A dipole field model for axisymmetric alfvén waves with finite ionosphere conductivities

An axisymmetric model for approximate solution of the magnetospheric Alfvén wave problem at latitudes above the plasmapause is proposed, in which a realistic dipole geometry is combined with finite anisotropic ionosphere conductivities, thus bringing together various ideas of previous authors. It is confirmed that the axisymmetric toroidal and poloidal modes interact via the ionospheric Hall effect, and an approximate method of solution is suggested using previously derived closed solutions of the uncoupled wave equations.A solution for zero Hall conductivity is obtained, which consists of sets of independent shell oscillations, regardless of the magnitude of the Pedersen conductivity. One set reduces to the classical solutions for infinite Pedersen conductivity, while another predicts a new set of harmonics of a quarter-wave fundamental, with longer eigenperiods than the classical solutions for a given L-shell.     

An integrated hydrological model for rain‐induced landslide prediction

This paper describes an extension to the Combined Hydrology And Stability Model (CHASM) to fully include the effects of vegetation and slope plan topography on slope stability. The resultant physically based numerical model is designed to be applied to site‐specific slopes in which a detailed assessment of unsaturated and saturated hydrology is required in relation to vegetation, topography and slope stability. Applications are made to the Hawke's Bay region in New Zealand where shallow‐seated instability is strongly associated with spatial and temporal trends in vegetation cover types, and the Mid‐Levels region in Hong Kong, an area subject to a variety of landslide mechanisms, some of which may be subject to strong topographic control. An improved understanding of process mechanism, afforded by the model, is critical for reliable and appropriate design of slope stabilization and remedial measures. Copyright © 2002 John Wiley & Sons, Ltd.     

Late Quaternary Upper Mississippi River alluvial episodes and their significance to the Lower Mississippi River system

