Search

Particle transport, acceleration and energisation are phenomena of major importance for both space and laboratory plasmas. Despite years of study, an accurate theoretical description of these effects is still lacking. Validating models with self-consistent, kinetic simulations represents today a new challenge for the description of weakly-collisional, turbulent plasmas. We perform two-dimensional (2D) hybrid-PIC simulations of steady-state turbulence to study the processes of diffusion and acceleration. The chosen plasma parameters allow to span different systems, going from the solar corona to the solar wind, from the Earth's magnetosheath to confinement devices. To describe the ion diffusion, we adapted the Nonlinear Guiding Center (NLGC) theory to the 2D case. Finally, we investigated the local influence of coherent structures on particle energisation and acceleration: current sheets play an important role if the ions Larmor radii are on the order of the current sheets size. This resonance-like process leads to the violation of the magnetic moment conservation, eventually enhancing the velocity-space diffusion., Comment: 24 pages, 16 figures

Electron-scale magnetic depressions in the terrestrial plasma sheet are studied using Cluster multispacecraft data. The structures, which have an observed duration of ~5–10 s, are approximately 200–300 km wide in the direction of propagation, and they show an average reduction in the background magnetic field of 10–20%. A majority of the events are also associated with an increase in the high-energy high pitch angle electron flux, which indicates that the depressions are presumably generated by electrons with relatively high velocity perpendicular to the background magnetic field. Differences in the recorded electron spectra in the four spacecraft indicates a possible nongyrotropic structure. Multispacecraft measurements show that a subset of events are cylindrical, elongated along the magnetic field, and with a field-parallel scale size of at a minimum 500 km. Other events seem to be better described as electron-scale sheets, about 200–300 km thick. We find that no single formation mechanism can explain this variety of events observed. Instead, several processes may be operating in the plasma sheet, giving rise to similar magnetic field structures in the single-spacecraft data, but with different 3-D structuring. The cylindrical structures have several traits that are in agreement with the electron vortex magnetic holes observed in 2-D particle-in-cell simulations of turbulent relaxation, whereas the sheets, which show nearly identical signatures in the multispacecraft data, are better explained by propagating electron solitary waves.

Particle dynamics are investigated in plasma turbulence, using self-consistent kinetic simulations, in two dimensions. In the steady state, the trajectories of single protons and proton pairs are studied, at different values of plasma $\ensuremath{\beta}$ (ratio between kinetic and magnetic pressure). For single-particle displacements, results are consistent with fluids and magnetic field line dynamics, where particles undergo normal diffusion for very long times, with higher $\ensuremath{\beta}$'s being more diffusive. In an intermediate time range, with separations lying in the inertial range, particles experience an explosive dispersion in time, consistent with the Richardson prediction. These results, obtained for the first time with a self-consistent kinetic model, are relevant for astrophysical and laboratory plasmas, where turbulence is crucial for heating, mixing, and acceleration processes.

Collisionless shocks are efficient particle accelerators. At Earth, ions with energies exceeding 100 keV are seen upstream of the bow shock when the magnetic geometry is quasi-parallel, and large-scale supernova remnant shocks can accelerate ions into cosmic rays energies. This energization is attributed to diffusive shock acceleration, however, for this process to become active the ions must first be sufficiently energized. How and where this initial acceleration takes place has been one of the key unresolved issues in shock acceleration theory. Using Cluster spacecraft observations, we study the signatures of ion reflection events in the turbulent transition layer upstream of the terrestrial bow shock, and with the support of a hybrid simulation of the shock, we show that these reflection signatures are characteristic of the first step in the ion injection process. These reflection events develop in particular in the region where the trailing edge of large-amplitude upstream waves intercept the local shock ramp and the upstream magnetic field changes from quasi-perpendicular to quasi-parallel. The dispersed ion velocity signature observed can be attributed to a rapid succession of ion reflections at this wave boundary. After the ions' initial interaction with the shock, they flow upstream along the quasi-parallel magnetic field. Each subsequent wave front in the upstream region will sweep the ions back toward the shock, where they gain energy with each transition between the upstream and the shock wave frames. Within three to five gyroperiods, some ions have gained enough parallel velocity to escape upstream, thus completing the injection process., 30 pages, 10 figures. Accepted for publication in The Astrophysical Journal

