Properties of coronal arches

The properties of coronal arches located on the peripheries of active regions, observed during a sounding rocket flight on March 8, 1973, are discussed. The arches are found to overlie filament channels and their footpoints are traced to locations on the perimeters of supergranulation cells. The arches have a wide range of lengths although their widths are well approximated by the value 2.2 × 10⁹ cm. Comparison of the size of the chromospheric footprint with the arch width indicates that arches do not always expand as they ascend into the corona. The electron temperatures and densities of the plasma contained in the arches were measured and the pressure calculated; typical values are 2 × 10⁶ K, 1 × 10⁹ cm^–3, and 2 × 10^–1 dyne cm^–2, respectively. The variation of these parameters with position along the length of the arch indicates that the arches are not in hydrostatic equilibrium.     

Rapid shearing in a rate-type soil surrounding a cylindrical cavity

The stress and deformation resulting from rapidly applied axial traction on the face of a cylindrical cavity in a saturated soil are analysed. A simple rate-type constitutive model is used for the effective stress response.     

The behaviour of an elastic non-homogeneous half-space. Part II–circular and strip footings

Solutions developed in the first part of this paper (i.e. describing the response of a non-homogeneous half-space subjected to surface point and line loads) are used in this part to obtain solutions for a variety of surface loadings. Consideration is given to a distributed load acting over a circular area or strip and a rigid disk or strip subjected to applied normal load and moment. It is established that the profiles of surface settlement due to uniformly distributed loads acting over a strip or circular area are strongly dependent on the degree of non-homogeneity. This dependency is reduced when the footing is rigid. When α = 1 the moduli variation is identical to the Gibson soil and the equivalence with the Winkler soil model is established.     

The cyclical variation of energy flux and photospheric magnetic field strength from coronal holes

We measured the average soft X-ray emission from coronal holes observed on images obtained during AS & E rocket flights from 1974 to 1981. The variation of this emission over the solar cycle was then compared with photospheric magnetic flux measurements within coronal holes over the same period. We found that coronal hole soft X-ray emission could be detected and that this emission appeared to increase with the rise of the sunspot cycle from activity minimum to maximum. Our quantitative results confirmed previous suggestions that the coronal brightness contrast between holes and large-scale structure decreased during this period of the cycle. Gas pressures at the hole base were estimated for assumed temperatures and found to vary from about 0.03 dyne cm^–2 in 1974 to 0.35 dyne cm^–2 in 1981. The increase in coronal hole X-ray emission was accompanied by a similar trend in the surface magnetic flux of near-equatorial holes between 1975 and 1980 (Harvey et al., 1982).     

A comparison of three-dimensional interpolation techniques for plume characterization

Interpolation of contaminant data can present a significant challenge due to sample clustering and sharp gradients in concentration. The research presented in this paper represents a study of commonly used interpolation schemes applied to three-dimensional plume characterization. Kriging, natural neighbor, and inverse distance weighted interpolation were tested on four actual data sets. The accuracy of each scheme was gauged using the cross-validation approach. Each scheme was compared to the other schemes and the effect of various interpolation parameters was studied. The kriging approach resulted in the lowest error at three of the four sites. The simpler and quicker inverse distance weighted approach resulted in a lower interpolation error on the other site and performed well overall. The natural neighbor method had the highest average error at all four sites in spite of the fact that it has been shown to perform well with clustered data. Another unexpected result was that the computationally expensive high order nodal functions resulted in reduced accuracy for the inverse distance weighted and natural neighbor approaches.     