The period in the Upper Mississippi Valley (UMV) from about 25 000 years B.P. until the time of strong human influence on the landscape beginning about 150–200 years ago can be characterized by three distinctly different alluvial episodes. The first episode is dominated by the direct and indirect effects of Late Wisconsin glacial ice in the basin headwaters. This period, which lasted until about 14 000 years B.P., was generally a time of progressive valley aggradation by a braided river system transporting large quantities of bedload sediment. An island braided system evolved during the second episode, which extended from about 14 000 to 9000 years B.P. The second episode is associated with major environmental changes of deglaciation when occurrences of major floods and sustained flows of low sediment concentration from drainage of proglacial lakes produced major downcutting. By the time of the beginning of the third episode about 9000 years B.P., most vegetation communities had established their approximate average Holocene locations. The change of climate and establishment of good vegetation cover caused upland landscapes of the UMV to become relatively stable during the Holocene in comparison to their relative instability during the Late Wisconsin. However, Holocene remobilization of Late Wisconsin age sediment stored in tributary valleys resulted in a return to long-term upper Mississippi River aggradation. The dominance of Holocene deposition over transportation reflects the abundance of sandy bedload sediment introduced from tributaries and the situation that energy conditions for floods and the hydraulic gradient of the upper Mississippi River are much less for the Holocene than they were for the Late Wisconsin and deglaciation periods.Outburst floods from glacial lakes appear to have been common in the UMV during the Late Wisconsin and especially during deglaciation. Magnitudes for the Late Wisconsin floods are generally poorly understood, but an estimate of 10 000–15 000 m³ s⁻¹ was determined for one of the largest events in the northern UMV based on heights of paleo-foreset beds in a flood unit deposited in the Savanna Terrace. For comparison, the great flood of 1993 on the upper Mississippi River was about 12 000 m³ s⁻¹ at Keokuk, Iowa, near the Des Moines River confluence where it represented the 500-year event in relation to modem flood series. Exceptionally large outburst floods derived from the rapid drainage of pro-glacial Lake Michigan and adjacent smaller proglacial lakes between about 16 000 and 15 500 years B.P. are a likely cause of the final diversion of the Mississippi River through the Bell City-Oran Gap at the upstream end of the Lower Mississippi Valley (LMV). The largest outburst flood from northern extremities of the UMV appears to have occurred between about 11700 and 10 800 years B.P. when the southern outlet of Lake Agassiz was incised. Based on the probable maximum capacity of the Agassiz flood channel 600 km downstream near the junction of the Wisconsin and Mississippi Rivers, the Agassiz flood discharge apparently did not exceed 30 000 m³ s⁻¹. However, if the Agassiz flood channel here is expanded to include an incised component, then the flood discharge maximum could have been as large as 100,000 to 125 000 m³ s⁻¹. The larger flood is presently viewed as unlikely, however, because field evidence suggests that the incised component of the cross-section probably developed after the main Agassiz flood event. Nevertheless, the large Agassiz flood between about 11 700 and 10 800 years B.P. produced major erosional downcutting and removal of Late Wisconsin sediment in the UMV. This flood also appears to be mainly responsible for the final diversion of the Mississippi River through Thebes Gap in extreme southwestern Illinois and the formation of the Charleston alluvial fan at the head of the LMV.After about 9000 years B.P. prairie-forest ecotones with associated steep seasonal climatic boundaries were established across the northern and southern regions of the UMV. The general presence of these steep climatically sensitive boundaries throughout the Holocene, in concert with the natural tendency for grasslands to be especially sensitive to climatic change, may partially explain why widespread synchroneity of Holocene alluvial episodes is recognized across the upper Mississippi River and Missouri River drainage systems. Comparison of estimated beginning ages of Holocene flood episodes and alluvial chronologies for upper Mississippi River and Missouri River systems with beginning ages for LMV meander belts and delta lobes shows a relatively strong correlation. At present, dating controls are not sufficiently adequate and confidence intervals associated with the identified ages representing system changes are too large to establish firm causal connections. Although the limitations of the existing data are numerous, the implicit causal connections suggested from existing information suggest that further exploration would be beneficial to improving the understanding of how upper valley hydrological and geomorphic events are influencing hydrological and geomorphic activity in the LMV. Since nearly 80% of the Mississippi River drainage system lies upstream of the confluence of the Mississippi and Ohio Rivers, there is a strong basis for supporting the idea that UMV fluvial activity should be having a strong influence on LMV fluvial activity. If this assertion is correct, then the traditional assignment of strong to dominant control by eustatic sea level variations for explaining channel avulsions, delta lobes, and meander belts in the LMV needs re-examination. A stronger role for upper valley fluvial activity as a factor influencing lower valley fluvial activity does not disregard the role of eustatic sea level, tectonic processes or other factors. Rather, upper valley fluvial episodes or specific events such as extreme floods may commonly serve as a “triggering mechanism” that causes a threshold of instability to be exceeded in a system that was poised for change due to sea level rise, tectonic uplift, or other environmental factors. In other situations, the upper valley fluvial activity may exert a more dominant control over many LMV fluvial processes and landforms as frequently was the case during times of glacial climatic conditions.     

Dauphiné twinning and texture memory in polycrystalline quartz. Part 3: texture memory during phase transformation

Samples of quartz-bearing rocks were heated above the α (trigonal)–β (hexagonal) phase transformation of quartz (625–950°C) to explore changes in preferred orientation patterns. Textures were measured both in situ and ex situ with neutron, synchrotron X-ray and electron backscatter diffraction. The trigonal–hexagonal phase transformation does not change the orientation of c- and a-axes, but positive and negative rhombs become equal in the hexagonal β-phase. In naturally deformed quartzites measured by neutron diffraction a perfect texture memory was observed, i.e. crystals returned to the same trigonal orientation they started from, with no evidence of twin boundaries. Samples measured by electron back-scattered diffraction on surfaces show considerable twinning and memory loss after the phase transformation. In experimentally deformed quartz rocks, where twinning was induced mechanically before heating, the orientation memory is lost. A mechanical model can explain the memory loss but so far it does not account for the persistence of the memory in quartzites. Stresses imposed by neighboring grains remain a likely cause of texture memory in this mineral with a very high elastic anisotropy. If stresses are imposed experimentally the internal stresses are released during the phase transformation and the material returns to its original state prior to deformation. Similarly, on surfaces there are no tractions and thus texture memory is partially lost.     

