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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

A self-consistent model for the long-term gamma-ray spectral variability of Cygnus X-1

The long-term transitions of the black hole candidate Cygnus X-1 (between the states gamma-1, gamma-2, and gamma-3) include the occasional appearance of a strong nearly-MeV bump (gamma-1), whose strength appears to be anticorrelated with the continuum flux (below 400 keV) due to the Compton upscattering of cold disk photons by the inner, hot corona. We develop a self-consistent disk picture in which the bump is seen as due to the self-Comptonization of bremsstrahlung photons emitted predominantly near the plane of the corona itself. A decrease by a factor of about two in the viscosity parameter alpha is responsible for quenching this bump and driving the system to the gamma-2 state, whereas a transition from gamma-2 to gamma-3 appears to be induced by an increase of about 25 percent in the accretion rate M. In view of the fact that most of the transitions observed in this source seem to be of the gamma-2-gamma-3 variety, we conclude that much of the long-term gamma-ray spectral variability in Cygnus X-1 is due to these small fluctuations in M. The unusual appearance of the gamma-1 state apparently reflects a change in the dissipative processes within the disk.

Melia, Fulvio↗
NASA NTRS1997

Self-Consistent Thermal Accretion Disk Corona Models for Compact Objects: Application to Cygnus X-1 - II

We apply our self-consistent accretion disk corona (ADC) model, with two different geometries, to the broadband X-ray spectrum of the black hole candidate Cygnus X-1. As shown in a companion paper, models in which the Comptonizing medium is a slab surrounding the cold accretion disk cannot have a temperature higher than about 140 keV for optical depths greater than 0.2, resulting in spectra that are much softer than the observed 10-30 keV spectrum of Cyg X-1. In addition, the slab-geometry models predict a substantial "soft excess" at low energies, a feature not observed for Cyg X-1, and Fe K-alpha fluorescence lines that are stronger than observed. Previous Comptonization models in the literature have invoked a slab geometry with optical depth tau(sub T) approx. greater than 0.3 and coronal temperature T(sub c) approx. 150 keV, but they are not self-consistent. Therefore, ADC models with a slab geometry are not appropriate for explaining the X-ray spectrum of Cyg X-1. Models with a spherical corona and an exterior disk, however, predict much higher self-consistent coronal temperatures than the slab-geometry models. The higher coronal temperatures are due to the lower amount of reprocessing of coronal radiation in the accretion disk, giving rise to a lower Compton cooling rate. Therefore, for the sphere-plus-disk geometry, the predicted spectrum can be hard enough to describe the observed X-ray continuum of Cyg X-1 while predicting Fe fluorescence lines having an equivalent width of approx. 40 eV. Our best-fit parameter values for the sphere-plus-disk geometry are tau(sub T) approx. equal to 1.5 and T(sub c) approx. equal to 90 keV.

Dove, James B.↗
NASA NTRS1998

A Multi-Year Light Curve of Scorpius X-1 Based on CGRO BATSE Spectroscopy Detector Observations

A multi-year light curve of the low mass X-ray binary, Scorpius X-1, is constructed based on the Compton Gamma-ray Observatory (CGRO) Burst and Transient Source Experiment (BATSE) Spectroscopy Detector (SD) data in the nominal energy range of 10-20 keV. A detailed discussion is given of the reduction process of the BATSE/SD data. Corrections to the SD measurements are made for off-axis pointings, spectral and bandpass changes, and differences in the eight SD sensitivities. The resulting 4.4 year Sco X-1 SD light curve is characterized in terms of the time scales over which various types of emission changes occur. This light curve is then compared with Sco X-1 light curves obtained by Axiel 5, the BATSE Large Area Detectors (LADs), and the RXTE all-sky monitor (ASM). Coincident temporal coverage by the BATSE/SD and RXTE/ASM allows a direct comparison of the behavior of Sco X-1 over a range of high energies to be made. These ASM light curves are then used to discuss model constraints on the Sco X-1 system.

