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11.
Sonia Tonarini Igor M. Villa Felix Oberli Martin Meier David A. Spencer Ugo Pognante John G. Ramsay 《地学学报》1993,5(1):13-20
A geochronological investigation of two rocks with an eclogitic assemblage (omphacite-garnet-quartz-rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and Ar/Ar methods is presented here. The first three methods outline a cooling history from the time of peak metamorphism at 49±6 Ma recorded by Sm/Nd in garnet-clinopyroxene to the closure of Rb/Sr in phengite at 43±1 Ma and U/Pb in rutile at 39–40 Ma. The Sm/Nd isotopic system was fully equilibrated during eclogitization and has not been disturbed since; its mineral ages may date the peak metamorphic conditions (650±50°C at 13–18 kbar: Pognante and Spencer, 1991). The Ar/Ar data reveal the presence of substantial amounts of excess 40Ar in hornblende, and yield a statistically acceptable but geologically meaningless phengite plateau age of 81.4±0.2 Ma, inconsistent with Sm/Nd, Rb/Sr and U/Pb. This questions the use of such a chronometer for the dating of high-pressure assemblages. The results imply a Late Palaeocene or Early Eocene subduction of the northern Indian plate margin in NW Himalaya. The fact that eclogites are restricted to NW Himalaya may be the result of a peculiar p-T-t path associated with a high convergence rate during the first indentation, in contrast to the later and slow subduction in Central and Eastern Himalaya. 相似文献
12.
The first observations of equatorial ionospheric emissions in the 800–1050 Å spectral region have been made from the STP 72-1 satellite. Analysis of these data and comparison with data taken simultaneously in the 1220–1400 Å wavelength range indicate a strong correlation between these emissions, as well as a pronounced dependence on season and dip latitude. Both nadir data and spin data are presented, and analyses of spin data indicate latitudinal variations of the F2 peak altitude. Theoretical calculations of emission in the 800–1050 Å spectral region show that the data are consistent with radiative recombination of O+ as a source of the observed emissions. 相似文献
13.
The TerraSAR-X (TSX) synthetic aperture radar (SAR) marks the recent emergence of a new generation of spaceborne radar sensors
that can for the first time lay claim to localization accuracies in the sub-meter range. The TSX platform’s extremely high
orbital stability and the sensor’s hardware timing accuracy combine to enable direct measurements of atmospheric refraction
and solid Earth movements. By modeling these effects for individual TSX acquisitions, absolute pixel geolocation accuracy
on the order of several centimeters can be achieved without need for even a single tiepoint. A 16-month time series of images
was obtained over a fixed test site, making it possible to validate both an atmospheric refraction and a solid Earth tide
model, while at the same time establishing the instrument’s long-term stability. These related goals were achieved by placing
trihedral corner reflectors (CRs) at the test site and estimating their phase centers with centimeter-level accuracy using
differential GPS (DGPS). Oriented in pairs toward a given satellite track, the CRs could be seen as bright “points” in the
images, providing a geometric reference set. SAR images from the high-resolution spotlight (HS) mode were obtained in alternating
ascending and descending orbit configurations. The highest-resolution products were selected for their small sample dimensions,
as positions can be more precisely determined. Based on the delivered product annotations, the CR image positions were predicted,
and these predictions were compared with their measured image positions both before and after compensation for atmospheric refraction and systematic solid Earth deviations. It was possible to show that when the atmospheric
distortion and Earth tides are taken into account, the TSX HS products have geolocation accuracies far exceeding the specified
requirements. Furthermore, this accuracy was maintained for the duration of the 16-month test period. It could be demonstrated
that with a correctly calibrated sensor, and after accounting for atmospheric and tidal effects, tiepoint-free geolocation
is possible with TSX with an absolute product accuracy of about 5 cm. 相似文献
14.
Ralf Döscher Klaus Wyser H. E. Markus Meier Minwei Qian René Redler 《Climate Dynamics》2010,34(7-8):1157-1176
The relative importance of regional processes inside the Arctic climate system and the large scale atmospheric circulation for Arctic interannual climate variability has been estimated with the help of a regional Arctic coupled ocean-ice-atmosphere model. The study focuses on sea ice and surface climate during the 1980s and 1990s. Simulations agree reasonably well with observations. Correlations between the winter North Atlantic Oscillation index and the summer Arctic sea ice thickness and summer sea ice extent are found. Spread of sea ice extent within an ensemble of model runs can be associated with a surface pressure gradient between the Nordic Seas and the Kara Sea. Trends in the sea ice thickness field are widely significant and can formally be attributed to large scale forcing outside the Arctic model domain. Concerning predictability, results indicate that the variability generated by the external forcing is more important in most regions than the internally generated variability. However, both are in the same order of magnitude. Local areas such as the Northern Greenland coast together with Fram Straits and parts of the Greenland Sea show a strong importance of internally generated variability, which is associated with wind direction variability due to interaction with atmospheric dynamics on the Greenland ice sheet. High predictability of sea ice extent is supported by north-easterly winds from the Arctic Ocean to Scandinavia. 相似文献
15.
