American Cities in Post- Apocalyptic Science Fiction

Visions of the American city in post-apocalyptic ruin permeate literary and popular fiction, across print, visual, audio and digital media. American Cities in Post-Apocalyptic Science Fiction explores the prevalence of these representations in American culture, drawing from a wide range of primary and critical works from the early-twentieth century to today. Beginning with science fiction in literary magazines, before taking in radio dramas, film, video games and expansive transmedia franchises, Robert Yeates argues that post-apocalyptic representations of the American city are uniquely suited for explorations of contemporary urban issues. Examining how the post-apocalyptic American city has been repeatedly adapted and repurposed to new and developing media over the last century, this book reveals that the content and form of such texts work together to create vivid and immersive fictional spaces in ways that would otherwise not be possible. Chapters present media-specific analyses of these texts, situating them within their historical contexts and the broader history of representations of urban ruins in American fiction. Original in its scope and cross-media approach, American Cities in Post-Apocalyptic Science Fiction both illuminates little-studied texts and provides provocative new readings of familiar works such as Blade Runner and The Walking Dead, placing them within the larger historical context of imaginings of the American city in ruins

Introduction to Field Line Helicity

Field line helicity measures the net linking of magnetic flux with a single magnetic field line. It offers a finer topological description than the usual global magnetic helicity integral, while still being invariant in an ideal evolution unless there is a flux of helicity through the domain boundary. In this chapter, we explore how to appropriately define field line helicity in different volumes in a way that preserves a meaningful topological interpretation. We also review the time evolution of field line helicity under both boundary motions and magnetic reconnection.Comment: 23 pages, 8 figures, to appear as chapter of AGU book "Helicities in Geophysics, Astrophysics and Beyond", published by Wiley, ISBN 111984168

Intrinsic winding of braided vector fields in tubular subdomains

Braided vector fields on spatial subdomains which are homeomorphic to the cylinder play a crucial role in applications such as solar and plasma physics, relativistic astrophysics, fluid and vortex dynamics, elasticity, and bio-elasticity. Often the vector field's topology—the entanglement of its field lines—is non-trivial, and can play a significant role in the vector field's evolution. We present a complete topological characterisation of such vector fields (up to isotopy) using a quantity called field line winding. This measures the entanglement of each field line with all other field lines of the vector field, and may be defined for an arbitrary tubular subdomain by prescribing a minimally distorted coordinate system. We propose how to define such coordinates, and prove that the resulting field line winding distribution uniquely classifies the topology of a braided vector field. The field line winding is similar to the field line helicity considered previously for magnetic (solenoidal) fields, but is a more fundamental measure of the field line topology because it does not conflate linking information with field strength

Optimal unstirred state of a passive scalar

This work was supported by Leverhulme Trust grant PRG-2017-169.Given a passive tracer distribution, what is the simplest unstirred pattern that may be reached under incompressible advection? This question is partially motivated by recent studies of three-dimensional (3-D) magnetic reconnection, in which the patterns of a topological invariant called the field line helicity greatly simplify until reaching a relaxed state. We test two approaches: a variational method with minimal constraints, and a magnetic relaxation scheme where the velocity is determined explicitly by the pattern of. Both methods achieve similar convergence for simple test cases. However, the magnetic relaxation method guarantees a monotonic decrease in the Dirichlet seminorm of, and is numerically more robust. We therefore apply the latter method to two complex mixed patterns modelled on the field line helicity of 3-D magnetic braids. The unstirring separates into a small number of large-scale regions determined by the initial topology, which is well preserved during the computation. Interestingly, the velocity field is found to have the same large-scale topology as. Similarity to the simplification found empirically in 3-D magnetic reconnection simulations supports the idea that advection is an important principle for field line helicity evolution.Publisher PDFPeer reviewe

Evolution of field line helicity in magnetic relaxation

This work was facilitated by Leverhulme Trust under Grant No. PRG-2017–169, with additional support from Science and Technology Facilities Council (UK) under consortium Grants Nos. ST/N000714, ST/N000781, and ST/S000321.Plasma relaxation in the presence of an initially braided magnetic field can lead to self-organization into relaxed states that retain non-trivial magnetic structure. These relaxed states may be in conflict with the linear force-free fields predicted by the classical Taylor theory, and remain to be fully understood. Here, we study how the individual field line helicities evolve during such a relaxation, and show that they provide new insights into the relaxation process. The line helicities are computed for numerical resistive-magnetohydrodynamic simulations of a relaxing braided magnetic field with line-tied boundary conditions, where the relaxed state is known to be non-Taylor. First, our computations confirm recent analytical predictions that line helicity will be predominantly redistributed within the domain, rather than annihilated. Second, we show that self-organization into a relaxed state with two discrete flux tubes may be predicted from the initial line helicity distribution. Third, for this set of line-tied simulations we observe that the sub-structure within each of the final tubes is a state of uniform line helicity. This uniformization of line helicity is consistent with Taylor theory applied to each tube individually. However, it is striking that the line helicity becomes significantly more uniform than the force-free parameter.Publisher PDFPeer reviewe

Phosphorus and sulphur interactions, 1978.

Yield results, 78AL3, 78KE4, 78M08, 78BY3, 78BA8, 78BU3, 78N04, 78A7, 78C4, 78B4, 78MA2. Soil sampling data