Reconstructing Holocene geomagnetic field variation: new methods, models and implications

Reconstructions of the Holocene geomagnetic field and how it varies on millennial timescales are important for understanding processes in the core but may also be used to study long-term solar-terrestrial relationships and as relative dating tools for geological and archaeological archives. Here, we present a new family of spherical harmonic geomagnetic field models spanning the past 9000 yr based on magnetic field directions and intensity stored in archaeological artefacts, igneous rocks and sediment records. A new modelling strategy introduces alternative data treatments with a focus on extracting more information from sedimentary data. To reduce the influence of a few individual records all sedimentary data are resampled in 50-yr bins, which also means that more weight is given to archaeomagnetic data during the inversion. The sedimentary declination data are treated as relative values and adjusted iteratively based on prior information. Finally, an alternative way of treating the sediment data chronologies has enabled us to both assess the likely range of age uncertainties, often up to and possibly exceeding 500 yr and adjust the timescale of each record based on comparisons with predictions from a preliminary model. As a result of the data adjustments, power has been shifted from quadrupole and octupole to higher degrees compared with previous Holocene geomagnetic field models. We find evidence for dominantly westward drift of northern high latitude high intensity flux patches at the core mantle boundary for the last 4000 yr. The new models also show intermittent occurrence of reversed flux at the edge of or inside the inner core tangent cylinder, possibly originating from the equator

The Low Quiescent X-Ray Luminosity of the Transient X-Ray Burster EXO 1747-214

We report on X-ray and optical observations of the X-ray burster EXO 1747-214. This source is an X-ray transient, and its only known outburst was observed in 1984-1985 by the EXOSAT satellite. We re-analyzed the EXOSAT data to derive the source position, column density, and a distance upper limit using its peak X-ray burst flux. We observed the EXO 1747-214 field in 2003 July with the Chandra X-ray Observatory to search for the quiescent counterpart. We found one possible candidate just outside the EXOSAT error circle, but we cannot rule out the possibility that the source is unrelated to EXO 1747-214. Our conclusion is that the upper limit on the unabsorbed 0.3-8 keV luminosity is L < 7E31 erg/s, making EXO 1747-214 one of the faintest neutron star transients in quiescence. We compare this luminosity upper limit to the quiescent luminosities of 19 neutron star and 14 black hole systems and discuss the results in the context of the differences between neutron stars and black holes. Based on the theory of deep crustal heating by Brown and coworkers, the luminosity implies an outburst recurrence time of >1300 yr unless some form of enhanced cooling occurs within the neutron star. The position of the possible X-ray counterpart is consistent with three blended optical/IR sources with R-magnitudes between 19.4 and 19.8 and J-magnitudes between 17.2 and 17.6. One of these sources could be the quiescent optical/IR counterpart of EXO 1747-214.Comment: 7 pages, accepted by the Astrophysical Journa

Ground-State and Domain-Wall Energies in the Spin-Glass Region of the 2D ±J\pm J Random-Bond Ising Model

The statistics of the ground-state and domain-wall energies for the two-dimensional random-bond Ising model on square lattices with independent, identically distributed bonds of probability $p$ of $J_{ij}= -1$ and $(1-p)$ of $J_{ij}= +1$ are studied. We are able to consider large samples of up to $320^2$ spins by using sophisticated matching algorithms. We study $L \times L$ systems, but we also consider $L \times M$ samples, for different aspect ratios $R = L / M$. We find that the scaling behavior of the ground-state energy and its sample-to-sample fluctuations inside the spin-glass region ($p_c \le p \le 1 - p_c$) are characterized by simple scaling functions. In particular, the fluctuations exhibit a cusp-like singularity at $p_c$. Inside the spin-glass region the average domain-wall energy converges to a finite nonzero value as the sample size becomes infinite, holding $R$ fixed. Here, large finite-size effects are visible, which can be explained for all $p$ by a single exponent $\omega\approx 2/3$, provided higher-order corrections to scaling are included. Finally, we confirm the validity of aspect-ratio scaling for $R \to 0$: the distribution of the domain-wall energies converges to a Gaussian for $R \to 0$, although the domain walls of neighboring subsystems of size $L \times L$ are not independent.Comment: 11 pages with 15 figures, extensively revise

Seeing motion and apparent motion

In apparent motion experiments, participants are presented with what is in fact a succession of two brief stationary stimuli at two different locations, but they report an impression of movement. Philosophers have recently debated whether apparent motion provides evidence in favour of a particular account of the nature of temporal experience. I argue that the existing discussion in this area is premised on a mistaken view of the phenomenology of apparent motion and, as a result, the space of possible philosophical positions has not yet been fully explored. In particular, I argue that the existence of apparent motion is compatible with an account of the nature of temporal experience that involves a version of direct realism. In doing so, I also argue against two other claims often made about apparent motion, viz. that apparent motion is the psychological phenomenon that underlies motion experience in the cinema, and that apparent motion is subjectively indistinguishable from real motion

