Some aspects of Sporadic-E at mid-latitudes

Evidence is presented to suggest that the ƒ₀E_s value tends to be high at positions in the E_s-layer where trough slope-lines and crest slope-lines of F2-layer irregularities meet the E_s-layer. These slope-lines are drawn through the troughs and crests, respectively, of the characteristic kinks in the F2-layer ionization contours, which are associated with F2-layer irregularities.

A rotating-loop direction-finding system has allowed an estimation of the distribution of ionization, which gives rise to Sporadic-E echoes. Analysis of Sporadic-E occurrence, on two occasions, suggests that the reflecting surfaces are frontal in nature, the fronts having a separation from each other of some tens of kilometres, and probably existing as closed curves, with diameters of the order of several hundreds of kilometres. A possible association between these structures and the occurrence of the green line of the airglow, is discussed.

A distribution of ionization, which will give contours showing “clouds” of ionization at some frequencies and a ripple structure at other frequencies, is proposed, in an endeavour to explain the apparent dual nature of Sporadic-E occurrence.

The evidence seems to indicate that the mechanism operating at the E_s-layer level, producing the phenomenon of Sporadic-E, is the same as that which produces the F2-layer irregularities which are responsible for “Spread-F”.     

An observational test of the stress accumulation model based on seismicity preceding the 1992 Landers, CA earthquake

We test the Bowman and King [Bowman, D.D., King, G.C.P., 2001a, Accelerating seismicity and stress accumulation before large earthquakes. Geophys. Res. Lett., 28 (21), 4039–4042, Bowman, D.D., King, G.C.P., 2001b. Stress transfer and seismicity changes before large earthquakes. C. R. Acad. Sci. Paris, 333, 591–599] Stress Accumulation model by examining the evolution of seismicity rates prior to the 1992 Landers, California earthquake. The Stress Accumulation (SA) model was developed to explain observations of accelerating seismicity preceding large earthquakes. The model proposes that accelerating seismicity sequences result from the tectonic loading of large fault structures through aseismic slip in the elasto-plastic lower crust. This loading progressively increases the stress on smaller faults within a critical region around the main structure, thereby causing the observed acceleration of precursory activity. A secondary prediction of the SA model is that the precursory seismicity rates should increase first at the edges of the critical region, with the rates gradually rising over time at closer distances to the main fault. We test this prediction by examining year-long seismicity rates between 1960 and 2004, as a function of distance from the Landers rupture. To quantify the significance of trends in the seismicity rates, we auto-correlate the data, using a range of spatial and temporal lags. We find weak evidence for increased seismicity rates propagating towards the Landers rupture, but cannot conclusively distinguish these results from those obtained for a random earthquake catalog. However, we find a strong indication of periodicity in the rate fluctuations, as well as high correlation between activity 130–170 km from Landers and seismicity rates within 50 km of the Landers rupture temporally offset 1.5–2 years. The implications of this spatio–temporal correlation will be addressed in future studies.     

Planview Geometry and morphological characteristics of pocket beaches on the Catalan coast (Spain)

Coastal planform studies are a relevant initial stage before launching detailed dynamic field experiments. The aim of this study is to define the planform characteristics of 72 Catalan pocket beaches, natural and man-made, and to determine their sheltering effect, embaymentization and their status of equilibrium. Planform measurements were performed on SIGPAC, 1:5000 orthophoto sets and wave climate was provided by Puertos del Estado (Wana model). Planform parameters were applied and coastal planview indexes were determined.The study shows that the Catalan pocket beaches display a wide range of indentation, suggesting that no single structural, tectonic or morphological control dominates their planform. The man-made pocket beaches typically display indentations which are smaller than those shown by natural pocket beaches. Headland spacing and beach area are positively correlated. The more indented bays are, the shorter their beaches become. Low-indented pocket beaches are the widest and the longest ones.Deep indentation contributes towards beach protection and energy dissipation which counteracts rip efficiency and inhibits the formation of mega-rips. Pocket beaches often show gradual and moderate alongshore changes in texture and beach morphology. One third of the Catalan pocket beaches are “sediment starved”, i.e., 60% and more of their embayed shorelines are deprived of beach sediments. Examination of the status of equilibrium demonstrates that most of the Catalan pocket beaches are in an unstable mode, with indentation ratios that are unrelated to the wave obliquity.     