McNamara, B. J.↗
NASA NTRS1998

Contamination Observed on the Passive Optical Sample Assembly (POSA)-1 Experiment

The Passive Optical Sample Assembly-1 (POSA-1), part of the Mir Environmental Effects Payload (MEEP), was designed to study the combined effects of contamination, atomic oxygen, ultraviolet radiation, vacuum, thermal cycling, and other constituents of the space environment on spacecraft materials. The MEEP program is a Phase 1 International Space Station Risk Mitigation Experiment. The payload was attached by EVA to the exterior of the Mir docking module during the Space Shuttle mission STS-76. It was removed during STS-86 after exposure to the Mir environment for 18 months. POSA-1 consists of nearly 400 samples of thermal control paints, chemical conversion coatings, mirrors, optics, and other candidate materials for International Space Station Alpha (ISSA). POSA-1 also flew state-of-the-art materials, such as electrically conductive thermal control coatings developed under contract by IITRI and AZ Technology, and passive instruments for monitoring the atomic oxygen and ultraviolet radiation dose to the experiment. Contamination was detected on the POSA-1 experiment. On the side facing space (-Z(sub B) direction using the Mir coordinate system), visible contamination was observed. The contamination was uniform as would occur from a slow photodeposition process. A very definite film was deposited on the optical samples. The deposition appears to have directionality with definite shadowing effects. On the side facing the main Mr core (+ZB direction using the Mir coordinate system), no visible contamination was noted. Pre- and post-flight characterization of candidate spacecraft materials is discussed, particularly the effect of contamination on total hemispherical spectral reflectance (250 - 2500 nm), infrared emittance, the bi-directional reflectance distribution function (BRDF), and vacuum ultraviolet spectral reflectance. Extensive photography under normal and black light was performed. Results of electron spectroscopy for chemical analysis (ESCA) and Fourier Transform Infrared Analysis (FTIR) on selected samples are also discussed.

Zwiener, James M.↗
NASA NTRS1998

X-Ray Emission from the Soft X-Ray Transient Aquila X-1

Aquila X-1 is the most prolific of soft X-ray transients. It is believed to contain a rapidly spinning neutron star sporadically accreting near the Eddington limit from a low-mass companion star. The interest in studying the repeated X-ray outbursts from Aquila X-1 is twofold: (1) studying the relation between optical, soft and hard X-ray emission during the outburst onset, development and decay; (2) relating the spectral component to thermal and non-thermal processes occurring near the magnetosphere and in the boundary layer of a time-variable accretion disk. Our investigation is based on the BATSE monitoring of Aquila X-1 performed by our group. We observed Aquila X-1 in 1997 and re-analyzed archival information obtained in April 1994 during a period of extraordinary outbursting activity of the source in the hard X-ray range. Our results allow, for the first time for this important source, to obtain simultaneous spectral information from 2 keV to 200 keV. A black body (T = 0.8 keV) plus a broken power-law spectrum describe accurately the 1994 spectrum. Substantial hard X-ray emission is evident in the data, confirming that the accretion phase during sub-Eddington limit episodes is capable of producing energetic hard emission near 5 x 10(exp 35) ergs(exp -1). A preliminary paper summarizes our results, and a more comprehensive account is being written. We performed a theoretical analysis of possible emission mechanisms, and confirmed that a non-thermal emission mechanism triggered in a highly sheared magnetosphere at the accretion disk inner boundary can explain the hard X-ray emission. An anticorrelation between soft and hard X-ray emission is indeed prominently observed as predicted by this model.

Tavani, Marco↗
NASA NTRS1999

Improvement of TOPEX/POSEIDON and Jason-1 Geophysical Data Record for Global Change Studies and Coastal Applications

The Earth's modem climate change has been characterized by interlinked changes in temperature, CO2, ice sheets and sea level. Global sea level change is a critical indicator for study of contemporary climate change. Sea level rise appears to have accelerated since the ice sheet retreats have stopped some 5000 years ago and it is estimated that the sea level rise has been approx. 15 cm over the last century. Contemporary radar altimeters represent the only technique capable of monitoring global sea level change with accuracy approaching 1 mm/yr and with a temporal scale of days and a spatial scale of 100 km or longer. This report highlights the major accomplishments of the TOPEX/POSEIDON (T/P) Extended Mission and Jason-1 science investigation. The primary objectives of the investigation include the calibration and improvement of T/P and Jason-1 altimeter data for global sea level change and coastal tide and circulation studies. The scientific objectives of the investigation include: (1) the calibration and improvement of T/P and Jason-1 data as a reference measurement system for the accurate cross-linking with other altimeter systems (Seasat, Geosat, ERS-1, ERS-2, GFO-1, and Envisat), (2) the improved determination and the associated uncertainties of the long-term (15-year) global mean sea level change using multiple altimeters, (3) the characterization of the sea level change by analyses of independent data, including tide gauges, sea surface temperature, and (4) the improvement coastal radar altimetry for studies including coastal ocean tide modeling and coastal circulation. Major accomplishments of the investigation include the development of techniques for low-cost radar altimeter absolute calibration (including the associated GPS-buoy technology), coastal ocean tide modeling, and the linking of multiple altimeter systems and the resulting determination of the 15-year (1985-1999) global mean sea level variations. The current rate of 15-year sea level rise observed by multiple satellite altimetry is +2.3 +/- 1.2 mm/yr, which is in general agreement with the analysis of sparsely distributed tide gauge measurements for the same data span, and represents the first such determination of sea level change in its kind.

