Radiometric measurements of the microwave emissivity of foam

Includes bibliographical references.Radiometric measurements of the microwave emissivity of foam were conducted during May 2000 at the Naval Research Laboratory's Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were performed at 36.5 GHz; horizontal, vertical, +45°, ­45°, left-circular, and right-circular polarization measurements were obtained at 10.8 GHz. These measurements were carried out over a range of incidence angles from 30° to 60°. Surface foam was generated by blowing compressed air through a matrix of gas-permeable tubing supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine the beam-fill fraction of the foam generator. Results show emissivities that were greater than 0.9 and approximately constant in value over the range of incidence angles for vertically polarized radiation at both 10.8 and 36.5 GHz, while emissivities of horizontally polarized radiation showed a gradual decrease in value as incidence angle increased. Emissivities at +45°, ­45°, left-circular, and right-circular polarizations were all very nearly equal to each other and were in turn approximately equal to the average values of the horizontal and vertical emissivities in each case.This work was sponsored by the Department of the Navy, Office of Naval Research under Award N0014-00-1-280 to the University of Massachusetts, Award N00014-00-0152 to the University of Washington, and Award N0001400WX21032 to the Naval Research Laboratory

Structure and diversity trends at Fagus timberline in central Italy

Structure and diversity trends (β-diversity and species richness) across the Fagus sylvatica timberline in the central Apennines were investigated. Twenty-three belt transects were laid out across the upper forest line in the Simbruini Mountains. Number of species, plant cover, and height of different layers were recorded in each quadrat. The moving split-window method was used to detect ecological discontinuities across beech timberlines. We show how β-diversity changes along timberlines and we put forward some hypotheses about the possible dynamics of these transitions. Fourmodels resulted from the analysis of β-diversity trends: two β-diversity peaks indicated a transition where shrubs, mainly Juniperus communis ssp. alpina, (two high peaks) or beech scrub (two small peaks) formed a mantle that could allow forest expansion. One high β-diversity peak referred to an anthropo-zoogenic boundary maintained by disturbance, without the presence of a mantle. A little peak indicated a gradual transition at the upper potential timberline limit where beech forest had lost its typical floristical composition and structural characteristics

Morphodynamic processes in the catchment area of the upper amazon in the light of the surface relief of quartz grains in the electron microscope

