Paulett C. Liewer

Measurements of microturbulence in tokamaks and comparisons with theories of turbulence and anomalous transport

A review of measurements of microscopic fluctuations and theories of turbulence and anomalous transport for tokamaks is given, and some comparisons between theory and experiment are presented. The results of the measurements indicate that all tokamaks have rather similar, broadband, incoherent microscopic fluctuations. Such fluctuations have been measured in the density, potential, electric field, and magnetic field. In the edge regions of three tokamaks, the particle transport caused by the turbulent electric field fluctuations has been measured directly. Although tokamak microturbulence has been studied extensively, neither its source nor its role in anomalous energy transport is yet understood. The incoherent, turbulent nature of the fluctuations has made it difficult to understand them theoretically. Recently, however, significant theoretical progress has been made in several areas including non-linear models of drift wave turbulence and transport, models of anomalous electron thermal conduction by stochastic magnetic field fluctuations, and non-linear models of localized resistive-MHD instabilities.

A general concurrent algorithm for plasma particle-in-cell simulation codes

Abstract We have developed a new algorithm for implementing plasma particle-in-cell (PIC) simulation codes on concurrent processors with distributed memory. This algorithm, named the general concurrent PIC algorithm (GCPIC), has been used to implement an electrostatic PIC code on the 32-node JPL Mark III Hypercube parallel computer. To decompose a PIC code using the GCPIC algorithm, the physical domain of the particle simulation is divided into sub-domains, equal in number to the number of processors, such that all sub-domains have roughly equal numbers of particles. For problems with non-uniform particle densities, these sub-domains will be of unequal physical size. Each processor is assigned a sub-domain and is responsible for updating the particles in its sub-domain. This algorithm has led to a very efficient parallel implementation of a well-benchmarked 1-dimensional PIC code. The dominant portion of the code, updating the particle positions and velocities, is nearly 100% efficient when the number of particles is increased linearly with the number of hypercube processors used so that the number of particles per processor is constant. For example, the increase in time spent updating particles in going from a problem with 11,264 particles run on 1 processor to 360,448 particles on 32 processors was only 3% (parallel efficiency of 97%). Although implemented on a hypercube concurrent computer, this algorithm should also be efficient for PIC codes on other parallel architectures and for large PIC codes on sequential computers where part of the data must reside on external disks.

CONNECTING SPEEDS, DIRECTIONS AND ARRIVAL TIMES OF 22 CORONAL MASS EJECTIONS FROM THE SUN TO 1 AU

Forecasting the in situ properties of coronal mass ejections (CMEs) from remote images is expected to strongly enhance predictions of space weather and is of general interest for studying the interaction of CMEs with planetary environments. We study the feasibility of using a single heliospheric imager (HI) instrument, imaging the solar wind density from the Sun to 1 AU, for connecting remote images to in situ observations of CMEs. We compare the predictions of speed and arrival time for 22 CMEs (in 2008-2012) to the corresponding interplanetary coronal mass ejection (ICME) parameters at in situ observatories (STEREO PLASTIC/IMPACT, Wind SWE/MFI). The list consists of front-and backsided, slow and fast CMEs (up to 2700 km s(-1)). We track the CMEs to 34.9 +/- 7.1 deg elongation from the Sun with J maps constructed using the SATPLOT tool, resulting in prediction lead times of - 26.4 +/- 15.3 hr. The geometrical models we use assume different CME front shapes (fixed-Phi, harmonic mean, self-similar expansion) and constant CME speed and direction. We find no significant superiority in the predictive capability of any of the three methods. The absolute difference between predicted and observed ICME arrival times is 8.1 +/- 6.3 hr (rms value of 10.9 hr). Speeds are consistent to within 284 +/- 288 km s(-1) . Empirical corrections to the predictions enhance their performance for the arrival times to 6.1 +/- 5.0 hr (rms value of 7.9 hr), and for the speeds to 53 +/- 50 km s(-1). These results are important for Solar Orbiter and a space weather mission positioned away from the Sun-Earth line.