Shum, C. K.↗
NASA NTRS1999

Preliminary Assessment of JERS-1 SAR to Discriminating Boreal Landscape Features for the Boreal Forest Mapping Project

This paper presents an overview of the JERS-1 North American Boreal Forest Mapping Project and a preliminary assessment of JERS-1 SAR imagery for application to discriminating features applicable to boreal landscape processes. The present focus of the JERS-1 North American Boreal Forest Mapping Project is the production of continental scale wintertime and summertime SAR mosaics of the North American boreal forest for distribution to the science community. As part of this effort, JERS-1 imagery has been collected over much of Alaska and Canada during the 1997-98 winter and 1998 summer seasons. To complete the mosaics, these data will be augmented with data collected during previous years. These data will be made available to the scientific community via CD ROM containing these and similar data sets compiled from companion studies of Asia and Europe. Regional landscape classification with SAR is important for the baseline information it will provide about distribution of woodlands, positions of treeline, current forest biomass, distribution of wetlands, and extent of major rivercourses. As well as setting the stage for longer term change detection, comparisons across several years provides additional baseline information about short-term landscape change. Rapid changes, including those driven by fire, permafrost heat balance, flooding, and insect outbreaks can dominate boreal systems. We examine JERS-1 imagery covering selected sites in Alaska and Canada to assess quality and applicability to such relevant ecological and hydrological issues. The data are generally of high quality and illustrate many potential applications. A texture-based classification scheme is applied to selected regions to assess the applicability of these data for distinguishing distribution of such landcover types as wetland, tundra, woodland and forested landscapes.

McDonald, Kyle↗
NASA NTRS2001

Fundamental Boiling and RP-1 Freezing Experiments

This paper describes results from experiments performed to help understand certain aspects of the MC-1 engine prestart thermal conditioning procedure. The procedure was constrained by the fact that the engine must chill long enough to get quality LOX at the LOX pump inlet but must be short enough to prevent freezing of RP-1 in the fuel pump. A chill test of an MC-1 LOX impeller was performed in LN2 to obtain data on film boiling, transition boiling and impeller temperature histories. The transition boiling data was important to the chill time so a subsequent experiment was performed chilling simple steel plates in LOX to obtain similar data for LOX. To address the fuel freezing concern, two experiments were performed. First, fuel was frozen in a tray and its physical characteristics were observed and temperatures of the fuel were measured. The result was physical characteristics as a function of temperature. Second was an attempt to measure the frozen thickness of RP-1 on a cold wall submerged in warm RP-1 and to develop a method for calculating that thickness for other conditions.

Goode, Brian↗
NASA NTRS2002

Materials Science Research Rack-1 Fire Suppressant Distribution Test Report

Fire suppressant distribution testing was performed on the Materials Science Research Rack-1 (MSRR-1), a furnace facility payload that will be installed in the U.S. Lab module of the International Space Station. Unlike racks that were tested previously, the MSRR-1 uses the Active Rack Isolation System (ARIS) to reduce vibration on experiments, so the effects of ARIS on fire suppressant distribution were unknown. Two tests were performed to map the distribution of CO2 fire suppressant throughout a mockup of the MSRR-1 designed to have the same component volumes and flowpath restrictions as the flight rack. For the first test, the average maximum CO2 concentration for the rack was 60 percent, achieved within 45 s of discharge initiation, meeting the requirement to reach 50 percent throughout the rack within 1 min. For the second test, one of the experiment mockups was removed to provide a worst-case configuration, and the average maximum CO2 concentration for the rack was 58 percent. Comparing the results of this testing with results from previous testing leads to several general conclusions that can be used to evaluate future racks. The MSRR-1 will meet the requirements for fire suppressant distribution. Primary factors that affect the ability to meet the CO2 distribution requirements are the free air volume in the rack and the total area and distribution of openings in the rack shell. The length of the suppressant flowpath and degree of tortuousness has little correlation with CO2 concentration. The total area of holes in the rack shell could be significantly increased. The free air volume could be significantly increased. To ensure the highest maximum CO2 concentration, the PFE nozzle should be inserted to the stop on the nozzle.