The article concerns the results of investigations of the surface relief of quartz grains in two samples of deposits coming from the upper part of the Amazon catchment area, presented in conditions of the natural environment prevailing nowadays and in those of the Pleistocene epoch. The analysis of the greatly magnified sand grains surface was done in a scanning electron microscope of the ISM-25 Jeol type, in the Institute of Geology, Warsaw. The samples of deposits were collected during field investigations in Peruvian Amazonia, in the summer of 1980. The first of the samples comes from the Huallaga valley (right tributary of the Maranón) near the town of Tingo María. It is an area of a warm and humid climate (over 3000 mm of precipitation per year) with intensive processes of chemical weathering, especially among limestone mountain massifs strewn with caves. Even in close proximity of Tingo María the climate is much differentiated; in the neighbouring Huánuco valley, crossed by the Huallaga river, precipitation does not exceed 400 mm yearly and the mountain-slopes are not densely covered with vegetation. The bottom of the Huallaga valley, the mid-channel and coastal sand-banks are built of thick, ill-sorted material containing pebbles, gravel and various-grain sands. The same material may be observed in the exposures of the river’s gorge section (between Huánuco and Tingo María), where it forms high terrace horizons now intensively destroyed by stone-falls and landslides. A cold and humid climate prevailed in the Andes in the glacial epoch. The Central Andes were then glaciated twice at least, and the traces of glaciation such as moraines and post-glacial lakes are common in the upper sections of river valleys of that region. Glaciers developed probably also in the Huallaga valley (above Huánuco) and in the upper parts of the valleys of some of its tributaries. It was probably at that time that the valleys of the Central Andes were highly filled up and among them the Huallaga valley in its gorge section. The analysis of the surface relief of quartz grains in the scanning microscope, by 400-1500 x magnification, has allowed to recognize the character of grain-forming processes and the order of their action. Smooth surfaces have conchoidal breakage with bunches of steps and block relief (Pl. I, photo 4, Pl. II. photo 1). On weathered surfaces there occur fragments covered with a crust (Pl. II, photo 2) which is an indication of dry climate in which silica and other minerals could be precipitated on the grain surface. Other fragments show forms directed in accordance with the crystallographic lattice which points to chemical etching processes (Pl. II, photo 3). There occurs sporadic „lichen” - type chipping (Pl. II, photo 4) observed in areas of intensive mechanical weathering. If more magnified (1500-5000 X) the surfaces modelled by weathering processes display deep corrosive pits (Pl. III, photo 1) frequently having a crystal-directed form (Pl. III. photo 2) and secondary crystallization on the surface. The alternation of the relief types allows to infer on the order in which the processes acted. Three stages may be distinguished: 1) intensive weathering in changing climatic conditions (humid, dry; of high and low temperature); 2) mechanical destruction of the grains; 3) renewed processes of chemical weathering. The time interpretation of those facts presents many difficulties. It cannot be excluded that the glacial period is indirectly recorded in the stage of intensive disintegration of quartz grains, separating two periods of active weathering processes. Thus the glaciation period would be preceded by a period of very dry climate and by another - of great humidity and presumably of high temperature. Uncluckily it is impossible to infer on the succession of those processes by the remaining forms of relief. The post-glacial period was marked only by chemical weathering. However, another origin of the fresh structures preserved on grain surfaces cannot be excluded. In strongly dynamic rivers, carrying masses of pebbles, quartz grains may undergo grinding and trituration like in rock mills. Thus the sharp-edged forms preserved on grains may be as well as relict of the time of flow of fluvioglacial water as they may be shaped at present in highly dynamic rivers to which the Huallaga belongs. The other sample of deposits comes from the Amazon Lowland. It was collected from the edge of a mid-channel sand-bank in the Amazon valley, near Iquitos. The place of collection lies ca 130 km below the confluence of the Ucayali and the Maranón. In this part of the Amazonia a humid, equatorial climate prevails, the mean temperature being 25°C and the precipitation - 2500 mm yearly. It is a low-lying and relatively flat area. The small differentiation of relief is connected with a relatively simple geological structure. Vast plateaux are built of terrigenic formations of the Upper Tertiary (Pebas Formation) and of younger deposits dating from the end of the Tertiary and the beginning of the Quaternary (Iquitos Formation) filling the valley-like depressions cut in older formations. Quaternary and contemporary deposits fill the valleys and build terrace horizons. It is very difficult to define precisely the age of Quaternary acumulation horizons. The phase of intensive erosion in the Amazon valley which descended, along the section from the river mouth to Manaus (1500 km), much below the present-day sea level, is connected with the period of pre-Flandrian regression corresponding with the Vistula glaciation. The climate of the present-day Amazonia was then drier than nowadays. In place of the now existing forests (selva) there grew a savannah with small spots of forests in more humid habitats. At that time, in the Andes foreland, there may have occurred the accumulation of large cones and terrace horizons. The formation of the flood-plain terrace (3-8 m high) in the Ucayali and the Amazon valleys can be probably connected with that period. Similar conditions must have prevailed in the farmer glaciation (the Riss) and the above-floodplain terrace (10-12 m high) could have been formed then. The rise of the ocean level in the Holocene (Flandrian transgression) and a more humid climate caused the expansion of forests and the set-back of erosion. A negative balance of alluvia due to dense vegetation did not allow the deposits to fill the Amazon valley in its lower part; moderate erosion cannot be excluded in the river’s upper part. Among the present-day processes modelling the relief in the river valleys, beside fluvial processes, the activity of the wind should be mentioned, which makes the water wave. This process is favoured by the considerable width of rivers and the existence of numerous overflow arms. The results of waving, such as beaches and cliffs reaching 1.5 m of height, can be seen on both sides of the river bed. Strong waving is also caused by the movement of boats. The analysis of quartz grains of the second sample has shown essential differences in the character of the relief forms of the grain surface as compared with the sample from the Huallaga valley. By 400 X magnification monadnock relief connected with weathering processes, and smooth, rounded edges (Pl. IV, photo 3) may be observed. Larger magnification (2000-7000 X) allows to interpret the relief type in its genetic sense and to connect it with appropriate processes. The oldest process recorded on the grain surface was that of precipitation of amorphous silica forming crusts (Pl. IV, photo 4, Pl. V, photo 1). It may be the record of weathering processes in dry, continental climate of the last glaciation. Another type of forms occurring on all the grains under investigation are V-shaped cuts chaotically disposed (Pl. VI, photos 3, 4, Pl. VII, photos 1, 2), characteristic of beach environment. Some of the V-shaped forms have developed on crust-covered surfaces, some - on fresh surfaces. At times accumulations of V-shaped forms may be observed together with crescent-shaped cuts (Pl. VII, photos 3, 4) which might indicate beach environment where intensive processes of chemical weathering cooperate. It may be supposed that both types of forms (forms connected with chemical weathering and forms developed through abrasion caused by waving) develop nowadays in the region of the upper Amazon. Waving and great changes of water level over the year cause strong side-erosion of higher terraces. This may explain the occurrence, in the alluvia, of crust-covered grains which probably lie on a secondary deposit

Trap-limited carrier recombination in single-walled carbon nanotube heterojunctions with fullerene acceptor layers

Single-walled carbon nanotube (SWCNT)-fullerene (C₆₀) bilayers represent an attractive “donor-acceptor” binary system for solar photoconversion, where the kinetics of photoinduced processes depend critically on the properties of the interface between the two materials. Using photoconductivity measurements we identify the kinetic scheme that describes the free carrier kinetics in such bilayers where the dominant SWCNT species is the (7,5) semiconducting nanotube. Following charge separation, the carrier kinetics, covering up to four orders of magnitude in volumetric hole density, are described by a recombination process that is limited by capture and emission at traps or states at the SWCNT-C₆₀ interface. The high-frequency mobility of holes in the (7,5) SWCNT phase is lower than in multichiral films, potentially due to differences in SWCNT defect density for nanotubes that have been purified more aggressively. The results obtained here provide fundamental insights into the transport and recombination of both charges and excitons within SWCNT thin films and bilayers, and point to several potential ways to improve SWCNT-C₆₀ photovoltaic devices.8 page(s