The delayed occurrence of equatorial ionospheric F2 layer post-sunset height decreases following auroral-zone substorm onsets

Nighttime F₂ layer height decreases have been examined for post-sunset intervals at the equatorial station, Huancayo. The analyses involved mainly R_z max years, (1957–1960), although R_z min years (1974–1977) were also used. The data were obtained from tabulations of the ionogram parameters h'F, f_○F₂ and f_○E_s. The height reductions are delayed by more than 7 h following geomagnetic substorm onsets for locations at longitudes to the east. The reduced occurrence of spread-F and f_○E_s enhancements is found to be associated. These enhancements are also recorded at the European station, Dourbes about 4.5 h before the Huancayo enhancements. It is proposed that an LS-TID which propagates in the 80 km sound channel may be involved. Also, the experimental evidence suggests that the westward propagation of the LS-TIDs allows equatorial disturbances to occur at local times (before midnight) which are similar to the times when the LS-TIDs are generated.     

Mechanisms of oxygen isotopic exchange and isotopic evolution of <Superscript>18</Superscript>O/<Superscript>16</Superscript>O-depleted periclase zone marbles in the Alta aureole,Utah: insights from ion microprobe analysis of calcite

Oxygen isotope ratios have been measured by ion microprobe and millimeter-scale dental drill along detailed sampling traverses across the boundary between periclase-bearing (δ¹⁸O = 11.8‰) and periclase-free (δ¹⁸O = 17.2‰) marble layers in the periclase (Per) zone of the Alta Stock aureole, Utah. These data define a steep, coherent gradient in δ¹⁸O that is displaced a short distance (~4 cm) into the periclase-free (Cal + Fo) layer. SEM and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty; ±0.27‰, 2SD). More poorly polished (pitted), texturally retrograde ‘turbid’-looking calcite has lower and more variable δ¹⁸O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Despite significant lowering of the δ¹⁸O values in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in δ¹⁸O. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion, 30,000–62,000 years at 575–600°C, exceed significantly the timescale (~1,250 years) estimated for the prograde development of the δ¹⁸O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains. Thus, the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Simulations of precipitation using NRCM and comparisons with satellite observations and CAM: annual cycle

The accurate representation of rainfall in models of global climate has been a challenging task for climate modelers owing to its small space and time scales. Quantifying this variability is important for comparing simulations of atmospheric behavior with real time observations. In this regard, this paper compares both the statistical and dynamically forced aspects of precipitation variability simulated by the high-resolution (36?km) Nested Regional Climate Model (NRCM), with satellite observations from the Tropical Rainfall Measuring Mission (TRMM) 3B42 dataset and simulations from the Community Atmosphere Model (CAM) at T85 spatial resolution. Six years of rainfall rate data (2000?C2005) from within the Tropics (30°S?C30°N) have been used in the analysis and results are presented in terms of long-term mean rain rates, amplitude and phase of the annual cycle and seasonal mean maps of precipitation. Our primary focus is on characterizing the annual cycle of rainfall over four land regions of the Tropics namely, the Indian Monsoon, the Amazon, Tropical Africa and the North American monsoon. The lower tropospheric circulation patterns are analyzed in both the observations and the models to identify possible causes for biases in the simulated precipitation. The 6-year mean precipitation simulated by both models show substantial biases throughout the global Tropics with NRCM/CAM systematically underestimating/overestimating rainfall almost everywhere. The seasonal march of rainfall across the equator, following the motion of the sun, is clearly seen in the harmonic vector maps. The timing of peak rainfall (phase) produced by NRCM is in closer agreement with the observations compared to CAM. However like the long-time mean, the magnitude of seasonal mean rainfall is greatly underestimated by NRCM throughout the Tropical land mass. Some of these regional biases can be attributed to erroneous circulation and moisture surpluses/deficits in the lower troposphere in both models. Overall, the results seem to indicate that employing a higher spatial resolution (36?km) does not significantly improve simulation of precipitation. We speculate that a combination of several physics parameterizations and lack of model tuning gives rise to the observed differences between NRCM and the observations.     