The report gives a defining description of the programming language Scheme. Scheme is a statically scoped and properly tail-recursive dialect of the Lisp programming language invented by Guy Lewis Steele, Jr. and Gerald Jay Sussman. It was designed to have an exceptionally clear and simple semantics and few different ways to form expressions. A wide variety of programming paradigms, including imperative, functional, and message passing styles, find convenient expression in Scheme. The introduction offers a brief history of the language and of the report. The first three chapters present the fundamental ideas of the language and describe the notational conventions used for describing the language and for writing programs in the language. Sections 5 and 6 describe the syntax and semantics of expressions, programs, and definitions. Section 7 describes Scheme‘s built-in procedures, which include all of the language‘s data manipulation and input/output primitives. Section 8 provides a formal syntax for Scheme written in extended BNF, along with a formal denotational semantics. An example of the use of the language follows the formal syntax and semantics. The report concludes with a list of references and an alphabetic index and is followed by a short list of clarifications and corrections.

Traditional compilation courses use formal methods for parsing, but treat the more important semantic aspects informally. We present a one semester course in which compiler development is reduced to a number of transformation steps, each of which is formally specified, easily tested, and clearly motivated by semantic considerations. The source language is substantial (essentially the host language of the compiler) and the target is a popular RISC architecture.

Turbulent behavior at sub-proton scales in magnetized plasmas is important for a full understanding of the energetics of astrophysical flows such as the solar wind. We study the formation of electron temperature anisotropy due to reconnection in the turbulent decay of sub-proton scale fluctuations using two dimensional, particle-in-cell (PIC) plasma simulations with realistic electron-proton mass ratio and a guide field out of the simulation plane. A fluctuation power spectrum with approximately power law form is created down to scales of order the electron gyroradius. In the dynamic magnetic field topology, which gradually relaxes in complexity, we identify the signatures of collisionless reconnection at sites of X-point field geometry. The reconnection sites are generally associated with regions of strong parallel electron temperature anisotropy. The evolving topology of magnetic field lines connected to a reconnection site allows spatial mixing of electrons accelerated at multiple, spatially separated reconnection regions. This leads to the formation of multi-peaked velocity distribution functions with a strong parallel temperature anisotropy. In a three-dimensional system, supporting the appropriate wave vectors, the multi-peaked distribution functions would be expected to be unstable to kinetic instabilities, contributing to dissipation. The proposed mechanism of anisotropy formation is also relevant to space and astrophysical systems where the evolution of the plasma is constrained by linear temperature anisotropy instability thresholds. The presence of reconnection sites leads to electron energy gain, nonlocal velocity space mixing and the formation of strong temperature anisotropy; this is evidence of an important role for reconnection in the dissipation of turbulent fluctuations., Comment: 27 pages, 14 figures. Accepted for publication in ApJ, Jan 3, 2014

The report gives a defining description of the programming language Scheme. Scheme is a statically scoped and properly tail-recursive dialect of the Lisp programming language invented by Guy Lewis Steele Jr. and Gerald Jay Sussman. It was designed to have an exceptionally clear and simple semantics and few different ways to form expressions. A wide variety of programming paradigms, including imperative, functional, and message passing styles, find convenient expression in Scheme.

This is a brief report of efforts to standardize the Scheme programming language. Scheme inherits Lisp's rich set of symbol manipulation primitives, latent storage allocation, dynamic type checking, and simple syntax. Scheme is distinguished from most Lisp dialects by a single variable environment, block structure with static scope, and uniform evaluation of the operator and operand positions of a procedure call. Since there is no storage penalty for tail-recursive procedure calls, they may be used to express iteration. Provision is made for a rich set of numerical types, and exact and inexact numbers are distinguished. The ability to create first-class escape procedures allows almost all known forms of sequential control to be expressed. Above all, Scheme achieves its expressive power through the simplicity and generality of its design, and not by the accumulation of features. (The draft standard is about 50 pages long.) The reader may wish to consult books by Abelson and Sussman [1], Springer and Friedman [2], and Dybvig [3], among others, for tutorial introductions to Scheme.

