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21.
Gordon H. Pettengill Steven J. Ostro Irwin I. Shapiro Brian G. Marsden Donald B. Campbell 《Icarus》1979,40(3):350-354
Radar observations of the asteroid 1580 Betulia, made at a wavelength of 12.6 cm, show a mean radar cross section of 2.2 ± 0.8 km2 and a total spectral bandwidth of 26.5 ± 1.5 Hz. Combining our bandwidth measurements with the optically determined rotation period sets a lower limit to the asteroid's radius of 2.9 ± 0.2 km. 相似文献
22.
A comparison is made between the flux-versus-time profile in the EUV band and the thick target electron flux profile as inferred from hard X-rays for a number of moderately large solar flares. This complements Kane and Donnelly's (1971) study of small flares. The hard X-ray data are from ESRO TD-1A and the EUV inferred from SFD observations.Use of a 2 minimising method shows that the best overall fit between the profile fine structures obtains for synchronism to 5 s which is within the timing accuracy. This suggests that neither conduction nor convection is fast enough as the primary mechanism of energy transport into the EUV flare and rather favours heating by the electrons themselves or by some MHD wave process much faster than acoustic waves.The electron power deposited, for a thick target model, is however far greater than the EUV luminosity for any reasonable assumptions about the area and depth over which EUV is emitted. This means that either most of the power deposited is conducted away to the optical flare or that only a fraction 1–10% of the X-ray emitting electrons are injected downwards. Recent work on H flare heating strongly favours the latter alternative - i.e. that electrons are mostly confined in the corona. 相似文献
23.
Surface sediments from 20 stations on the Scotian Shelf, collected on a transect from Halifax to Emerald Bank and around Sable Island, have been analyzed for hydrocarbon content and composition by gas chromatography and fluorescence spectrophotometry. Some samples were taken near abandoned exploratory drilling sites in the Sable Island area. Hydrocarbons appear to be mainly derived from biogenic terrestrial sources as evidenced by an inverse correlation of concentration with increasing distance from the mainland and a strong odd carbon preference in the n-alkanes. Contribution from petroleum sources, while minor, was most noticeable between Halifax and Emerald Bank. There is also evidence that the hydrocarbon composition at abandoned exploratory drilling sites has been slightly altered. 相似文献
24.
Donald B. Campbell John F. Chandler Steven J. Ostro Gordon H. Pettengill Irwin I. Shapiro 《Icarus》1978,34(2):254-267
Radar observations of the Galilean satellites, made in late 1976 using the 12.6-cm radar system of the Arecibo Observatory, have yielded mean geometric albedos of 0.04 ± , 0.69 ± 0.17, 0.37 ± 0.09, and 0.15 ± 0.04, for Io, Europa, Ganymede, and Callisto, respectively. The albedo for Io is about 40% smaller than that obtained approximately a year earlier, while the albedos for the outer three satellites average about 70% larger than the values previously reported for late 1975, raising the possibility of temporal variation. Very little dependence on orbital phase is noted; however, some regional scattering inhomogeneities are seen on the outer three satellites. For Europa, Ganymede, and Callisto, the ratios of the echo received in one mode of circular polarization to that received in the other were: 1.61 ± 0.20 1.48 ± 0.27, and 1.24 ± 0.19, respectively, with the dominant component having the same sence of circularity as that transmitted. This behavior has not previously been encountered in radar studies of solar system objects, whereas the corresponding observations with linear polarization are “normal.” Radii determined from the 1976 radar data for Europa and Ganymede are: 1530 ± 30 and 2670 ± 50 km, in fair agreement with the results from the 1975 radar observations and the best recent optical determinations. Doppler shifts of the radar echoes, useful for the improvement of the orbits of Jupiter and some of the Galilean satellites, are given for 12 nights in 1976 and 10 nights in 1975. 相似文献
25.
Abstract: This paper discusses three walking tour guides prepared for the 12th International Conference of Historical Geographers. It argues that the guides are innovative in terms of format, content and ironic sensibility. It construes them as devices for re-imagining Auckland's urban heritage. 相似文献
26.
