, I will show our recent measurements of plasma fluctuations on spatial scales comparable to Coulomb mean free path. In contrast to expectations of hydrodynamic models with pure Coulomb collision rates, we observe a continuation of the power spectrum of fluctuations with a slope that is consistent with the Kolmogorov model. This implies that the effective isotropic viscosity is orders of magnitude smaller than the Spitzer value. This also indicates an enhanced collision rate in the plasma due to the presence of plasma instabilities, or that the transport processes are anisotropic with respect to the local magnetic field.   

The physics of fast electrons in the solar wind

The electron velocity distribution function in the solar wind typically contains a population of energetic, suprathermal electrons that form a beam (strahl) along the magnetic field lines. These electrons streaming from the hot solar corona along the Parker-spiral magnetic field lines, experience very weak Coulomb collisions, and they can contribute to the energy transport and heating of the solar wind. I will discuss some recent analytic results [1-4] on the formation of the electron strahl, and its role in the solar wind heating. [1] Horaites, K., Boldyrev, S., Krasheninnikov, S.I., Salem, C., Bale, S.D., Pulupa, M., Phys. Rev. Lett 114, 245003 (2015); [2] Horaites, K., Astfalk, P., Boldyrev, S., Jenko, F., MNRAS 480, 1499 (2018); [3] Horaites, K., Boldyrev, S., Medvedev, M. V., MNRAS 484, 2474 (2019); [4] Boldyrev, S, Horaites, K, MNRAS (2019) submitted.   

Plasma Turbulence Near The Sun: New Observations From Parker Solar Probe

In August 2018, the Parker Solar Probe spacecraft was launched on its journey to the solar corona, and in November it reached a heliocentric distance of 0.17 AU, nearly twice as close as any previous spacecraft. It has now completed its first two orbits and returned a wealth of data, enabling a detailed in situ study of the solar wind plasma physics in this unexplored region as well as its radial evolution out to 1 AU. In this talk, I will present results on the radial evolution of solar wind turbulence at MHD scales as measured by PSP, as well as a detailed examination of the properties of kinetic-scale turbulence at 0.17 AU. The results will be compared to our current theoretical understanding of plasma turbulence and its role near the Sun, to reveal both the extent to which it operates as a universal process, and to enable its potential role in the long-standing problems of coronal heating and solar wind acceleration to be investigated.   

Using Field-Particle Correlations to Diagnose Particle Energization in Turbulence, Magnetic Reconnection, and Shocks

The recently devised field-particle correlation technique is a generally applicable kinetic approach that can be used to understand the energization of particles due to a broad range of fundamental plasma processes, including turbulence, magnetic reconnection, and shocks. The technique generates a signature of the energization of particles as a function of velocity space, providing valuable information about the mechanisms governing the energization of particles in weakly collisional plasmas, ultimately yielding plasma heating or particle acceleration. In many cases, these velocity-space signatures are unique, providing a means for identifying and distinguishing the processes responsible for the particle energization. Furthermore, the technique requires only information about the electromagnetic fields and particle velocity distributions at a single-point in space, making it possible to apply the method to in situ spacecraft observations. Here we will present a number of applications of the field-particle correlation technique to different problems, including the dissipation of turbulence, acceleration of auroral electrons, energization of particles in magnetic reconnection, and energization of particles at collisionless shocks.   

Magneto-kinetic turbulence and particle heating in collisionless, high-beta plasmas

The transport of energy and momentum and the heating of plasma particles by waves and turbulence are key ingredients in many problems at the frontiers of heliospheric and astrophysics research. This includes the heating and acceleration of the solar wind; the observational appearance of black-hole accretion flows on event-horizon scales; and the properties of the hot, diffuse plasmas that fill dark-matter halos. All of these plasmas are magnetized and weakly collisional, with plasma beta parameters of order unity or even much larger. In this regime, magnetic-field-biased deviations from local thermodynamic equilibrium (i.e., pressure anisotropies) and the kinetic instabilities they excite can dramatically change the propagation and damping of Alfv\'{e}nic and compressive waves from predictions based on fluid (MHD) and linear kinetic theory. In this talk, results from hybrid-kinetic simulations are used to demonstrate this physics in the context of driven, steady-state turbulence, with a focus on energy spectra and nonlocal energy transfer, particle energization and velocity-space structure, and effective collisionality due to particle scattering off ion-Larmor-scale instabilities. Testable predictions are made for pressure-anisotropic parts of the solar wind and magnetosheath.   