We report the properties of a novel type of sub-proton scale magnetic hole found in two dimensional PIC simulations of decaying turbulence with a guide field. The simulations were performed with a realistic value for ion to electron mass ratio. These structures, electron vortex magnetic holes (EVMHs), have circular cross-section. The magnetic field depression is associated with a diamagnetic azimuthal current provided by a population of trapped electrons in petal-like orbits. The trapped electron population provides a mean azimuthal velocity and since trapping preferentially selects high pitch angles, a perpendicular temperature anisotropy. The structures arise out of initial perturbations in the course of the turbulent evolution of the plasma, and are stable over at least 100 electron gyroperiods. We have verified the model for the EVMH by carrying out test particle and PIC simulations of isolated structures in a uniform plasma. It is found that (quasi-)stable structures can be formed provided that there is some initial perpendicular temperature anisotropy at the structure location. The properties of these structures (scale size, trapped population etc.) are able to explain the observed properties of magnetic holes in the terrestrial plasma sheet. EVMHs may also contribute to turbulence properties, such as intermittency, at short scale lengths in other astrophysical plasmas., Accepted by Physics of Plasmas

We propose a practical path-based framework for deriving and simplifying source-tracking information for term unification in the empty theory. Such a framework is useful for debugging unification-based systems, including the diagnosis of ill-typed programs and the generation of success and failure proofs in logic programming.

Computer programming languages (and their dominance as the means of software development) have been one of the most high-profile topics in the 50-year history of computing --- reaching mature technology and achievements for language definition, translation, usage, analysis, pedagogical approaches, etc. and in terms of their general impact on computing systems This reality has continuously driven the content of Computer Science curriculums to feature programming languages, their definition techniques, and their translation methods as the subject of many core courses, and their usage as a tool to carry out the teaching of the principles in most other courses. The objective of this panel is to, at just past the mid-point of the first century of computing, debate whether this pre-eminent role of programming languages will continue straightforwardly in the second half-century, if radically new styles of programming languages will emerge and dominate, or if programming languages will recede to minor importance compared to alternate software-development paradigms in both usage and teaching impact. And, for each of those propositions, to assess the impacts for change in typical CS curriculums.For most software domains today, so-called 3rd-generation programming languages (e.g., Ada, Scheme, Pascal, C/C++, Modula 2, COBOL, FORTRAN, and recently, Java) enjoy considerable portability between computers and are now widely accepted. In these "procedural" languages programmers still are responsible for designing and coding the software architectures, algorithms, logic, and most data structures to solve the problems. For some well-understood domains (e.g., report writing, inventory control, and some financial programming), 4th-generation languages (4GLs) or "non-procedural" languages are established which essentially allow users to specify "requirements" instead of software solutions, with 4GL compilers that generate code satisfying the requirements; the number of domains for which 4GLs are appropriate increases steadily as domain understanding increases. 5th-generation or automatic programming approaches have been a research topic for many years, with some promising results demonstrated in constrained domains, although the distinction from a 4GL to a 5th-generation language (5GL) is less clear than the earlier transitions. Technologies for formally representing and translating programming languages, and the concepts and methods for teaching their effective application, have continually evolved with the programming language trends, and are generally regarded in the body of crystallized core CS concepts.

An ML-style type system with variable-arity procedures is defined that supports both optional arguments and arbitrarily-long argument sequences. A language with variable-arity procedures is encoded in a core-ML variant with infinitary tuples. We present an algebra of infinitary tuples and solve its unification problem. The resulting type discipline preserves principal typings and has a terminating type reconstruction algorithm. The expressive power of infinitary tuples is illustrated.

Continuations, when made available to the programmer as first class objects, provide a general control abstraction for sequential computation. The power of first class continuations is demonstrated by implementing a variety of coroutine mechanisms using only continuations and functional abstraction. The importance of general abstraction mechanisms such as continuations is discussed.

The traditional Lisp macro expansion facility inhibits several important forms of expansion control. These include selective expansion of subexpressions, expansion of subexpressions using modified expansion functions, and expansion of application and variable expressions. Furthermore, the expansion algorithm must treat every special form as a separate case. The result is limited expressive power and poor modularity. We propose an alternative facility that avoids these problems, using a technique called expansion-passing style (EPS). The critical dierence between the facility proposed here and the traditional macro mechanism is that expansion functions are passed not only an expression to be expanded but also another expansion function. This function may or may not be used to perform further expansion. The power of this technique is illustrated with several examples. Most Lisp systems may be adapted to employ this technique.