Centennial scale benthic foraminiferal record of late Holocene oceanographic variability in Disko Bugt,West Greenland

A new centennial scale benthic foraminiferal record of late Holocene climate variability and oceanographic changes off West Greenland (Disko Bugt) highlights substantial subsurface water mass changes (e.g. temperature and salinity) of the West Greenland Current (WGC) over the past 3.6 ka BP. Benthic foraminifera reveal a long-term late Holocene cooling trend, which may be attributed to increased advection of cold, low-salinity water masses derived from the East Greenland Current (EGC). Cooling becomes most pronounced from c. 1.7 ka BP onwards. At this point the calcareous Atlantic benthic foraminiferal fauna decrease significantly and is replaced by an agglutinated Arctic fauna. Superimposed on this cooling trend, centennial scale variability in the WGC reveals a marked cold phase at c. 2.5 ka BP, which may correspond to the 2.7 ka BP cooling-event recorded in marine and terrestrial archives elsewhere in the North Atlantic region. A warm phase recognized at c. 1.8 ka BP is likely to correspond to the ‘Roman Warm Period’ and represents the warmest bottom water conditions. During the time period of the ‘Medieval Climate Anomaly’ we observe only a slight warming of the WGC. A progressively more dominant cold water contribution from the EGC on the WGC is documented by the prominent rise in abundance of agglutinated Arctic water species from 0.9 ka BP onwards. This cooling event culminates at c. 0.3 ka BP and represents the coldest episode of the ‘Little Ice Age’.Gradually increased influence of cold, low-salinity water masses derived from the EGC may be linked to enhanced advection of Polar and Arctic water by the EGC. These changes are possibly associated with a reported shift in the large-scale North Atlantic Oscillation atmospheric circulation pattern towards a more frequent negative North Atlantic Oscillation mode during the late Holocene.     

The data‐driven approach as an operational real‐time flood forecasting model

Accurate water level forecasts are essential for flood warning. This study adopts a data‐driven approach based on the adaptive network–based fuzzy inference system (ANFIS) to forecast the daily water levels of the Lower Mekong River at Pakse, Lao People's Democratic Republic. ANFIS is a hybrid system combining fuzzy inference system and artificial neural networks. Five ANFIS models were developed to provide water level forecasts from 1 to 5 days ahead, respectively. The results show that although ANFIS forecasts of water levels up to three lead days satisfied the benchmark, four‐ and five‐lead‐day forecasts were only slightly better in performance compared with the currently adopted operational model. This limitation is imposed by the auto‐ and cross‐correlations of the water level time series. Output updating procedures based on the autoregressive (AR) and recursive AR (RAR) models were used to enhance ANFIS model outputs. The RAR model performed better than the AR model. In addition, a partial recursive procedure that reduced the number of recursive steps when applying the AR or the RAR model for multi‐step‐ahead error prediction was superior to the fully recursive procedure. The RAR‐based partial recursive updating procedure significantly improved three‐, four‐ and five‐lead‐day forecasts. Our study further shows that for long lead times, ANFIS model errors are dominated by lag time errors. Although the ANFIS model with the RAR‐based partial recursive updating procedure provided the best results, this method was able to reduce the lag time errors significantly for the falling limbs only. Improvements for the rising limbs were modest. Copyright © 2011 John Wiley & Sons, Ltd.     

From Celsius to Chaos to Cyclones: Using Temperature-Conversion Equations to Introduce Advanced Mathematical Concepts in Earth Science Courses

The Fahrenheit-to-Celsius temperature-conversion equation is a basic component of many introductory earth science courses. Despite its simplicity, it presents a challenge to students and instructors alike because residents of the United States are unfamiliar with the Celsius scale. By solving for the point at which these two temperature scales are equal, it is possible to use the equations for temperature conversion as a springboard to more advanced topics. It is demonstrated that temperature-conversion equations and chaotic equations can be solved using identical numerical and graphical techniques. As a result, the fundamental concepts of chaos theory and numerical methods can be introduced to students in the context of the simplest equations in the earth sciences. These solution methods are applied to the quantitative theory of the extratropical cyclone as an example of the utility and broad scope of this educational approach.