The light ion trough

A distinct feature of the ion composition results from the OGO-2, 4 and 6 satellites is the light ion trough, wherein the mid latitude concentrations of H⁺ and He⁺ decrease sharply with latitude, dropping to levels of 10³ ions/cm³ or less near 60° dipole latitude (L=4). In contrast to the ‘main trough’ in electron density, N_e, observed primarily as a nightside phenomenon, the light ion trough persists during both day and night. For daytime winter hemisphere conditions and for all seasons during night, the mid latitude light ion concentration decrease is a pronounced feature. In the dayside summer and equinox hemispheres, the rate of light ion decrease with latitude is comparatively gradual, and the trough boundary is less well defined, particularly for quiet magnetic conditions. In response to magnetic storms, the light ion trough minimum moves equatorward, and deepens, consistent with earlier evidence of the contraction of the plasmasphere in response to storm time enhancements in magnetospheric plasma convection. The fact that a pronounced light ion trough is observed under conditions for which the dominant ion O⁺ may exhibit little or no simultaneous decrease appears to explain why earlier studies of the ‘main trough’ in topside distributions of N_e and N_i may, at times, have been inconclusive in relating the total ionization minimum with the mechanism of the plasmapause. In particular, the topside distribution of N_i appears to be the complex resultant of several variables within the ion composition, being governed by the competing processes of chemical production and loss, loss through magnetospheric convection, and large-scale dynamic transport resulting from neutral winds and electric fields. The net result is that in general, the light ion trough, rather than N_i, provides a more fundamental parameter for examining the structure and behavior of the plasmapause.     

Serendipity is not a strategy: the impact of national climate programmes on greenhouse-gas emissions

This article examines the efficacy of national climate programmes (packages of policies introduced by governments to meet emissions reduction targets set out in the Kyoto Protocol) by considering emissions trends before and after their implementation. Analysis reveals that only four of 21 countries with defined programmes demonstrate improved emissions trends following their inception and in only one is the change statistically significant. The reasons for this are manifold but serendipity appears to play as large a part as strategy in determining national emissions trends in the early years of climate programmes. Inflated claims of success by national governments are unhelpful for effective policy analysis and development.     

Propagating decadal sea surface temperature signal identified in modern proxy records of the tropical Pacific

Analysis of 86 years of multiple modern coral δ¹⁸O records in the tropical Pacific reveals a basin-scale decadal pattern of variability. Although coral δ¹⁸O records the effects of both temperature and seawater δ¹⁸O variability due to salinity effects, in practice, most of the records used here agree well with observations of sea surface temperature on longer timescales. These coral proxy records reveal strong variability near a 12-year period. Their relative phasing suggests a signal propagating from the southwestern subtropical Pacific to other regions. The results are consistent with recent studies based on instrumental data and with coupled climate model studies, in which advection of thermal anomalies leads to El Niño/Southern Oscillation-like variability on decadal timescales. Additionally, there is evidence for a significant shift in many of the time series, along with a decrease in the decadal variability, occurring in the early 1940s. Finally, the proxy records indicate the presence of strong teleconnections between the eastern tropical Pacific and high latitude climate.     

Organic geochemistry of impactites from the Haughton impact structure, Devon Island, Nunavut, Canada

Organic matter in impactites from the 24 km wide and 39 Ma old Haughton impact structure, Canadian High Arctic, is a mixture of fossil and modern biological components. The fossil component represents a conventional oil that was generated from Lower Palaeozoic marine source material before impact and permeates bedrock dolomites. Biomarker maturity parameters record the thermal effect of the mid-Tertiary impact. Maturity-influenced sterane, rearranged hopanoid, and triaromatic steroid ratios all increase towards the centre of the impact structure, where thermal alteration was greatest. The heating was probably dominated by an impact-related hydrothermal system, as such systems last long enough for kinetically-based thermal alteration to occur. Kinetically-related biomarker data suggest that the hydrothermal heating lasted for c. 5000 years. Biomarkers are also preserved in dolomite clasts within impact melt breccia, and indicate strong thermal alteration. Modern biological contamination of the rocks is responsible for the superposition of two geochemical signatures (which could be cyanobacteria, non-marine algae, or higher plant matter) onto the fossil component, but they can be recognized and distinguished. The data show that the impact structure system holds a record of both the pre-impact organic signature and the thermal signature of the impact, and thereby indicates that organic geochemistry is a valuable tool in documenting the response of rocks to impacts.     