Wieland, P. O.↗
NASA NTRS2002

A Sensitive VLA Search for Small-Scale Glycine Emission Toward OMC-1

We have conducted a deep Q-band (lambda-7 mm) search with the Very Large Array (VLA) toward OMC-1 for the lowest energy conformation (conformer I) of glycine (NH2CH2COOH) in four rotational transitions: the 6(sub 15)- 5(sub 14), 6(sub 24)-5(sub 23), 7(sub 17- 6(sub 16), and 7(sub 07)-6(sub 06). Our VLA observations sample the smallest-scale structures to date in the search for glycine toward OMC-1. No glycine emission features were detected. Thus if glycine exists in OMC-1, either it is below our detection limit, or it is more spatially extended than other large molecules in this source, or it is primarily in its high energy form (conformer II). Our VLA glycine fractional abundance limits in OMC-1 are comparable to those determined from previous IRAM 30m measurements -- somewhat better or worse depending on the specific source model -- and the entire approximately 1 foot primary beam of the VLA was searched while sensitive to an areal spatial scale approximately 150 times smaller than the 24 inch beam of the IRAM single-element telescope. In the course of this work, we detected and imaged the 4(sub 14)-3(sub 13) A and E transitions of methyl formate (HCOOCH3) and also the 2(sub 02) - 1(sub 01) transition of formic acid (HCOOH). Since formic acid is a possible precursor to glycine, our glycine limits and formic acid results provide a constraint on this potential formation chemistry route for glycine in OMC-1.

Hollis, J. M.↗
NASA NTRS2004

Uncertainty Analysis on Heat Transfer Correlations for RP-1 Fuel in Copper Tubing

NASA is studying kerosene (RP-1) for application in Next Generation Launch Technology (NGLT). Accurate heat transfer correlations in narrow passages at high temperatures and pressures are needed. Hydrocarbon fuels, such as RP-1, produce carbon deposition (coke) along the inside of tube walls when heated to high temperatures. A series of tests to measure the heat transfer using RP-1 fuel and examine the coking were performed in NASA Glenn Research Center's Heated Tube Facility. The facility models regenerative cooling by flowing room temperature RP-1 through resistively heated copper tubing. A Regression analysis is performed on the data to determine the heat transfer correlation for Nusselt number as a function of Reynolds and Prandtl numbers. Each measurement and calculation is analyzed to identify sources of uncertainty, including RP-1 property variations. Monte Carlo simulation is used to determine how each uncertainty source propagates through the regression and an overall uncertainty in predicted heat transfer coefficient. The implications of these uncertainties on engine design and ways to minimize existing uncertainties are discussed.

Driscoll, E. A.↗
NASA NTRS2002

Expression of the ctenophore Brain Factor 1 forkhead gene ortholog (ctenoBF-1) mRNA is restricted to the presumptive mouth and feeding apparatus: implications for axial organization in the Metazoa

Ctenophores are thoroughly modern animals whose ancestors are derived from a separate evolutionary branch than that of other eumetazoans. Their major longitudinal body axis is the oral-aboral axis. An apical sense organ, called the apical organ, is located at the aboral pole and contains a highly innervated statocyst and photodetecting cells. The apical organ integrates sensory information and controls the locomotory apparatus of ctenophores, the eight longitudinal rows of ctene/comb plates. In an effort to understand the developmental and evolutionary organization of axial properties of ctenophores we have isolated a forkhead gene from the Brain Factor 1 (BF-1) family. This gene, ctenoBF-1, is the first full-length nuclear gene reported from ctenophores. This makes ctenophores the most basal metazoan (to date) known to express definitive forkhead class transcription factors. Orthologs of BF-1 in vertebrates, Drosophila, and Caenorhabditis elegans are expressed in anterior neural structures. Surprisingly, in situ hybridizations with ctenoBF-1 antisense riboprobes show that this gene is not expressed in the apical organ of ctenophores. CtenoBF-1 is expressed prior to first cleavage. Transcripts become localized to the aboral pole by the 8-cell stage and are inherited by ectodermal micromeres generated from this region at the 16- and 32-cell stages. Expression in subsets of these cells persists and is seen around the edge of the blastopore (presumptive mouth) and in distinct ectodermal regions along the tentacular poles. Following gastrulation, stomodeal expression begins to fade and intense staining becomes restricted to two distinct domains in each tentacular feeding apparatus. We suggest that the apical organ is not homologous to the brain of bilaterians but that the oral pole of ctenophores corresponds to the anterior pole of bilaterian animals.

