The effects of ionospheric ‘phase mixing’ on a distributed driven shear Alfvén ulf pulsation’

A numerical simulation study of the ultra-low frequency (ULF) H-component magnetic field at the Earth’s surface arising from a perturbation ionospheric Hall current has been developed. The Hall current system is driven by field-aligned currents (FACs) associated with shear Alfvén field line resonances (FLRs) driven by fast mode global cavity oscillations. The ionospheric phase mixing of the Hall current manifests itself in a number of ways in the ground field, these are: (i) Smoothing the spectral maxima of the ground signal: (ii) Loss in clarity of the harmonic structure of the spectra: (iii) A small increase in the damping rate of the ULF wave at the resonance latitude and (iv) small localised minimum in the spectra at the resonance latitude.     

CRRES/Ground-based multi-instrument observations of an interval of substorm activity

Observations are presented of data taken during a 3-h interval in which five clear substorm onsets/intensifications took place. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were recorded. In addition data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation, are available. The locations and movements of the substorm current system in latitude and longitude, determined from ground and spacecraft magnetic field data, have been correlated with the locations and propagation of increased particle precipitation in the E-region at EISCAT, increased particle fluxes measured by CRRES and DMSP, with auroral luminosity and with ionospheric convection velocities. The onsets and propagation of the injection of magnetospheric particle populations and auroral luminosity have been compared. CRRES was within or very close to the substorm expansion phase onset sector during the interval. The onset region was observed at low latitudes on the ground, and has been confirmed to map back to within L=7 in the magnetotail. The active region was then observed to propagate tailward and poleward. Delays between the magnetic signature of the substorm field aligned currents and field dipolarisation have been measured. The observations support a near-Earth plasma instability mechanism for substorm expansion phase onset.     

An evaluation of the hysteresis in chemical concentration–discharge (C–Q) relationships from drained,intensively managed grasslands in southwest England

This study examined the hysteresis exhibited in concentration–discharge (C–Q) relationships in the runoff from four hydrologically separated fields (catchments) at an intensively managed grassland. The objectives were to examine C–Q relationships constructed from high-resolution time series of flow, temperature, pH, conductivity, nitrate and turbidity, and their implications for hydrological processes. High-resolution datasets from the quality assured records of the Rothamsted Research North Wyke Farm Platform in the UK were examined using a graphical method and cross-correlation statistics. The study found that storm events based C–Q hysteresis reflects the cross-correlation that is generally hidden in time series analysis of large datasets, and that although Q and water quality variables can be effectively influenced by catchment size, the C–Q relationship is less significantly influenced. The dominant C–Q relationships of the water variables in the study area reflect that saturated overland flow was prevalent during the study period in the catchments, while the CCF results indicate coupled transfer of sediments and solute in the area at lag ≥ 0.

EDITOR D. Koutsoyiannis

ASSOCIATE EDITOR M. D. Fidelibus     

Approaches to optimal aquifer management and intelligent control in a multiresolutional decision support system

Despite remarkable new developments in stochastic hydrology and adaptations of advanced methods from operations research, stochastic control, and artificial intelligence, solutions of complex real-world problems in hydrogeology have been quite limited. The main reason is the ultimate reliance on first-principle models that lead to complex, distributed-parameter partial differential equations (PDE) on a given scale. While the addition of uncertainty, and hence, stochasticity or randomness has increased insight and highlighted important relationships between uncertainty, reliability, risk, and their effect on the cost function, it has also (a) introduced additional complexity that results in prohibitive computer power even for just a single uncertain/random parameter; and (b) led to the recognition in our inability to assess the full uncertainty even when including all uncertain parameters. A paradigm shift is introduced: an adaptation of new methods of intelligent control that will relax the dependency on rigid, computer-intensive, stochastic PDE, and will shift the emphasis to a goal-oriented, flexible, adaptive, multiresolutional decision support system (MRDS) with strong unsupervised learning (oriented towards anticipation rather than prediction) and highly efficient optimization capability, which could provide the needed solutions of real-world aquifer management problems. The article highlights the links between past developments and future optimization/planning/control of hydrogeologic systems.

