Turbulence and Magnetic Fields in Astrophysics (original) (raw)
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Magnetohydrodynamic turbulence and turbulent dynamo in partially ionized plasma
New Journal of Physics, 2017
Astrophysical fluids are turbulent, magnetized, and frequently partially ionized. As an example of astrophysical turbulence, the interstellar turbulence extends over a remarkably large range of spatial scales and participates in key astrophysical processes happening on different ranges of scales. Significant progress has been achieved in the understanding of the magnetohydrodynamic (MHD) turbulence since the turn of the century, and this enables us to better describe turbulence in magnetized and partially ionized plasmas. In fact, the modern revolutionized picture of MHD turbulence physics facilitates the development of various theoretical domains, including the damping process for dissipating MHD turbulence and the dynamo process for generating MHD turbulence with many important astrophysical implications. In this paper, we review some important findings from our recent theoretical works to demonstrate the interconnection between the properties of MHD turbulence and those of turbulent dynamo in a partially ionized gas. We also briefly exemplify some new tentative studies on how the revised basic processes influence the associated outstanding astrophysical problems in areas such as magnetic reconnection, cosmic ray scattering, and magnetic field amplification in both the early and present-day universe. 1. Turbulent, magnetized, and partially ionized interstellar medium Astrophysical plasmas, e.g., in the low solar atmosphere and molecular clouds, are commonly partially ionized and magnetized (see the book by Draine 2011 for a list of the partially ionized interstellar medium phases). The presence of neutrals affects the magnetized plasma dynamics and induces damping of MHD turbulence (see studies by e.g., Piddington 1956, Kulsrud and Pearce 1969). On the other hand, astrophysical plasmas are characterized by large Reynolds numbers, and therefore they are expected to be turbulent (see e.g., Schekochihin et al 2002a, Mac Low and Klessen 2004, McKee and Ostriker 2007). This expectation is consistent with the turbulent spectrum of electron density fluctuations measured in the interstellar medium (ISM)
Recent results on magnetic plasma turbulence
2013
Magnetic plasma turbulence is observed over a broad range of scales in the solar wind. We discuss the results of high-resolution numerical simulations of magnetohydrodynamic (MHD) turbulence that models plasma motion at large scales and the results of numerical simulations of kinetic-Alfvén turbulence that models plasma motion at small, sub-proton scales. The simulations, with numerical resolutions up to 2048 3 mesh points in the MHD case and 512 3 points in kinetic-Alfvén case and statistics accumulated over 30 to 150 eddy turnover times, constitute, to the best of our knowledge, the largest statistical sample of steadily driven three dimensional MHD and kinetic-Alfvén turbulence to date.
Magnetohydrodynamic turbulence and turbulent dynamo in a partially ionized plasma
Cornell University - arXiv, 2018
Astrophysical fluids are turbulent, magnetized and frequently partially ionized. As an example of astrophysical turbulence, the interstellar turbulence extends over a remarkably large range of spatial scales and participates in key astrophysical processes happening on different ranges of scales. A significant progress has been achieved in the understanding of the magnetohydrodynamic (MHD) turbulence since the turn of the century, and this enables us to better describe turbulence in magnetized and partially ionized plasmas. In fact, the modern revolutionized picture of the MHD turbulence physics facilitates the development of various theoretical domains, including the damping process for dissipating MHD turbulence and the dynamo process for generating MHD turbulence with many important astrophysical implications. In this paper, we review some important findings from our recent theoretical works to demonstrate the interconnection between the properties of MHD turbulence and those of turbulent dynamo in a partially ionized gas. We also briefly exemplify some new tentative studies on how the revised basic processes influence the associated outstanding astrophysical problems in, such as, magnetic reconnection, cosmic ray scattering, magnetic field amplification in both the early and the present-day universe.
Magnetohydrodynamic Turbulence in the Solar Wind
Annual Review of Astronomy and Astrophysics, 1995
Recent work in describing the solar wind as an MHD turbulent fluid has shown that the magnetic fluctuations are adequately described as time stationary and to some extent as spatially homogeneous. Spectra of the three rugged invariants of incompressible MHD are the principal quantities used to characterize the velocity and magnetic field fluctuations. Unresolved issues concerning the existence of actively developing turbulence are discussed. INTRODUCTION To describe a fluid system as turbulent is to say that the dynamical fluid variables exhibit complex and essentially non-reproducible behavior as a function of time. This is generally due to the presence of nonlinearities in the fluid equations which strongly couple a large number of degrees of freedom. Turbulent systems are usually very far from equilibrium states for which detailed analytically tractable theories might exist. By all appearances, the solar wind plasma flow and the interplanetary magnetic field carried along with it are such a turbulent system. In the zero momentum frame, the magnetic and velocity field fluctuations are energetically comparable to the mean magnetic field over length scales of order I AU and display the type of complicated behavior expected of turbulence. velocity and magnetic fields in magnetohydrodynamie turbulence, EOS,
Interplanetary and interstellar plasma turbulence
Plasma Physics and Controlled Fusion, 2007
Theoretical approaches to low-frequency magnetized turbulence in collisionless and weakly collisional astrophysical plasmas are reviewed. The proper starting point for an analytical description of these plasmas is kinetic theory, not fluid equations. The anisotropy of the turbulence is used to systematically derive a series of reduced analytical models. Above the ion gyroscale, it is shown rigourously that the Alfvén waves decouple from the electron-density and magnetic-field-strength fluctuations and satisfy the Reduced MHD equations. The density and field-strength fluctuations (slow waves and the entropy mode in the fluid limit), determined kinetically, are passively mixed by the Alfvén waves. The resulting hybrid fluid-kinetic description of the lowfrequency turbulence is valid independently of collisionality. Below the ion gyroscale, the turbulent cascade is partially converted into a cascade of kinetic Alfvén waves, damped at the electron gyroscale. This cascade is described by a pair of fluid-like equations, which are a reduced version of the Electron MHD. The development of these theoretical models is motivated by observations of the turbulence in the solar wind and interstellar medium. In the latter case, the turbulence is spatially inhomogeneous and the anisotropic Alfvénic turbulence in the presence of a strong mean field may coexist with isotropic MHD turbulence that has no mean field.