Nonlinear Reconnection in Magnetized Turbulence

Recent analytical works on strong magnetized plasma turbulence have hypothesized the existence of a range of scales where the tearing instability may govern the energy cascade. In this talk, we discuss the conditions under which such tearing may give rise to full nonlinear magnetic reconnection, thereby enabling significant energy conversion and dissipation. When those conditions are met, a new turbulence regime is accessed where reconnection-driven energy dissipation becomes the norm, rather than a sporadic event.   

Why a fractal current distribution is a lowest-energy state

Because ideal MHD is flux-conserving and has no intrinsic scale, pinching, kinks, and sausage instabilities occur at all scales. However, when the scale shrinks to the ion skin depth, Hall and electron inertia terms come into play, ideal MHD fails, and radically different physics occurs (e.g., fast reconnection). Flux-conserving magnetic energy scales inversely with inductance. Thus, it is energetically favorable for current to be diffuse rather than in a thin layer because a diffuse current has more inductance than a thin current. Since flux conservation precludes current spreading in an axisymmetric plasma, a sequence of non-axisymmetric instabilities of progressively smaller scale must develop to achieve minimum system energy. This sequence causes the current to be distributed like insulated strands of a wire rope where the smallest strand has radius of the order of the ion skin depth. The current distribution is then fractal because a current rope at any specific scale is both (i) an insulated strand of some larger-scale rope and (ii) composed of insulated strands of smaller-scale ropes. This is analogous to Litz wire, a commercial product made of braided insulated fine wires resulting in less inductance than a corresponding solid wire.   

The Emergence of Two Invariants from One in MHD Turbulence

In incompressible MHD flows, it is only total energy that is conserved and not magnetic or kinetic energy separately. As a manifestation of order emerging out of chaos (or permanence out of turbulence), we have found in [1] that they are in fact conserved separately over a range of scales in turbulent flows. This essentially gives us two global invariants (kinetic energy and magnetic energy) instead of just one (total energy). I will discuss this seemingly counter-intuitive result and how it is to be reconciled with the strong magnetic-flow coupling and the role of waves in MHD turbulence. I will put this result in the context of cascade theories and briefly examine its potential implications on the energetics and dissipation of plasma flows, magnetic reconnection, and also on modeling efforts. [1] X. Bian and H. Aluie, Phys. Rev. Lett. 122, 135101 (2019).   

Inverse magnetic energy transfer through magnetic reconnection

A wide range of space and astrophysical systems, such as the solar corona, heliosheath and Weibel-produced magnetic field in supernova shocks, of which the dynamics are governed by turbulence and reconnection, can be conceptualized as an ensemble of interacting flux ropes. We investigate magnetic field dynamics in a system of parallel flux ropes as well as more generic magnetically-dominated turbulent systems, focusing on the inverse magnetic energy transfer. An analytical model is introduced and shown to capture the evolution of the main quantities of interest, as borne out by our 2D and 3D reduced magnetohydrodynamics (RMHD) and 2D particle-in-cell simulations. Magnetic reconnection is identified as the key mechanism enabling the inverse transfer and setting its properties: magnetic energy decays as $\tilde t^{-1}$, where $\tilde t$ is time normalized to the reconnection timescale; and the field correlation length grows as $\tilde t^{1/2}$. Critical balance is shown (by magnetic structure functions) to govern the aspect ratio of the flux ropes in 3D RMHD simulations. This quantitative description of inverse energy transfer could improve our understanding of longstanding problems such as coronal heating, galactic magnetogenesis, and high-energy emission in gamma-ray bursts.   

Reconnection and ion heating in low-$\beta$ hybrid-kinetic plasma turbulence

Turbulence and kinetic processes in collisionless, magnetized plasmas have been investigated extensively over the past decades via theoretical models, {\em in situ} spacecraft measurements in the heliosphere, and numerical simulations. Alongside the debate about the nature of ion- and electron-scale fluctuations in solar-wind turbulence, one of the fundamental open questions concerns how turbulent energy is partitioned between ions and electrons. In space and astrophysical plasmas, this "turbulent heating" may involve a wide variety of collisionless plasma processes, with their relative importance depending on several plasma parameters. In this talk, we present results from 3D hybrid particle-in-cell (PIC) simulations of continuously driven, critically balanced plasma turbulence at low $\beta$. The interplay between the spectral anisotropy of the fluctuations and different ion-heating mechanisms -- and how to possibly diagnose and/or disentangle them -- will be discussed. Our results have implications for the differential heating of ion and electrons in turbulent low-$\beta$ plasmas such as the solar wind.