The effects of a local interstellar magnetic field on voyager 1 and 2 observations

We show that an interstellar magnetic field can produce a north-south asymmetry in the solar wind termination shock. Using Voyager 1 and 2 measurements, we suggest that the angle α between the interstellar wind velocity and the magnetic field is 30° < α < 60°. The distortion of the shock is such that termination shock particles could have streamed outward along the spiral interplanetary magnetic field connecting Voyager 1 to the shock when the spacecraft was within ~2 AU of the shock. The shock distortion is larger in the southern hemisphere, and Voyager 2 could be connected to the shock when it is within ~5 AU of the shock, but with particles from the shock streaming inward along the field. Tighter constraints on the interstellar magnetic field should be possible when Voyager 2 crosses the shock in the next several years.

Alfvén wave propagation and ion cyclotron interactions in the expanding solar wind: One‐dimensional hybrid simulations

We carry out one-dimensional hybrid simulations of Alfven waves propagating along the magnetic field in the presence of a mean radial spherically expanding plasma outflow, representing fast solar wind streams. The equations for particle ions of multiple species and fluid electrons are solved using the Expanding Box Model, a locally Cartesian representation of motion in spherical coordinates, in a frame moving with the local average wind speed. The model gives a minimally consistent description of the effects associated with such motion on particle dynamics, e.g., the flux-conserving decrease of magnetic field intensity and consequent decrease of cyclotron frequency with increasing distance from the Sun. The cyclotron frequency decreases faster than Alfven wave frequency, allowing fluctuations below the cyclotron frequency at smaller distance from the Sun to come into cyclotron resonance at greater distances. The hybrid treatment yields a fully self-consistent description of the consequent cyclotron wave-particle interaction in a multi-ion plasma. We present results for cases of monochromatic circularly polarized Alfven waves propagating radially outward and for initially well developed Alfvenic spectra with and without alpha particles. When both alpha particles and protons are present, the alpha particles, which come into resonance first as the wind expands, are observed to be preferentially heated and accelerated. For high beta (equal to ratio of ion pressure to magnetic field pressure) the amount of alpha particles acceleration and heating is limited by the available wave power. For low beta cases the amount of heating and acceleration is limited, not by the wave power, but by the depletion of the distribution function in the resonance region by pitch-angle scattering. The implication of these results for solar wind models is discussed.

Motion of the termination shock in response to an 11 Year variation in the solar wind

A two-dimensional hydrodynamic numerical model has been used to study the motion of the termination shock in response to an 11 year variation in the solar wind ram pressure. We find that for a total variation in the ram pressure by a factor of 2, a termination shock at 89 AU moves inward and outward about ±8% of its distance with a typical velocity of 12 km/sec. This movement may be understood in terms of the various time scales associated with the response of the termination shock and heliopause to variations in the solar wind ram pressure.

Hydrodynamic instability of the heliopause driven by plasma-neutral charge-exchange interactions

Results from time-dependent two-dimensional hydrodynamic simulations of the global heliosphere suggest that drag between the plasma ions and the interstellar neutrals, caused by charge-exchange collisions, may cause the heliopause to be hydrodynamically unstable. Both ions and neutrals are treated as fluids coupled by charge-exchange collisions. The neutral-ion drag is proportional to the plasma density and introduces an effective gravity in the direction of the neutral flow, which, because the interstellar plasma is much denser than the heliosheath plasma, causes a Rayleigh-Taylor-like instability to develop. The heliopause is unstable only near the stagnation point at the “nose” of the heliosphere. In the simulations, the heliopause is seen to oscillate nonlinearly about its equilibrium position with a timescale of the order of a hundred years and amplitudes of tens of AUs. Growth rates from the simulations are in reasonable agreement with theoretical estimates. The possible stabilizing influence of energetic solar wind neutrals, neglected in the present model, is discussed. Implications of this instability on the interpretation of the Voyager 2–3 kHz emissions are also discussed.

3D Electromagnetic Plasma Particle Simulations on a MIMD Parallel Computer

Abstract A three-dimensional electromagnetic PIC code has been developed on the 512 node Intel Touchstone Delta MIMD parallel computer. This code uses a standard relativistic leapfrog scheme to push particles and a local finite-difference time-domain method to update the electromagnetic fields. The code is implemented using the General Concurrent PIC algorithm which uses a domain decomposition to divide the computation among the processors. The 3D simulation domain can be partitioned into 1-, 2-, or 3-dimensional subdomains. Particles must be exchanged between processors as they move among the subdomains. The Intel Delta allows one to use this code for very-large-scale simulations (i.e. over 10 8 particles and 10 6 grid cells). The parallel efficiency of this code is measured, and the overall code performance on the Delta is compared with that on Cray supercomputers. It is shown that our code runs with a high parallel efficiency of ≥ 95% for large size problems. The particle push time achieved is 115 ns/particle/time step for 162 million particles on 512 nodes. Compared with the performance on a single CPU Cray C90, this represents a factor of 58 speedup. It is also shown that the finite-difference method for the field solve is significantly more efficient than transform methods on parallel computers. The field solve time is