A note on anti-coordination and social interactions

This note confirms a conjecture of [Bramoull\'{e}, Anti-coordination and social interactions, Games and Economic Behavior, 58, 2007: 30-49]. The problem, which we name the maximum independent cut problem, is a restricted version of the MAX-CUT problem, requiring one side of the cut to be an independent set. We show that the maximum independent cut problem does not admit any polynomial time algorithm with approximation ratio better than $n^{1-\epsilon}$, where $n$ is the number of nodes, and $\epsilon$ arbitrarily small, unless P=NP. For the rather special case where each node has a degree of at most four, the problem is still MAXSNP-hard.Comment: 7 page

Targeting pericytes for therapeutic approaches to neurological disorders

Many central nervous system diseases currently lack effective treatment and are often associated with defects in microvascular function, including a failure to match the energy supplied by the blood to the energy used on neuronal computation, or a breakdown of the blood-brain barrier. Pericytes, an under-studied cell type located on capillaries, are of crucial importance in regulating diverse microvascular functions, such as angiogenesis, the blood-brain barrier, capillary blood flow and the movement of immune cells into the brain. They also form part of the "glial" scar isolating damaged parts of the CNS, and may have stem cell-like properties. Recent studies have suggested that pericytes play a crucial role in neurological diseases, and are thus a therapeutic target in disorders as diverse as stroke, traumatic brain injury, migraine, epilepsy, spinal cord injury, diabetes, Huntington's disease, Alzheimer's disease, diabetes, multiple sclerosis, glioma, radiation necrosis and amyotrophic lateral sclerosis. Here we report recent advances in our understanding of pericyte biology and discuss how pericytes could be targeted to develop novel therapeutic approaches to neurological disorders, by increasing blood flow, preserving blood-brain barrier function, regulating immune cell entry to the CNS, and modulating formation of blood vessels in, and the glial scar around, damaged regions

Local behavior of p-harmonic Green's functions in metric spaces

We describe the behavior of p-harmonic Green's functions near a singularity in metric measure spaces equipped with a doubling measure and supporting a Poincar\'e inequality

An accelerating high-latitude jet in Earth's core

Observations of the change in Earth's magnetic field, the secular variation, provide information on the motion of liquid metal within the core that is responsible for its generation. The very latest high-resolution observations from ESA's Swarm satellite mission show intense field change at high-latitude localised in a distinctive circular daisy-chain configuration centred on the north geographic pole. Here we explain this feature with a localised, non-axisymmetric, westwards jet of 420 km width on the tangent cylinder, the cylinder of fluid within the core that is aligned with the rotation axis and tangent to the solid inner core. We find that the jet has increased in magnitude by a factor of three over the period 2000--2016 to about 40 km/yr, and is now much stronger than typical large-scale flows inferred for the core. The current accelerating phase may be a part of a longer term fluctuation of the jet causing both eastwards and westwards movement of magnetic features over historical periods, and may contribute to recent changes in torsional wave activity and the rotation direction of the inner core

Implementation of deep soil mixing at the Kansas City Plant

In July 1996, the US Department of Energy (DOE) Kansas City Plant (KCP), AlliedSignal Federal Manufacturing and Technologies, and Oak Ridge National Laboratory (ORNL), conducted field-scale tests of in situ soil mixing and treatment technologies within the Northeast Area (NEA) of the KCP at the Former Ponds site. This demonstration, testing, and evaluation effort was conducted as part of the implementation of a deep soil mixing (DSM) innovative remedial technology demonstration project designed to test DSM in the low-permeability clay soils at the KCP. The clay soils and groundwater beneath this area are contaminated by volatile organic compounds (VOCs), primarily trichloroethene (TCE) and 1,2-dichloroethene (1,2-DCE). The demonstration project was originally designed to evaluate TCE and 1,2-DCE removal efficiency using soil mixing coupled with vapor stripping. Treatability study results, however, indicated that mixed region vapor stripping (MRVS) coupled with calcium oxide (dry lime powder) injection would improve TCE and 1,2-DCE removal efficiency in saturated soils. The scope of the KCP DSM demonstration evolved to implement DSM with the following in situ treatment methodologies for contaminant source reduction in soil and groundwater: DSM/MRVS coupled with calcium oxide injection; DSM/bioaugmentation; and DSM/chemical oxidation using potassium permanganate. Laboratory treatability studies were started in 1995 following collection of undisturbed soil cores from the KCP. These studies were conducted at ORNL, and the results provided information on optimum reagent concentrations and mixing ratios for the three in situ treatment agents to be implemented in the field demonstration