Non-NASA Center↗
NASA NTRS2001

Connexin45 interacts with zonula occludens-1 in osteoblastic cells

Connexin43 (Cx43) and Cx45 are co-expressed in a number of different tissues. Studies demonstrated that Cx45 transfected ROS (ROS/Cx45) cells, were less permeable to low molecular weight dyes than untransfected ROS cells, that have gap junctions made of Cx43. This suggests that there may be a functionally important interaction between Cx43 and Cx45 in these cells. One way in which these proteins may interact is by associating with the same set of proteins. In order to isolate connexin interacting proteins, we isolated Cx45 from Cx45 transfected ROS cells (ROS/Cx45 cells) under mild detergent conditions. These studies showed that Cx45 co-purified with the tight junction protein, ZO-1. Immunofluorescence studies of ROS/Cx45 cells simultaneously stained with polyclonal Cx45 antibody and a monoclonal ZO-1 antibody showed that Cx45 and ZO-1 colocalized in ROS/Cx45 cells. Furthermore we found that ZO-1 could bind to peptides derived from the carboxyl terminal of Cx45 that had been covalently bound to an agarose resin. These data suggests that Cx45 and ZO-1 directly interact in ROS/Cx45 cells.

Non-NASA Center↗
NASA NTRS1997

Alteration of mineral crystallinity and collagen cross-linking of bones in osteopetrotic toothless (tl/tl) rats and their improvement after treatment with colony stimulating factor-1

A common feature of various types of mammalian osteopetroses is a marked increase in bone mass accompanied by spontaneous bone fractures. The toothless (tl/tl) rat osteopetrotic mutation is characterized by drastically reduced bone resorption due to a profound deficiency of osteoclasts and their precursors. An altered bone morphology has also been observed. The mutants cannot be cured by bone marrow transplantation, but skeletal defects are greatly reduced after treatment with colony stimulating factor 1 (CSF-1). The objectives of this study were to characterize mineral and collagen matrices in cancellous and compact bone isolated from long bones of 6-week-old normal littermates, tl/tl osteopetrotic mutants and mutants (tl/tl) treated with CSF-1. There were no differences in bone mineral content, but a significant decrease in the crystallinity of mineral evaluated by the method based on electron paramagnetic resonance spectrometry was observed in all bones of tl/tl mutants as compared to that of controls. Within the collagen matrix, slight decreases in the labile cross-links, but significant increases in the content of the stable cross-links, pyridinoline, and deoxypyridinoline, were observed in both cancellous and compact bone of osteopetrotic mutants. In tl/tl mutants treated with human recombinant CSF-1, the normalization of the crystallinity of bone mineral as well as collagen cross-links was found. Our results indicate that remodeling of bone matrix in tl/tl mutants is highly suppressed, but that after treatment with CSF-1, this activity recovers significantly. Taken together, these data provide further support for the hypothesis that CSF-1 is an essential factor for normal osteoclast differentiation and bone remodelling.

NASA Discipline Musculoskeletal↗
NASA NTRS2005

Biomechanical Evaluation of Russian BD-1 Treadmill during Non-Motorized Treadmill Locomotion in a Weightless Environment (KC-135)

Subjects are able to successfully perform locomotion exercise on the BD-1 treadmill in weightlessness. The BD-1 bungee system has limited adjustability and provides higher loading for small subjects and lower loading for larger subjects. Some subjects have difficulty reaching and maintaining higher speeds starting at around 14 km/h. The ground reaction force profile for BD-1 locomotion shows peak propulsive forces comparable to 1g, but the heel strike peak appears to be absent during running on the BD-1 in weightlessness. These observations have implications for exercise prescriptions for BD-1 treadmill use on-board ISS.