Thomas N. Woods Phillip C. Chamberlin W. K. Peterson R. R. Meier Phil G. Richards Douglas J. Strickland Gang Lu Liying Qian Stanley C. Solomon B. A. Iijima A. J. Mannucci B. T. Tsurutani 《Solar physics》2008,250(2):235-267
Solar soft X-ray (XUV) radiation is highly variable on all time scales and strongly affects Earth’s ionosphere and upper atmosphere;
consequently, the solar XUV irradiance is important for atmospheric studies and for space weather applications. Although there
have been several recent measurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially
its variability during flares, has been hampered by the broad bands measured in the XUV range. In particular, the simple conversion
of the XUV photometer signal into irradiance, in which a static solar spectrum is assumed, overestimates the flare variations
by more than a factor of two as compared to the atmospheric response to the flares. To address this deficiency in the simple
conversion, an improved algorithm using CHIANTI spectral models has been developed to process the XUV Photometer System (XPS)
measurements with its broadband photometers. Model spectra representative of quiet Sun, active region, and flares are combined
to match the signals from the XPS and produce spectra from 0.1 to 40 nm in 0.1-nm intervals for the XPS Level 4 data product.
The two XPS instruments are aboard NASA’s Solar Radiation and Climate Experiment (SORCE) and Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) satellites. In addition, the XPS responsivities have been updated for the latest XPS data processing version. The
new XPS results are consistent with daily variations from the previous simple conversion technique used for XPS and are also
consistent with spectral measurements made at wavelengths longer than 27 nm. Most importantly, the XPS flare variations are
reduced by factors of 2 – 4 at wavelengths shorter than 14 nm and are more consistent, for the first time, with atmospheric
response to solar flares. Along with the details of the new XPS algorithm, several comparisons to dayglow and photoelectron
measurements and model results are also presented to help verify the accuracy of the new XUV irradiance spectra. 相似文献
16.
Mark B. Dyurgerov & Mark F. Meier 《Geografiska Annaler: Series A, Physical Geography》1999,81(4):541-554
Seasonal mass balance components bw (winter balance) and bs (summer balance) as well as ct (total accumulation) and at (total ablation), can be used directly to infer climate variables. In contrast, ac (net balance of the accumulation area) and aa (net balance of the ablation area), and ba or bn (annual or net balance) can not. The traditional Alpine system of observations of ac and aa , however, can be converted to true seasonal values bw and bs if both pairs of components are simultaneously observed for some years, because a correlation between the two pairs of components exists. We analyzed bw and bs data and their mean, standard deviations and ratios of these to the corresponding net or annual balances for 50 glaciers with relatively long records representing different regions in the northern hemisphere. We also investigated correlations between seasonal components. A negative correlation between bw and bs exists at many glaciers. About two-thirds of the glaciers show insignificant correlations (?0.3 < r < 0.3), implying independence of summer and winter balances. In a few unusual cases the correlations are positive. These different correlations, or lack thereof, may offer insight into feedback conditions that must exist in this climate-related system. The correspondence of the bw and ct , and bs and at , appears to depend largely on the relative amounts of summer snowfall, a function of their climatic environment expressed as [α = (bw+bs)/2]. The contribution of variability of bs to the net balance increases markedly with decreasing values of α. The variability of bw and bs , and therefore the net balance, has been increasing with time; whether this is due to an increase in climate variability or to other causes is not clear. It appears that bw has been increasing with time at the highest altitudes, but bs has been increasing more rapidly especially at low altitudes; the many-glacier average net balance is becoming more negative. 相似文献
17.
We dispute the claim by Alexeev (2010) that the numerous fossil meteorites found in mid-Ordovician sediments in southern Sweden
can be explained by a single large meteorite shower some 470 million years ago, and—in particular—that the reported variable
cosmic ray exposure ages derived from cosmogenic 21Ne in chromite grains extracted from the fossil meteorites actually reflect variable contributions of nucleogenic 21Ne instead. 相似文献
18.
Monika Bischoff Alpan Cete Ralf Fritschen Thomas Meier 《Pure and Applied Geophysics》2010,167(1-2):63-75
Over the last 25 years mining-induced seismicity in the Ruhr area has continuously been monitored by the Ruhr-University Bochum. About 1,000 seismic events with local magnitudes between 0.7 ≤ M L ≤ 3.3 are located every year. For example, 1,336 events were located in 2006. General characteristics of induced seismicity in the entire Ruhr area are spatial and temporal correlation with mining activity and a nearly constant energy release per unit time. This suggests that induced stresses are released rapidly by many small events. The magnitude–frequency distribution follows a Gutenberg–Richter relation which is a result from combining distributions of single longwalls that themselves show large variability. A high b-value of about 2 was found indicating a lack of large magnitude events. Local analyses of single longwalls indicate that various factors such as local geology and mine layout lead to significant differences in seismicity. Stress redistribution acts very locally since differences on a small scale of some hundreds of meters are observed. A regional relation between seismic moment M 0 and local magnitude M L was derived. The magnitude–frequency distribution of a single longwall in Hamm was studied in detail and shows a maximum at M L = 1.4 corresponding to an estimated characteristic source area of about 2,200 m2. Sandstone layers in the hanging or foot wall of the active longwall might fail in these characteristic events. Source mechanisms can mostly be explained by shear failure of two different types above and below the longwall. Fault plane solutions of typical events are consistent with steeply dipping fracture planes parallel to the longwall face and nearly vertical dislocation in direction towards the goaf. We also derive an empirical relation for the decay of ground velocity with epicenter distance and compare maximum observed ground velocity to local magnitude. This is of considerable public interest because about 30 events larger than M L ≥ 1.2 are felt each month by people living in the mining regions. Our relations, for example, indicate that an event in Hamm with a peak ground velocity of 6 mm/s which corresponds to a local magnitude M L between 1.7 and 2.3 is likely to be felt within about 2.3 km radius from the event. 相似文献
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