John E. Gordon Igor J. Dvorák Christer Jonasson Melanie Josefsson Milena Kociánová & Des B.A. Thompson 《Geografiska Annaler: Series A, Physical Geography》2002,84(3&4):193-203
Montane (alpine) areas are generally of high value for nature conservation. Such environments and the habitats they support are dynamic and often fragile. They are vulnerable to disturbance from a range of human activities and are responsive to climate changes over short and long timescales. Biodiversity and conservation values are closely linked to geological history, geomorphological processes and soils, and it is crucial that management systems are based on understanding these links.
There are many similarities between the Cairngorm Mountains (Scotland), the Giant Mountains (Czech Republic) and Abisko Mountains (Sweden) in terms of geology, geomorphology, ecology, links with biodiversity and high conservation importance. Comparable pressures and management issues involve, to varying degrees, a history of human use and impacts from deforestation, pasturing, grazing, recreation and atmospheric pollution. Landscape change therefore involves a complex interplay between natural and anthropogenic factors. Managing such change requires better understanding of the geo–ecological processes involved and the factors that determine landscape sensitivity. This is illustrated through a simple framework and examples from the three areas. Comparison of landscape sensitivity between similar montane areas, but in different geographic locations and climatic environments, should allow more informed management planning and a precautionary approach in advance of further changes in human activity and from predicted global warming scenarios. 相似文献
There are many similarities between the Cairngorm Mountains (Scotland), the Giant Mountains (Czech Republic) and Abisko Mountains (Sweden) in terms of geology, geomorphology, ecology, links with biodiversity and high conservation importance. Comparable pressures and management issues involve, to varying degrees, a history of human use and impacts from deforestation, pasturing, grazing, recreation and atmospheric pollution. Landscape change therefore involves a complex interplay between natural and anthropogenic factors. Managing such change requires better understanding of the geo–ecological processes involved and the factors that determine landscape sensitivity. This is illustrated through a simple framework and examples from the three areas. Comparison of landscape sensitivity between similar montane areas, but in different geographic locations and climatic environments, should allow more informed management planning and a precautionary approach in advance of further changes in human activity and from predicted global warming scenarios. 相似文献
27.
28.
Smart optics means much more than adaptive optics on telescopes: these new technologies are changing the way space instruments are built and operated and are bringing new technologies into everyday life in the form of cheaper, lighter and more robust optical systems. Steve Welch, Peter Doel, Alan Greenaway and Gordon Love summarize the work of the Smart Optics Faraday Partnership in the UK. 相似文献
29.
Zachary A. Garbow Gordon Erlebacher David A. Yuen John M. Boggs Fabien W. Dubuffet 《Visual Geosciences》2003,8(1):1-20
This paper describes the issues related to using handheld devices to interrogate wirelessly large-scale geophysical data over the Web using a client–server configuration. A modification of our current Web-based data interrogation program addresses specifically the limitations of current handheld devices with wireless access, such as processor speed, screen size and resolution, memory, and network speeds. The system uses a combination of Java, Python, and C in order to run remotely from the handheld device, while incorporating the power and memory capacity of a large workstation server. 相似文献
30.
Boreal temperature variability inferred from maximum latewood density and tree-ring width data, Wrangell Mountain region, Alaska 总被引:3,自引:0,他引:3
Variations in both width and density of annual rings from a network of tree chronologies were used to develop high-resolution proxies to extend the climate record in the Wrangell Mountain region of Alaska. We developed a warm-season (July–September) temperature reconstruction that spans A.D. 1593–1992 based on the first eigenvector from principal component analysis of six maximum latewood density (MXD) chronologies. The climate/tree-growth model accounts for 51% of the temperature variance from 1958 to 1992 and shows cold in the late 1600s–early 1700s followed by a warmer period, cooling in the late 1700s–early 1800s, and warming in the 20th century. The 20th century is the warmest of the past four centuries. Several severely cold warm-seasons coincide with major volcanic eruptions. The first eigenvector from a ring-width (RW) network, based on nine chronologies from the Wrangell Mountain region (A.D. 1550–1970), is correlated positively with both reconstructed and recorded Northern Hemisphere temperatures. RW shows a temporal history similar to that of MXD by increased growth (warmer) and decreased growth (cooler) intervals and trends. After around 1970 the RW series show a decrease in growth, while station data show continued warming, which may be related to increasing moisture stress or other factors. Both the temperature history based on MXD and the growth trends from the RW series are consistent with well-dated glacier fluctuations in the Wrangell Mountains and some of the temperature variations also correspond to variations in solar activity. 相似文献