Journal of Geophysical Research, 2008
This paper studies the turbulent cascade of magnetic energy in weakly collisional magnetized plasmas. A cascade model is presented, based on the assumptions of local nonlinear energy transfer in wavenumber space, critical balance between linear propagation and nonlinear interaction times, and the applicability of linear dissipation rates for the nonlinearly turbulent plasma. The model follows the nonlinear cascade of energy from the driving scale in the MHD regime, through the transition at the ion Larmor radius into the kinetic Alfvén wave regime, in which the turbulence is dissipated by kinetic processes. The turbulent fluctuations remain at frequencies below the ion cyclotron frequency due to the strong anisotropy of the turbulent fluctuations, k ≪ k ⊥ (implied by critical balance). In this limit, the turbulence is optimally described by gyrokinetics; it is shown that the gyrokinetic approximation is well satisfied for typical slow solar wind parameters. Wave phase velocity measurements are consistent with a kinetic Alfvén wave cascade and not the onset of ion cyclotron damping. The conditions under which the gyrokinetic cascade reaches the ion cyclotron frequency are established. Cascade model solutions imply that collisionless damping provides a natural explanation for the observed range of spectral indices in the dissipation range of the solar wind. The dissipation range spectrum is predicted to be an exponential fall off; the power-law behavior apparent in observations may be an artifact of limited instrumental sensitivity. The cascade model is motivated by a programme of gyrokinetic simulations of turbulence and particle heating in the solar wind.
Comment on “Kinetic Simulations of Magnetized Turbulence in Astrophysical Plasmas”
Physical Review Letters, 2008
This letter presents the first ab initio, fully electromagnetic, kinetic simulations of magnetized turbulence in a homogeneous, weakly collisional plasma at the scale of the ion Larmor radius (ion gyroscale). Magnetic and electric-field energy spectra show a break at the ion gyroscale; the spectral slopes are consistent with scaling predictions for critically balanced turbulence of Alfvén waves above the ion gyroscale (spectral index −5/3) and of kinetic Alfvén waves below the ion gyroscale (spectral indices of −7/3 for magnetic and −1/3 for electric fluctuations). This behavior is also qualitatively consistent with in situ measurements of turbulence in the solar wind. Our findings support the hypothesis that the frequencies of turbulent fluctuations in the solar wind remain well below the ion cyclotron frequency both above and below the ion gyroscale.
Astrophysical Hydromagnetic Turbulence
Space Science Reviews, 2013
Recent progress in astrophysical hydromagnetic turbulence is being reviewed. The physical ideas behind the now widely accepted Goldreich-Sridhar model and its extension to compressible magnetohydrodynamic turbulence are introduced. Implications for cosmic ray diffusion and acceleration is being discussed. Dynamo-generated magnetic fields with and without helicity are contrasted against each other. Certain turbulent transport processes are being modified and often suppressed by anisotropy and inhomogeneities of the turbulence, while others are being produced by such properties, which can lead to new largescale instabilities of the turbulent medium. Applications of various such processes to astrophysical systems are being considered.
Introduction to Turbulence in Magnetised Plasmas
AIP Conference Proceedings, 2008
The ideas of turbulence of small fluctuations on a background as a statistical phenomenon are outlined. Basic properties such as three-wave interactions and spatial scale cascades are derived from the basic equations. Passive scalar dynamics is treated. The special case of dissipative coupling between the fluid and an otherwise passive scalar, of central relevance to magnetised plasmas, is used as an example.
Fluid-like dissipation of magnetic turbulence at electron scales in the solar wind
arXiv (Cornell University), 2011
The turbulent spectrum of magnetic fluctuations in the solar wind displays a spectral break at ion characteristic scales. At electron scales the spectral shape is not yet completely established. Here, we perform a statistical study of 102 spectra at plasma kinetic scales, measured by the Cluster/STAFF instrument in the free solar wind. We show that the magnetic spectrum in the high frequency range, [1, 400] Hz, has a form similar to what is found in hydrodynamics in the dissipation range ∼ Ak −α exp (−kℓ d). The dissipation scale ℓ d is found to be correlated with the electron Larmor radius ρe. The spectral index α varies in the range [2.2, 2.9] and is anti-correlated with ℓ d , as expected in the case of the balance between the energy injection and the energy dissipation. The coefficient A is found to be proportional to the ion temperature anisotropy, suggesting that local ion instabilities may play some rôle for the solar wind turbulence at plasma kinetic scales. The exponential spectral shape found here indicates that the effective dissipation of magnetic fluctuations in the solar wind has a wave number dependence similar to that of the resistive term in collisional fluids ∼ △δB ∼ k 2 δB.