Analysis of the climate signal contained within δO and growth rate parameters in two Ethiopian stalagmites

We combine surface and cave climate monitoring with multiple stalagmite parameters to help understand and calibrate the climate records contained within stalagmites from a region with strong rainfall seasonality. Two actively growing stalagmites from Ethiopia were analysed in order to investigate the climate signal contained within δ¹⁸O and growth rate parameters. The δ¹⁸O and growth rate of the two stalagmites give different responses to surface climate due to variations in the climate signal transfer. Both stalagmites (Merc-1 and Asfa-3) have a climate response that is seasonal; however this signal is subsequently smoothed by the mixing of event and storage water within the aquifer. Merc-1 responds more to high frequency (‘event’) climate, due to a greater ratio of event to storage water in this sample, whereas Asfa-3 responds more to low frequency (‘storage’) climate. In addition, different parameters respond to different seasons. For example, stalagmite Asfa-3, from greater depth from the surface and with a slow drip rate, has a growth rate that responds to the amount of summer rain. In contrast, Merc-1, closer to the surface and with a faster drip rate, exhibits no clear response to surface climate, probably due to a more complex climate signal transfer. δ¹⁸O response varies with stalagmite due to the interplay between rainfall forcing factors (amount, seasonality) and disequilibrium kinetics, with opposing correlations between seasonal rainfall and δ¹⁸O between the samples. Our results demonstrate that analysis of seasonal climate forcing, and transfer functions reflecting the mixing of event and storage water, may be the most appropriate approach to develop of transfer functions appropriate for high-resolution, stalagmite climate reconstruction.     

In situ U-series dating by laser-ablation multi-collector ICPMS: new prospects for Quaternary geochronology

The capabilities and potential applications of in situ dating of Quaternary materials using laser ablation-MC-ICPMS are explored. ²³⁴U/²³⁸U and ²³⁰Th/²³⁴U can be measured with precision sufficient for dating at a spatial resolution of 100 μm or better in samples that contain as a little as 1 ppm uranium. Moreover, U and Th concentrations and U-series isotope ratios can be continuously profiled to determine changes in age that occur with sample growth (e.g. in speleothems). These capabilities additionally permit the dating of bones, teeth and possibly molluscs, which are subject to post-mortem open-system behaviour of U-series isotopes, and can be employed to elucidate processes of U-series migration during weathering and diagenesis. A drawback of laser ablation-MC-ICPMS is that it cannot in general provide U-series age estimates with the high precision and accuracy of conventional TIMS or solution MC-ICPMS methods. However, sample preparation is straightforward, the amount of sample consumed negligible, and it can be used to rapidly characterise or screen and select samples from which more precise and accurate dates can be obtained using conventional methods. Given further instrumental developments and the establishment of suitable matrix-matched standards for carbonates and other materials, we foresee that laser ablation-MC-ICPMS will play an increasingly important role in Quaternary dating research.     

Vertical impulse measurements of mines buried in saturated sand

The ultimate aim of our overall task, of which the effort described in this paper is a part, is to be able to model the impulsive output of buried charges and the response of targets of interest. It is not practical or cost-effective to determine the response of all targets of interest to buried charges of all sizes by testing them. In order to have confidence in our models, however, they must be validated by a modest number of tests. A critical element in modelling the response of a target is the ability to model the loading function. The load a buried charge applies to a target above it when the charge detonates can be characterized in terms of the vertical impulse. The vertical impulse is a function of the size of the charge, its depth of burial, and the properties of the soil in which it is buried. The primary objective of the effort described in this paper is to determine the load a known charge places on a non-responding target so the data can be used to validate our models.