MAGNETOSPHERIC AND PLASMA SCIENCE WITH CASSINI-HUYGENS

Magnetospheric and plasma science studies at Saturn offer a unique opportunity to explore in-depth two types of magnetospheres. These are an ‘induced’ magnetosphere generated by the interaction of Titan with the surrounding plasma flow and Saturns ‘intrinsic’ magnetosphere, the magnetic cavity Saturns planetary magnetic field creates inside the solar wind flow. These two objects will be explored using the most advanced and diverse package of instruments for the analysis of plasmas, energetic particles and fields ever flown to a planet. These instruments will make it possible to address and solve a series of key scientific questions concerning the interaction of these two magnetospheres with their environment.The flow of magnetospheric plasma around the obstacle, caused by Titans atmosphere/ionosphere, produces an elongated cavity and wake, which we call an ‘induced magnetosphere’. The Mach number characteristics of this interaction make it unique in the solar system. We first describe Titans ionosphere, which is the obstacle to the external plasma flow. We then study Titans induced magnetosphere, its structure, dynamics and variability, and discuss the possible existence of a small intrinsic magnetic field of Titan.Saturns magnetosphere, which is dynamically and chemically coupled to all other components of Saturns environment in addition to Titan, is then described. We start with a summary of the morphology of magnetospheric plasma and fields. Then we discuss what we know of the magnetospheric interactions in each region. Beginning with the innermost regions and moving outwards, we first describe the region of the main rings and their connection to the low-latitude ionosphere. Next the icy satellites, which develop specific magnetospheric interactions, are imbedded in a relatively dense neutral gas cloud which also overlaps the spatial extent of the diffuse E ring. This region constitutes a very interesting case of direct and mutual coupling between dust, neutral gas and plasma populations. Beyond about twelve Saturn radii is the outer magnetosphere, where the dynamics is dominated by its coupling with the solar wind and a large hydrogen torus. It is a region of intense coupling between the magnetosphere and Saturns upper atmosphere, and the source of Saturns auroral emissions, including the kilometric radiation. For each of these regions we identify the key scientific questions and propose an investigation strategy to address them.Finally, we show how the unique characteristics of the CASSINI spacecraft, instruments and mission profile make it possible to address, and hopefully solve, many of these questions. While the CASSINI orbital tour gives access to most, if not all, of the regions that need to be explored, the unique capabilities of the MAPS instrument suite make it possible to define an efficient strategy in which in situ measurements and remote sensing observations complement each other.Saturns magnetosphere will be extensively studied from the microphysical to the global scale over the four years of the mission. All phases present in this unique environment — extended solid surfaces, dust and gas clouds, plasma and energetic particles — are coupled in an intricate way, very much as they are in planetary formation environments. This is one of the most interesting aspects of Magnetospheric and Plasma Science studies at Saturn. It provides us with a unique opportunity to conduct an in situ investigation of a dynamical system that is in some ways analogous to the dusty plasma environments in which planetary systems form.

Alfven wave heating of heavy ions in the expanding solar wind: Hybrid simulations

We present hybrid expanding box simulations of the interaction of left-handed Alfven waves with protons, alpha particles and a tenuous population of oxygen O 5+ . The Alfven waves are initially nonresonant with the ions and the expansion bring them to the cyclotron resonance with O 5+ ions, then with alpha particles, and finally with protons. Th e simulations show that O 5+ ions are efficiently heated in the directions perpendicular to the background mag- netic field, but are only slightly accelerated. Oxygen scatt ering has a finite time span and sat- urates mainly due to the marginal stabilization with respec t to the oxygen cyclotron instabil- ity generated by the temperature anisotropy. During the sca ttering oxygen ions are able to ab- sorb only a limited amount of available fluctuating energy an d, for the parameters used in the simulations, their presence has a minimum influence on alpha particles and protons.