Supernova 1993k — first photometric results

The 61 cm Lowell telescope at Perth Observatory has been automated by the Perth Astronomy Research Group (PARG) for the detection, confirmation and follow-up studies of supernovae and other transient events. This search program complements similar searches operating in the Northern Hemisphere, such as the Berkeley Supernova Search. The first supernova found by our search program, SN 1993k, was discovered on March 28, 1993 (IAUC 5733). The discovery has been made with the telescope in fully automated mode, including automated image analysis. We are currently imaging some 80 galaxies per night. Photometric methods for the analysis of lightcurves are at present being implemented.     

The photoabsorption coefficients of CO and CO2 in the region 350 to 650Å

The photoabsorption coefficients of CO and CO₂ have been measured to ± 3% accuracy in the 350–650 Å spectral region.     

Effect of vegetation cover and sediment type on 3D subsurface structure and shear strength in saltmarshes

The vulnerability of saltmarshes to lateral erosion at their margin depends on the local biogeomorphological properties of the substrate. In particular, the 3D architecture of pore and root systems is expected to influence shear strength, with repercussions for the wider-scale stability of saltmarshes. We apply X-ray computed microtomography (μCT) to visualize and quantify subsurface structures in two UK saltmarshes at Tillingham Farm, Essex (silt/clay rich substrate) and Warton Sands (sand-rich substrate), with four types of ground cover: bare ground, Spartina spp, Salicornia spp and Puccinellia spp. We extracted μCT structural parameters that characterize pore and root morphologies at each station, and compared them with field measurements of shear strength using a principal component analysis and correlation tests. The 3D volumes show that species-dependent variations in root structures, plant colonization events and bioturbation activity control the morphology of macropores, while sediment cohesivity determines the structural stability and persistence of these pore structures over time, even after the vegetation has died. Areas of high porosity and high mean pore thickness were correlated to lower values of shear strength, especially at Tillingham Farm, where well-connected vertical systems of macropores were associated with current or previous colonization by Spartina spp. However, while well-connected systems of macropores may lower the local deformation threshold of the sediment, they also encourage drainage, promote vegetation growth and reduce the marsh vulnerability to hydrodynamic forces. The highest values of shear strength at both sites were found under Puccinellia spp, and were associated with a high density of mesh-like root structures that bind the sediment and resist deformation. Future studies of marsh stability should ideally consider time series of vegetation cover, especially in silt/clay-dominated saltmarshes, in order to consider the potential effect of preserved buried networks of macropores on water circulation, marsh functioning and cliff-face erosion.     

How Many CMEs Have Flux Ropes? Deciphering the Signatures of Shocks, Flux Ropes, and Prominences in Coronagraph Observations of CMEs

We intend to provide a comprehensive answer to the question on whether all Coronal Mass Ejections (CMEs) have flux rope structure. To achieve this, we present a synthesis of the LASCO CME observations over the last 16 years, assisted by 3D MHD simulations of the breakout model, EUV and coronagraphic observations from STEREO and SDO, and statistics from a revised LASCO CME database. We argue that the bright loop often seen as the CME leading edge is the result of pileup at the boundary of the erupting flux rope irrespective of whether a cavity or, more generally, a three-part CME can be identified. Based on our previous work on white light shock detection and supported by the MHD simulations, we identify a new type of morphology, the ‘two-front’ morphology. It consists of a faint front followed by diffuse emission and the bright loop-like CME leading edge. We show that the faint front is caused by density compression at a wave (or possibly shock) front driven by the CME. We also present highly detailed multi-wavelength EUV observations that clarify the relative positioning of the prominence at the bottom of a coronal cavity with a clear flux rope structure. Finally, we visually check the full LASCO CME database for flux rope structures. In the process, we classify the events into two clear flux rope classes (‘three-part’, and ‘Loop’), jets and outflows (no clear structure). We find that at least 40 % of the observed CMEs have clear flux rope structures and that ～?29 % of the database entries are either misidentifications or inadequately measured and should be discarded from statistical analyses. We propose a new definition for flux rope CMEs (FR-CMEs) as a coherent magnetic, twist-carrying coronal structure with angular width of at least 40^° and able to reach beyond 10 R_⊙ which erupts on a time scale of a few minutes to several hours. We conclude that flux ropes are a common occurrence in CMEs and pose a challenge for future studies to identify CMEs that are clearly not FR-CMEs.     