The need for a programming language abstraction for timed preemption is argued, and several possibilities for such an abstraction are presented. One, called engines, is adopted. Engines are an abstraction of bounded computation, not a process abstraction in the usual sense. However, in conjuction with first class continuations, engines allow a language to be extended with time-sharing implementations for a variety of process abstraction facilities. We present a direct implementation of hiaton streams. Engine nesting refers to the initiation of an engine computation by an already running engine. We consider the need for engine nesting and show how it may be accomplished in a manner that charges a parent engine for the computation of its offspring. We conclude by discussing the importance of simple and general abstractions such as engines.

Continuations, when available as first-class objects, provide a general control abstraction in programming languages. They liberate the programmer from specific control structures, increasing programming language extensibility. Such continuations may be extended by embedding them in procedural objects. This technique is first used to restore a fluid environment when a continuation object is invoked. We then consider techniques for constraining the power of continuations in the interest of security and efficiency. Domain mechanisms, which create dynamic barriers for enclosing control, are implemented using fluids. Domains are then used to implement an unwind-protect facility in the presence of first-class continuations. Finally, we present two mechanisms, wind-unwind and dynamic-wind, that generalize unwind-protect.

We propose a path-based framework for deriving and simplifying source-tracking information for first-order term unification in the empty theory. Such a framework is useful for diagnosing unification-based systems, including debugging of type errors in programs and the generation of success and failure proofs in logic programming. The objects of source-tracking are deductions in the logic of term unification. The semantics of deductions are paths over a unification graph whose labels form the suffix language of a semi-Dyck set. Based on this idea of unification paths, two algorithms for generating proofs are presented: the first uses context-free labeled shortest-path algorithms to generate optimal (shortest) proofs in time O(n3) for a fixed signature, where n is the number of vertices of the unification graph. The second algorithm integrates easily with standard unification algorithms, entailing an overhead of only a constant factor, but generates non-optimal proofs. These non-optimal proofs may be further simplified by group rewrite rules.

Progress in programming language design has often been achieved by making an abstraction a “first class object”, one that can be passed to and returned from procedures and entered in data structures. For example, the importance of functional parameters has long been recognized, though it is only more recently that actor semantics [2] and object oriented programming have demonstrated the power of first class functional objects. This paper illustrates, with a novel example, the power of first class control objects, called continuations.

Continuations, when available as first-class objects, provide a general control abstraction in programming languages. They liberate the programmer from specific control structures, increasing programming language extensibility. Such continuations may be extended by embedding them in functional objects. This technique is first used to restore a fluid environment when a continuation object is invoked. We then consider techniques for constraining the power of continuations in the interest of security and efficiency. Domain mechanisms, which create dynamic barriers for enclosing control, are implemented using fluids. Domains are then used to implement an unwind-protect facility in the presence of first-class continuations. Finally, we demonstrate two mechanisms, wind-unwind and dynamic-wind, that generalize unwind-protect.

Males (n = 12, M age 25.6 yrs) with clear experiences (CE) of both "pure consciousness" (unbounded inner awareness without thoughts) and the TM--Sidhi techniques (mental procedures derived from Vedic Science to enhance cognitive, perceptual and other abilities) were compared with unclear experience (UE) males (n = 10, M age 25.5 yrs) on creativity (ideational fluency, Torrance, Novel Uses, Verbal) and EEG coherence, a measure of phase stability derived from Fourier series analysis. Multivariate analysis of variance of coherence between four pairs of EEG derivations (F3F4, F3C3, F4C4, C3C4) and a t-test on the creativity variable showed that CE subjects had higher alpha (8--12 Hz) coherence (p = 0.052) and higher creativity (p = 0.011) than UE subjects. Several of the alpha coherence variables were positively correlated with creativity: Bilateral Frontal (F3F4), r = 0.65, p = 0.001; Homolateral Right, r = 0.50, p = 0 .011; mean of the four alpha variables, r = 0.66, p = 0.001; Dominant Alpha (area of highest alpha coherence for each subject) r = 0.64, p = 0.001. It is concluded that: (1) information processing, at least to the extent measured by ideational fluency, is enhanced in those with clear experiences of pure consciousness and the TM--Sidhi techniques, (2) that EEG coherence is a psychophysiological correlate of this subject variable and (3) that the results may be generalizable to the field of information processing and "peak experiences" described in Humanistic psychology.