Showing papers by "William H. Matthaeus published in 2002"

Perpendicular Transport of Charged Particles in Composite Model Turbulence: Recovery of Diffusion

Abstract: The computation of charged particle orbits in model turbulent magnetic fields is used to investigate the properties of particle transport in the directions perpendicular to the large-scale magnetic field. Recent results by Qin, Matthaeus, & Bieber demonstrate that parallel scattering suppresses perpendicular diffusion to a subdiffusive level when the turbulence lacks transverse structure. Here numerical computations are used to show that in turbulence in which there is substantial transverse structure, a second regime of diffusive transport can be established. In both the subdiffusion regime and this second diffusion regime, perpendicular transport is intrinsically nonlinear. The regime of second diffusion persists for long times and may therefore be of interest in astrophysical transport problems such as the scattering and solar modulation of cosmic rays.

Subdiffusive transport of charged particles perpendicular to the large scale magnetic field

Abstract: [1] Transport of charged particles across the mean magnetic field due to broad band, powerlaw-distributed magnetic fluctuations is examined by direct computation of a large number of charged test particle trajectories. Fluctuations consist of a one dimensional slab spectrum with an additional 0.01 percent level of two dimensional fluctuations; the net field has no ignorable directions. We find that, at long times (t), the mean square displacements subdiffusively scale as ∼t1/2. This confirms that perpendicular diffusion is suppressed when turbulence lacks strong three dimensional structure, in agreement with theories of Urch and of Kota and Jokipii.

Evidence for weak MHD turbulence in the middle magnetosphere of Jupiter

Abstract: In this paper we study the small scale-magnetic eld fluctuations of the middle Jovian magnetosphere that are embedded in the strong background eld 0 of Jupiter in the framework of turbulence theory. We perform a statistical analysis of these fluctuations using a ve year set of Galileo spacecraft magnetic eld data. Calculating power spectra of the fluctuations, we identify for the rst time a spectral index of minus two for wave vectors perpendicular to 0. These results strongly support a description of the fluctuations within the framework of weak magnetic turbulence and recent theoretical developments therein by Galtier et al. (2000). In addition, we show that there is little variation of the spectral index upon radial distance in the middle magnetosphere. We comment also on how the presence of such fluctuating elds might have interesting consequences concerning transport properties in the Jovian magnetosphere.

Energetic Particles From Three-Dimensional Magnetic Reconnection Events In The Swarthmore Spheromak Experiment

Abstract: Measurements are presented from the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] showing a population of superthermal, super-Alfvenic ions with Ē≅90 eV and Emax⩾200 eV accelerated by reconnection activity in three-dimensional magnetic structures. These energetic ions are temporally and spatially correlated with three-dimensional magnetic reconnection events (measured with a 3D probe array) and are accelerated along the X-line normal to the local 2D plane of reconnection. In a typical SSX discharge, the peak reconnection electromotive force E=vBL⩽(105 m/s)(0.05 T)(0.1 m)=500 V consistent with our observations. In addition, test particle simulations using magnetohydrodynamic (MHD) data from SSX simulations and run with dimensionless parameters similar to the experiment (S=1000, β=0.1) show acceleration of ions up to 2vAlf in a few Alfven times consistent with the measurement. The process includes two phases—a strong but short duration direct acceleration in the quasi-ste...

Pickup ion acceleration by turbulent field‐aligned electric fields in the slow low‐latitude solar wind

Abstract: [1] Recent spacecraft observations reveal the continuous presence of energetic tails in solar wind and pickup ion spectra in the quiet, slow low-latitude solar wind. To explain this phenomenon, we take into account growing evidence from observations, theory, and simulations that MHD turbulence in the solar wind has a dominant two-dimensional (2-D) component. Of special significance is that 2-D MHD turbulence simulations feature turbulent field-aligned electric fields due to turbulent convection of 2-D magnetic field fluctuations. Such electric fields have been observed in the low-latitude solar wind. We explore pickup ion propagation and acceleration between the Sun and Earth with a newly developed kinetic numerical model. It includes a random distribution of large-scale field-aligned electric field fluctuations consistent with observations in the low-latitude solar wind. The main result is the qualitative reproduction of observed accelerated interstellar pickup He + spectra at Earth. We find that a spectral "knee" connects the pickup ion core with the accelerated "tail." Like observations, the simulated tail is approximately a power law f(v) ν - 4 . 5 until it reaches particle speeds ∼10 times the solar wind speed, where a rollover occurs. The energetic tail particles originate closer to the Sun, where acceleration is most efficient. The knee forms because the local pickup ion source particles are abundant but weakly accelerated. Further predictions, such as large knees for pickup ion species formed from interstellar neutrals with large ionization cavities, and the absence of knees in accelerated spectra of inner source pickup ions are also discussed.