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Resumen  A pesar de nuevos avances notables en hidrología estocástica y las adaptaciones de métodos avanzados de investigación de operaciones, control estocástico, e inteligencia artificial, las soluciones de problemas complejos del mundo real en hidrogeología han sido bastante limitadas. La principal razón es la dependencia definitiva en modelos de primer-principio que conducen a ecuaciones parciales diferencias de parámetro distribuido complejas (PDE) a una escala dada. Mientras que la adición de incertidumbre, y por lo tanto, estocasticidad o aleatoriedad ha incrementado la profundidad y resaltado relaciones importantes entre la incertidumbre, confiabilidad, riesgo, y su efecto en la función de costo, la adición también ha permitido (a) introducir complejidad adicional que resulta en potencia computacional excesiva aún para un solo parámetro incierto/aleatorio; y (b) llevar a reconocer nuestra discapacidad para evaluar la incertidumbre completa aún cuando se incluyen todos los parámetros inciertos. Se introduce un cambio paradigmático: una adaptación de nuevos métodos de control de inteligencia que relajará la dependencia en PDE estocásticas, rígidas y de uso computacional intensivo, cambiando el énfasis hacia un sistema de apoyo de decisiones de propósitos múltiples (MRDS) adaptivo, flexible, y orientado a objetivos con fuerte aprendizaje sin supervisión (orientado a la anticipación más que a la predicción) con fuerte capacidad de optimización eficiente, lo cual podría aportar las soluciones necesarias a los problemas de manejo reales con los acuíferos. El artículo resalta los vínculos entre desarrollos pasados y control/planificación/optimización futura de sistemas hidrogeológicos.

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Résumé  Malgré de remarquables nouveaux développements en hydrologie stochastique ainsi que de remarquables adaptations de méthodes avancées pour les opérations de recherche, le contrôle stochastique, et lintelligence artificielle, solutions pour les problèmes complexes en hydrogéologie sont restées assez limitées. La principale raison est lultime confiance en les modèles qui conduisent à des équations partielles complexes aux paramètres distribués (PDE) à une échelle donnée. Alors que laccumulation dincertitudes et, par conséquent, la stockasticité ou laléat a augmenté la perspicacité et a mis en lumière dimportantes relations entre lincertitude, la fiabilité, le risque, et leur effet sur les coûts de fonctionnement, il a également (a) introduit une complexité additionnelle qui résulte dans un pouvoir prohibitif des moyens de calcul informatique même pour une simple estimation de lincertitude; et (b) a conduit a une reconnaissance de notre manque daptitude à maîtriser lincertitude totale même en introduisant tous les paramètres connus de lincertitude. La représentation du changement est introduit: une adaptation de nouvelles méthodes de contrôle intelligent qui va relâcher la dépendance à la rigidité des algorithmes, aux calculs informatiques intensifs, à la PDE stockastique, et qui modifiera lemphase entre les MRDS—systèmes interactifs daide à la décision de multiresolutionelle (flexibles, adaptables et orientables selon les objectifs)—avec un fort apprentissage non (orienté vers lanticipation plutôt que la prédiction), et une capacité doptimisation efficiente très élevée, qui pourrait apporter le besoin de solutions pour la modélisation des problèmes de management des aquifères réalistes. Cet article met en lumière les liens entre les développements passés et les futurs moyens doptimisation, de gestion et de contrôle des systèmes hydrogéologiques.

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Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain

We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u _* when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t<inf>θ</inf> less than about h /(10u _*). Standard Monin–Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained for the surface layer, when there are small changes in h /|L_MO| over the time scale t_MO~|L _MO|/(10u_*) (where L _MO and t _MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong surface heating is suddenly applied, resulting in a large change in h /|L _MO| (>>1), over the time scale t _MO. The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence at the ground. A low-level wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable to the ABL passing over spatially varying surface heat fluxes.     

Cyclic spattering,seismic tremor,and surface fluctuation within a perched lava channel,Kīlauea Volcano

In late 2007, a perched lava channel, built up to 45 m above the preexisting surface, developed during the ongoing eruption near Pu‘u ‘Ō‘ō cone on Kīlauea Volcano’s east rift zone. The lava channel was segmented into four pools extending over a total of 1.4 km. From late October to mid-December, a cyclic behavior, consisting of steady lava level rise terminated by vigorous spattering and an abrupt drop in lava level, was commonly observed in pool 1. We use geologic observations, video, time-lapse camera images, and seismicity to characterize and understand this cyclic behavior. Spattering episodes occurred at intervals of 40–100 min during peak activity and involved small (5–10-m-high) fountains limited to the margins of the pool. Most spattering episodes had fountains which migrated downchannel. Each spattering episode was associated with a rapid lava level drop of about 1 m, which was concurrent with a conspicuous cigar-shaped tremor burst with peak frequencies of 4–5 Hz. We interpret this cyclic behavior to be gas pistoning, and this is the first documented instance of gas pistoning in lava well away from the deeper conduit. Our observations and data indicate that the gas pistoning was driven by gas accumulation beneath the visco-elastic component of the surface crust, contrary to other studies which attribute similar behavior to the periodic rise of gas slugs. The gas piston events typically had a gas mass of about 2,500 kg (similar to the explosions at Stromboli), with gas accumulation and release rates of about 1.1 and 5.7 kg s⁻¹, respectively. The time-averaged gas output rate of the gas pistoning events accounted for about 1–2% of the total gas output rate of the east rift zone eruption.