Chemical and stable isotopic models for boundary creek warm springs, southwestern Yellowstone National Park, Wyoming

Thermal springs of the Boundary Creek hydrothermal system in the southwestern part of Yellowstone Park outside the caldera boundary vary in chemical and isotopic composition, and temperature. The diversity may be accounted for by a combination of processes including boiling of a deep thermal water, mixing of the deep thermal water with cool meteoric water and/or with condensed steam or steam-heated meteoric water, and chemical reactions with surrounding rocks. Dissolved-silica, Na⁺, K⁺ and Ca²⁺ contents of the thermal springs could result from a thermal fluid with a temperature of 200 ± 20°C. Chloride-enthalpy and silica-enthalpy mixing models suggest mixing of 230°C, 220 mg/l Cl⁻ thermal water with cool, low-Cl⁻ components. A 350 to 390°C component with Cl⁻ ≥ 300 mg/l is possibly present in thermal springs inside the caldera but is not required to fit observed spring chemical and isotopic compositions. Irreversible mass transfer models in which a low-temperature water reacts with volcanic glass as it percolates downward and warms, can account for observed pH and dissolved-silica, K⁺, Na⁺, Ca²⁺ and Mg²⁺ concentrations, but produces insufficient Cl⁻ or F⁻ for measured concentrations in the warm springs. The ratio of a_Na/a_H, and Cl⁻ are best accounted for in mixing models. The water-rock interaction model fits compositions of acid-sulfate waters observed at Summit Lake and of low-Cl⁻ waters involved in mixing.The cold waters collected from southwestern Yellowstone Park have δD values ranging from −118 to −145 per mil and δ¹⁸O values of −15.9 to −19.4 per mil. Two samples from nearby Island Park have δD values of −112 and −114 per mil and δ¹⁸O values of −15.1 and −15.3 per mil. All samples of thermal water plot significantly to the right of the meteoric water line. The low Cl⁻ and variable δD values of the thermal waters indicate isotopic compositions are derived by extensive dilution with cold meteoric water and by steam separation on ascent to the surface. Many of the hot springs with higher δD values may contain in addition a significant amount of high-D, low-Cl⁻, acid-sulfate or steam-heated meteoric water. Mixing models, Cl⁻ content and isotopic compositions of thermal springs suggest that 30% or less of a deep thermal component is present. For example, the highest-temperature springs from Three Rivers, Silver Scarf and Upper Boundary Creek thermal areas contain up to 70% cool meteoric water and 30% hot water components, springs at Summit Lake and Middle Boundary Creek spring 57 are acid-sulfate or steam-heated meteoric water; springs 27 and 48 from Middle Boundary Creek and 49 from Mountain Ash contain in excess of 50% acid-sulfate water; and Three Rivers spring 46 and Phillips could result from mixing hot water with 55% cool meteoric water followed by mixing of acid-sulfate water. Extensive dilution by cool meteoric water increases the uncertainties in quantity and nature of the deep meteoric, thermal component.     

The declining but non-rejuvenating base level—The Lisan lake,the Dead Sea area,Israel

Laboratory experiments investigating the effects of a lowering base level do not simulate natural eustatic lowering along concave river profiles. The field data on this issue are also quite limited. In evaluating the control exercised by lowering base level on a drainage network, distinction must be made between its influence and those of other hydromorphological processes operating within the basin. Field data on morphological relations, based on photogrammetric mapping and longitudinal profiling, have been gathered in the Dead Sea area, Israel, where a rapid fall in base level has occurred during the Holocene. The study area is an entrenched fan delta with a sequence of 14 unpaired fan terraces which die out at intersection points within an alluvial fan system. The results suggest that the intersection points did not function as base levels. The receding base level played only a passive role, allowing entrenchment without transmitting a head-cutting feedback basinwards. The arid environment caused a delay in transmission of information through the system. The results support the model of short, episodic, and discontinuous erosional events, inherent in the evolution of drainage basins. It is suggested that base level effects in temperate and humid regions are not transferrable to arid zones.