The interaction of turbulence with shock waves: A basic model

Abstract: The interaction of turbulence and shock waves is considered self-consistently so that the back-reaction of the turbulence and its associated reaction on the turbulence is addressed. This approach differs from previous studies which considered the interaction of linear modes with a shock. The most basic model of hypersonic flow, described by the inviscid form of Burgers’ equation, is used. An energy-containing model which couples the turbulent energy density and correlation length of the flow with the mean flow is developed. Upstream turbulence interacting with a shock wave is found to mediate the shock by (1) increasing the mean shock speed, and (2) decreasing the efficiency of turbulence amplification at the shock as the upstream turbulence energy density is increased. The implication of these results is that the energy in upstream turbulent fluctuations, while being amplified at the shock, is also being converted into mean flow energy downstream. The variance in both the shock speed and position is comp...

Experimental Observation of Energetic Ions Accelerated by Three-dimensional Magnetic Reconnection in a Laboratory Plasma

Abstract: Magnetic reconnection is widely believed responsible for heating the solar corona as well as for generating X-rays and energetic particles in solar flares. On astrophysical scales, reconnection in the intergalactic plasma is a prime candidate for a local source (!100 Mpc) of cosmic rays exceeding the Greisen-Zatsepin-Kuzmin cutoff (∼10 19 eV). In a laboratory astrophysics experiment, we have made the first observation of particles accelerated by magnetic reconnection events to energies significantly above both the thermal and the characteristic magneto- hydrodynamic energies. These particles are correlated temporally and spatially with the formation of three- dimensional magnetic structures in the reconnection region. Subject headings: acceleration of particles — cosmic rays — magnetic fields — methods: laboratory — MHD — plasmas Magnetic reconnection occurs when two bodies of highly conductive plasma bearing oppositely directed embedded mag- netic fields merge (Brown 1999; Priest & Forbes 2000). In the simplest, two-dimensional picture, the interface where the in- flowing magnetofluid stagnates contains a current sheet to sup- port the curl of the magnetic field and an electric field to support the consumption of magnetic flux (see Fig. 1). Within the bulk of each inflow, the motion of the field and fluid are coupled owing to the high conductivity. At the interface, this condition no longer holds, and field lines convected into this region break and reconnect across the layer, producing a global change in field topology. The reconnection outflow, coplanar and trans- verse to the inflow, exits the reconnection region with a speed not exceeding that of a magnetohydrodynamic (MHD) or Alf- ven wave of the coupled fluid and field. In 1/2

Sensitivity of cosmic ray modulation to the correlation length

Abstract: [1] Recent efforts to improve and complete modulation theory are described, emphasizing factors that arise in attempts to understand the diffusion tensor based upon turbulence theory and the theory of charged particle scattering. Direct numerical solutions of the transport equations are used to study sensitivity of cosmic ray modulation to the correlation length. It is shown that modulation is strongly influenced by the radial variation of the correlation length. This variation is still poorly understood, in part owing to the uncertain impact of pickup ion driven turbulence in the outer heliosphere and to practical difficulties in measuring the parallel correlation length in the outer heliosphere.

Mini-conference on plasma turbulence in the corona, heliosphere and interstellar medium

Abstract: This paper provides a summary of some major physics issues discussed in the Mini-conference on plasma turbulence in the corona, heliosphere and interstellar medium. This is one of two Mini-conferences sponsored by the Topical Group on Plasma Astrophysics held as part of the American Physical Society Division of Plasma Physics Fall 2001 Meeting, 30 October–2 November 2001

One-point statistics of the induced electric field in quasinormal magnetofluid turbulence.

Abstract: We study one-point statistical properties of the induced turbulent electric field for a magnetohydrodynamic (MHD) plasma under the quasinormal approximation. Assuming exact Gaussianity for both the velocity field and the magnetic field, and different degrees of correlations between their Cartesian components, we derive the probability distribution function (PDF) for the Cartesian components of the electric field e i . We show that the PDF reduces in some canonical cases to an exponential function of the form exp(-‖e i ‖). To study deviations from these results in the more realistic case in which the velocity and magnetic fields are not exactly normal but quasinormal instead, we perform three-dimensional numerical simulations of the MHD equations at moderate Reynolds numbers. For turbulent relaxation from an initial condition, we find that the analytical results give a very good first-order approximation to the computed PDF.

A Quasi-linear Kinetic Theory for Charged-Particle Transport in Two-dimensional Turbulence

Abstract: There is growing evidence for the presence of a strong two-dimensional MHD turbulence component in the solar wind. The theory for energetic charged-particle transport and energization in such two-dimensional turbulence, however, is not well developed. In particular, the role of turbulent electric fields, generated by the turbulent convective motions of the two-dimensional magnetic field fluctuations, needs to be clarified. We present a new quasi-linear kinetic theory of super-Alfvenic charged-particle transport and energization in quasi-static homogeneous incompressible two-dimensional MHD turbulence, including the associated turbulent electric fields. We find for a nearly gyrotropic particle distribution that field-aligned turbulent electric fields are mainly responsible for causing particle pitch angle scattering and momentum diffusion. The new theory allows for efficient scattering through a 90° pitch angle whereby a long-standing problem in standard magnetostatic quasi-linear theory is addressed anew.