Effects of gasoline and diesel additives on kaolinite

Centrifuge tests were carried out to confirm and determine the effect of different pure alcohols, methyl t-butyl ether (MTBE) and mixtures of alcohols with gasoline and diesel on a thin disc of consolidated clay. The evolution of changes in the clay hydraulic conductivity with time was investigated and other structural changes due to chemical attack were monitored. The findings presented here demonstrate that the hydraulic conductivity of the clay appear to be generally related to the polarity of the chemicals and the dielectric constant. The cracking effect of butanol and MTBE on consolidated clay at low flow rate and low stress level was observed. The addition of ethanol or MTBE to diesel increased the clay permeability and the migration of organic chemical. The addition of ethanol to gasoline also caused an increase in the clay hydraulic conductivity. The effect of the association of alcohols with gasoline or diesel on the clay hydraulic conductivity is discussed, with a view to improving current pollution remediation techniques.     

Subspace system identification of support‐excited structures—part I: theory and black‐box system identification

This paper reviews the theoretical principles of subspace system identification as applied to the problem of estimating black‐box state‐space models of support‐excited structures (e.g., structures exposed to earthquakes). The work distinguishes itself from past studies by providing readers with a powerful geometric interpretation of subspace operations that relates directly to theoretical structural dynamics. To validate the performance of subspace system identification, a series of experiments are conducted on a multistory steel frame structure exposed to moderate seismic ground motions; structural response data is used off‐line to estimate black‐box state‐space models. Ground motions and structural response measurements are used by the subspace system identification method to derive a complete input–output state‐space model of the steel frame system. The modal parameters of the structure are extracted from the estimated input–output state‐space model. With the use of only structural response data, output‐only state‐space models of the system are also estimated by subspace system identification. The paper concludes with a comparison study of the modal parameters extracted from the input–output and output‐only state‐space models in order to quantify the uncertainties present in modal parameters extracted from output‐only models. Copyright © 2012 John Wiley & Sons, Ltd.     

Decentralized ℋ︁∞ controller design for large‐scale civil structures

Complexities inherent to large‐scale modern civil structures pose many challenges in the design of feedback structural control systems for dynamic response mitigation. With the emergence of low‐cost sensors and control devices creating technologies from which large‐scale structural control systems can deploy, a future control system may contain hundreds, or even thousands, of such devices. Key issues in such large‐scale structural control systems include reduced system reliability, increasing communication requirements, and longer latencies in the feedback loop. To effectively address these issues, decentralized control strategies provide promising solutions that allow control systems to operate at high nodal counts. This paper examines the feasibility of designing a decentralized controller that minimizes the ??_∞ norm of the closed‐loop system. ??_∞ control is a natural choice for decentralization because imposition of decentralized architectures is easy to achieve when posing the controller design using linear matrix inequalities. Decentralized control solutions are investigated for both continuous‐time and discrete‐time ??_∞ formulations. Numerical simulation results using a 3‐story and a 20‐story structure illustrate the feasibility of the different decentralized control strategies. The results also demonstrate that when realistic semi‐active control devices are used in combination with the decentralized ??_∞ control solution, better performance can be gained over the passive control cases. It is shown that decentralized control strategies may provide equivalent or better control performance, given that their centralized counterparts could suffer from longer sampling periods due to communication and computation constraints. Copyright © 2008 John Wiley & Sons, Ltd.     

Quaternary climate,environment, and the human occupation of the south-central Andes

This paper reviews the paleoenvironmental context for Paleoindian and Early Archaic settlement of the south-central Andes. I attempt to reconcile proposals for late-glacial and early Holocene environmental changes that have originated in several Quaternary sciences. Most useful are the records of changing lake levels, geomorphological indicators of glacial advance and retreat, changes in vegetation as seen through pollen stratigraphy, the record of large mammal extinctions, and the archaeological settlement pattern itself. High lake stands in the central and south-central Andes, c. 12,500 to 11,000 B.P., seem to correspond to glacial retreat. Palynological studies also suggest higher temperatures, coincident with greater summer precipitation from 11,500 B.P. to perhaps 10,000 B.P., followed by some reduction in temperatures and then widespread aridity from about 8500 to 5000 B.P. Environmental change at the Pleistocene/Holocene boundary was at least a major contributing factor to the extinction of horse and sloth around 10,000 B.P. Archaeology of the salt puna and Punta Negra region conforms well with the environmental record, showing human entry by the Early Archaic (11,000-8000 B.P.), subsequent depopulation and perhaps abandonment, then readaptation to more localized resources in the Late Archaic (5500-4000 B.P.).