Schaffner, Grant↗
NASA NTRS2005

JSC-1: Lunar Simulant of Choice for Geotechnical Applications and Oxygen Production

Lunar simulant JSC-1 was produced as the result of a workshop held in 1991 to evaluate the status of simulated lunar material and to make recommendations on future requirements and production of such material (McKay et al., 1991). JSC-1 was prepared from a welded tuff that was mined, crushed, and sized from the Pleistocene San Francisco volcanic field, northern Arizona. As the initial production of approxiamtely 12,300kgs is nearly depleted, new production has commenced. The mineralogy and chemical properties of JSC-1 are described in McKay et al. (1994) and Hill et al. (this volume); description of its geotechnical properties appears in Klosky et al. (1996). Although other lunar-soil simulants have been produced (e.g., MLS-1: Weiblen et al., 1990; Desai et al., 1992; Chua et al., 1994), they have not been as well standardized as JSC-I; this makes it difficult to standardize results from tests performed on these simulants. Here, we provide an overview of the composition, mineralogy, strength and deformation properties, and potential uses of JSC-1 and outline why it is presently the 'lunar simulant of choice' for geotechnical applications and as a proxy for lunar-oxygen production.

Taylor, Lawrence A.↗
NASA NTRS2005

Comparison of Morphologies of Apollo 17 Dust Particles with Lunar Simulant, JSC-1

Lunar dust (< 20 microns) makes up approx.20 wt.% of the lunar soil. Because of the abrasive and adhering nature of lunar soil, a detailed knowledge of the morphology (size, shape and abundance) of lunar dust is important for dust mitigation on the Moon. This represents a critical step towards the establishment of long-term human presence on the Moon (Taylor et al. 2005). Machinery design for in-situ resource utilization (ISRU) on the Moon also requires detailed information on dust morphology and general physical/chemical characteristics. Here, we report a morphological study of Apollo 17 dust sample 70051 and compare it to lunar soil stimulant, JSC-1. W e have obtained SEM images of dust grains from sample 70051 soil (Fig. 1). The dust grains imaged are composed of fragments of minerals, rocks, agglutinates and glass. Most particles consist largely of agglutinitic impact glass with their typical vesicular textures (fine bubbles). All grains show sub-angular to angular shapes, commonly with sharp edges, common for crushed glass fragments. There are mainly four textures: (1) ropey-textured pieces (typical for agglutinates), (2) angular shards, (3) blocky bits, and (4) Swiss-cheese grains. This last type with its high concentration of submicron bubbles, occurs on all scales. Submicron cracks are also present in most grains. Dust-sized grains of lunar soil simulant, JSC-1, were also studied. JSC-1 is a basaltic tuff with relatively high glass content (approx.50%; McKay et al. 1994). It was initially chosen in the early 90s to approximate the geotechnical properties of the average lunar soil (Klosky et al. 1996). JSC-1 dust grains also show angular blocky and shard textures (Fig. 2), similar to those of lunar dust. However, the JSC-1 grains lack the Swiss-cheese textured particles, as well as submicron cracks and bubbles in most grains.

Liu, Yang↗
NASA NTRS1994

(abstract) Monitoring the Freeze/Thaw Transitions in Taiga Forests Using ERS-1 SAR

Automated recording stations have been installed at the Bonanza Creek Experimental Forest, a Long Term Ecological Research (LTER) site located near Fairbanks, Alaska, in a forest stand of the Tanana River floodplain underlain by discontinuous permafrost. These stations provide a continuous record of dielectric constant and temperature of tree trunks, and soil moisture and temperature profiles down to the root zone. Along with the weather stations deployed at the same location, these measurements provide a continuous record of the environmental and phenologic conditions of the forest during a complete seasonal cycle. At the same time, ERS-1 SAR imaged the study site repeatedly from space to provide radar backscatter measurements of the forest approximately three times a month. Here, we examine the temporal dynamic of ERS-1 SAR measurements in relation with the changing environmental and phenologic state of the forest canopy and of the forest ground layers during the winter/spring and fall/winter transitions of 1992 and 1993. During these transitions, we examine whether changes in radar backscatter observed by ERS-1 may be related to freezing or thawing of the soil and vegetation in order to determine the start and end of the growing season for the forest. The results of this analysis are used in turn to determine whether similar changes are observed over larger regions. Mosaics of SAR data generated along three different North-South Alaskan ERS-1 transects that intercept with our study site are used in combination with hourly air temperature and daily precipitation rates gathered at airport weather stations by the National Weather Service. Results obtained using ERS-1 data collected from January 1992 to mid-1993 will be discussed.

permafrost temporal dynamics Alaska ecology radar ↗