Rift-phase extensional fabrics of the back-arc Drummond Basin, eastern Australia

The base of the Late Devonian–Early Carboniferous Drummond Basin, a major backarc extensional feature in eastern Australia which formed in response to detachment faulting, is extensively exposed in central Queensland. Here a crystalline basin floor is overlain by the Silver Hills Volcanics, a synrift sequence of predominantly silicic ash flow tuffs and lavas ranging to over 2 km in thickness. Detailed mapping of faults and stratigraphic logging of thickness changes within the Silver Hills Volcanics have allowed the rift-phase structural architecture that accompanied initial subsidence near the basin margin to be resolved. A complex mosaic of block faults with throws of up to 1 km is indicated. Locally developed mosaics may conform to, or depart from, the configuration predicted by the detachment faulting model. Structural fabric of the basement was a critical determinant of the extensional geometry. Distributed shear along pre-existing penetrative planar fabrics is considered to have accommodated hangingwall extension at lower strain rates whereas the propagation of tension fractures and the development of block faults by failure on pre-existing, brittle, basement dislocations facilitated extension at higher strain rates. The detachment fault inferred to lie beneath the extended hangingwall carapace has not been mapped at the surface and is thought to dissipate into a broad zone of distributed shear within basement to the east of the basin. Volcanism coincided with the initiation of extensional movements at which time deep crustal repositories for evolved magma were tapped by extensional fractures. The main extensional faults cutting the basinal succession were not used as conduits for magmatic products which were sourced from the basin margin and from extended hinterland to the east.     

An analysis of the accuracy of magnetopause reconnection rate variations deduced from cusp ion dispersion characteristics

We present an analysis of the accuracy of the method introduced by Lockwood et al. (1994) for the determination of the magnetopause reconnection rate from the dispersion of precipitating ions in the ionospheric cusp region. Tests are made by applying the method to synthesised data. The simulated cusp ion precipitation data are produced by an analytic model of the evolution of newly-opened field lines, along which magnetosheath ions are firstly injected across the magnetopause and then dispersed as they propagate into the ionosphere. The rate at which these newly opened field lines are generated by reconnection can be varied. The derived reconnection rate estimates are then compared with the input variation to the model and the accuracy of the method assessed. Results are presented for steady-state reconnection, for continuous reconnection showing a sine-wave variation in rate and for reconnection which only occurs in square wave pulses. It is found that the method always yields the total flux reconnected (per unit length of the open-closed field-line boundary) to within an accuracy of better than 5%, but that pulses tend to be smoothed so that the peak reconnection rate within the pulse is underestimated and the pulse length is overestimated. This smoothing is reduced if the separation between energy channels of the instrument is reduced; however this also acts to increase the experimental uncertainty in the estimates, an effect which can be countered by improving the time resolution of the observations. The limited time resolution of the data is shown to set a minimum reconnection rate below which the method gives spurious short-period oscillations about the true value. Various examples of reconnection rate variations derived from cusp observations are discussed in the light of this analysis.     

Decaying Scalars Emitted By A Forest Canopy: A Numerical Study

A large-eddy simulation is modified to include multiple scalars emitted by a plant canopy. Each of these scalars is subjected to varying rates of chemical loss. Presented is a detailed comparison between conservedspecies and species undergoing first- and second-order chemical loss.Profiles of mean mixing ratio, mixing-ratio variance and vertical mixing-ratio flux reveal the influence of chemical reactivity. Distribution of thescalar source through the depth of the canopy is shown to locally reducethe reaction rate for second-order species. Transport efficiencies, diffusioncoefficients, and mean source heights also exhibit chemical dependencies.Budgets of mixing-ratio variance and flux elucidate the mechanisms throughwhich chemistry modifies each. Instantaneous fields show the existence ofintermittently occurring coherent structures that are thought to enhancespecies segregation.     

Analyzing individual presolar grains with CHARISMA

Isotopic analysis of heavy elements in individual stardust grains is important in testing and constraining theories of stellar nucleosynthesis. These analyses are challenging in that the grains are very small, the largest being perhaps a few microns in diameter, and contain only trace concentrations of heavy elements, generally on the order of ppm. In addition, isotopic analysis requires the suppression of isobaric interferences. We describe a unique instrument, based on resonant ionization mass spectrometry, that has successfully characterized such grains for the past several years, and report on some recent upgrades that significantly enhance the instrumental capabilities. The fundamental principles and operational details are discussed, along with illustrative results and plans for future modifications.