For model validation, a large number of detonator-scale experiments have been conducted by the University of Maryland (Fourney et al. [1]). It was also necessary to conduct a modest number of experiments at a larger scale, nine in total, to ensure that the results of the detonator-scale tests can be satisfactorily scaled up. Of the nine large-scale experiments conducted, seven were conducted with 5 or 10 lb cast TNT charges. All experiments were conducted in sand that was as nearly fully water-saturated as possible. The objective of the experiments was to determine the vertical impulse applied to a non-deforming target plate above the charge.

The large-scale experiments were conducted using the Vertical Impulse Measurement Fixture (VIMF) at the Army Research Laboratory, Aberdeen, MD. The VIMF is a unique facility that has been designed specifically to measure accurately the vertical impulse from buried charges weighing up to 8 kg.

This paper describes the VIMF and its instrumentation, test methods and test results. The results obtained demonstrate that in some cases, when the soil is saturated sand, explosive 'bubble' effects similar to those encountered in shallow water are encountered.     

Cluster Observations of the CUSP: Magnetic Structure and Dynamics

This paper reviews Cluster observations of the high altitude and exterior (outer) cusp, and adjacent regions in terms of new multi-spacecraft analysis and the geometry of the surrounding boundary layers. Several crossings are described in terms of the regions sampled, the boundary dynamics and the electric current signatures observed. A companion paper in this issue focuses on the detailed plasma distributions of the boundary layers. The polar Cluster orbits take the four spacecraft in a changing formation out of the magnetosphere, on the northern leg, and into the magnetosphere, on the southern leg, of the orbits. During February to April the orbits are centred on a few hours of local noon and, on the northern leg, generally pass consecutively through the northern lobe and the cusp at mid- to high-altitudes. Depending upon conditions, the spacecraft often sample the outer cusp region, near the magnetopause, and the dayside and tail boundary layer regions adjacent to the central cusp. On the southern, inbound leg the sequence is reversed. Cluster has therefore sampled the boundaries around the high altitude cusp and nearby magnetopause under a variety of conditions. The instruments onboard provide unprecedented resolution of the plasma and field properties of the region, and the simultaneous, four-spacecraft coverage achieved by Cluster is unique. The spacecraft array forms a nearly regular tetrahedral configuration in the cusp and already the mission has covered this region on multiple spatial scales (100–2000 km). This multi-spacecraft coverage allows spatial and temporal features to be distinguished to a large degree and, in particular, enables the macroscopic properties of the boundary layers to be identified: the orientation, motion and thickness, and the associated current layers. We review the results of this analysis for a number of selected crossings from both the North and South cusp regions. Several key results have been found or have confirmed earlier work: (1) evidence for magnetically defined boundaries at both the outer cusp/magnetosheath interface and the␣inner cusp/lobe or cusp/dayside magnetosphere interface, as would support the existence of a distinct exterior cusp region; (2) evidence for an associated indentation region on the magnetopause across the outer cusp; (3) well defined plasma boundaries at the edges of the mid- to high-altitude cusp “throat”, and well defined magnetic boundaries in the high-altitude “throat”, consistent with a funnel geometry; (4) direct control of the cusp position, and its extent, by the IMF, both in the dawn/dusk and North/South directions. The exterior cusp, in particular, is highly dependent on the external conditions prevailing. The magnetic field geometry is sometimes complex, but often the current layer has a well defined thickness ranging from a few hundred (for the inner cusp boundaries) to 1000 km. Motion of the inner cusp boundaries can occur at speeds up to 60 km/s, but typically 10–20 km/s. These speeds appear to represent global motion of the cusp in some cases, but also could arise from expansion or narrowing in others. The mid- to high-altitude cusp usually contains enhanced ULF wave activity, and the exterior cusp usually is associated with a substantial reduction in field magnitude.