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131.
Tetsuo Hasegawa Takaaki Arai Nobuyuki Yamaguchi Fumio Sato 《Astrophysics and Space Science》2008,313(1-3):91-94
We report mapping observations of a 35 pc × 35 pc region covering the Sgr B2 molecular cloud complex in the 13CO (3-2) and the CS (7-6) lines using the ASTE 10 m telescope with high angular resolution. The central region was mapped
also in the C18O (3-2) line. The images not only reproduce the characteristic structures noted in the preceding millimeter observations,
but also highlight the interface of the molecular clouds with a large velocity jump of a few tens of km s−1. These new results further support the scenario that a cloud–cloud collision has triggered the formation of massive cloud
cores, which form massive stars of Sgr B2. Prospects of exciting science enabled by ALMA are discussed in relation to these
observations. 相似文献
132.
Lingyu Wang Michael Rowan-Robinson Issei Yamamura Hiroshi Shibai Rich Savage Seb Oliver Matthew Thomson Nurur Rahman Dave Clements Elysandra Figueredo Tomotsugu Goto Sunao Hasegawa Woong-Seob Jeong Shuji Matsuura Thomas G. Müller Takao Nakagawa Chris P. Pearson Stephen Serjeant Mai Shirahata Glenn J. White 《Monthly notices of the Royal Astronomical Society》2008,387(2):601-615
We present a careful analysis of the point-source detection limit of the AKARI All-Sky Survey in the WIDE-S 90-μm band near the North Ecliptic Pole (NEP). Timeline analysis is used to detect IRAS ( Infrared Astronomy Satellite ) sources and then a conversion factor is derived to transform the peak timeline signal to the interpolated 90-μm flux of a source. Combined with a robust noise measurement, the point-source flux detection limit at signal-to-noise ratio (S/N) > 5 for a single detector row is 1.1 ± 0.1 Jy which corresponds to a point-source detection limit of the survey of ∼0.4 Jy.
Wavelet transform offers a multiscale representation of the Time Series Data ( tsd ). We calculate the continuous wavelet transform of the tsd and then search for significant wavelet coefficients considered as potential source detections. To discriminate real sources from spurious or moving objects, only sources with confirmation are selected. In our multiscale analysis, IRAS sources selected above 4σ can be identified as the only real sources at the Point Source Scales. We also investigate the correlation between the non- IRAS sources detected in timeline analysis and cirrus emission using wavelet transform and contour plots of wavelet power spectrum. It is shown that the non- IRAS sources are most likely to be caused by excessive noise over a large range of spatial scales rather than real extended structures such as cirrus clouds. 相似文献
Wavelet transform offers a multiscale representation of the Time Series Data ( tsd ). We calculate the continuous wavelet transform of the tsd and then search for significant wavelet coefficients considered as potential source detections. To discriminate real sources from spurious or moving objects, only sources with confirmation are selected. In our multiscale analysis, IRAS sources selected above 4σ can be identified as the only real sources at the Point Source Scales. We also investigate the correlation between the non- IRAS sources detected in timeline analysis and cirrus emission using wavelet transform and contour plots of wavelet power spectrum. It is shown that the non- IRAS sources are most likely to be caused by excessive noise over a large range of spatial scales rather than real extended structures such as cirrus clouds. 相似文献
133.
Keika Horiguchi Sadato Ueki Yuji Sano Naoto Takahata Akira Hasegawa George Igarashi 《Island Arc》2010,19(1):60-70
In order to study the precise geographical distribution of helium isotope ratios in northeastern Japan and compare it with geophysical data, we collected 43 gas and water samples from hot and mineral springs in the region where the ratio had never been reported, and measured the 3 He/4 He and 4 He/20 Ne ratios of these samples. It was found that the 3 He/4 He ratios show clear contrasts between the forearc and the back-arc regions in the Tohoku district in northeastern Japan. In the forearc region, the ratios are smaller than 1 RA (1 RA = 1.4 × 10−6 ; RA means the 3 He/4 He ratio of the atmosphere). On the other hand, those along the volcanic front and in the back-arc region are apparently higher. Moreover, we found a variation in the 3 He/4 He ratios along the volcanic front. In Miyagi Prefecture (38–39°N), the ratios range from 2 to 5 RA . On the other hand, the ratios are less than 1 RA in and around the southern border between Iwate and Akita Prefectures (39–39.5°N). Comparing the distribution of helium isotope ratios to results of recent geophysical studies, we found that the features in geographical distribution of helium isotope ratios are similar to those of seismic low-velocity zone distributions and high Qp−1 distributions in the uppermost mantle. These observations strongly suggest that the helium isotope ratios reflect the distribution of melts in the uppermost mantle and are a useful tool for investigating the origin, behavior, and distribution of deep fluids and melts. 相似文献
134.
Antidunes and their sedimentary structures can be useful in reconstructing paleo‐hydraulic conditions, especially for large discharge events. However, three‐dimensional (3D) antidunes in sand‐sized sediments have not yet been studied extensively, as compared to either two‐dimensional (2D) antidunes or antidunes in gravel‐sized sediments. In this study, we estimated formative conditions of gravel step‐pool morphologies and applied them to the formation of 3D antidunes over a sand bed. Formative conditions are expressed in terms of a relationship between the water discharge per unit width and the bed slope. Flume experiments demonstrated that 3D mound‐like antidune configurations and their associated internal sedimentary structures could be preserved. Internal sedimentary structures were characterized by shallow lens‐like structures whose bases were erosional. Although gently‐dipping concave‐upward lamination was dominant, convex‐upward lamination was occasionally observed. The dimensions of lenticular lamina‐sets can be used to estimate antidune geometry. Thus if 3D antidunes can be interpreted in the stratigraphic record, it is possible to estimate the paleo‐hydraulic parameters such as water discharge and bed slope more precisely than previously. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
135.
Representative rainfall thresholds for landslides in the Nepal Himalaya 总被引:14,自引:0,他引:14
Measuring some 2400 km in length, the Himalaya accommodate millions of people in northern India and Pakistan, Nepal, Bhutan, and parts of other Asian nations. Every year, especially during monsoon rains, landslides and related natural events in these mountains cause tremendous damage to lives, property, infrastructure, and environment. In the context of the Himalaya, however, the rainfall thresholds for landslide initiation are not well understood. This paper describes regional aspects of rainfall thresholds for landslides in the Himalaya. Some 677 landslides occurring from 1951 to 2006 were studied to analyze rainfall thresholds. Out of the 677 landslides, however, only 193 associated with rainfall data were analyzed to yield a threshold relationship between rainfall intensity, rainfall duration, and landslide initiation. The threshold relationship fitted to the lower boundary of the field defined by landslide-triggering rainfall events is I = 73.90D− 0.79 (I = rainfall intensity in mm h− 1 and D = duration in hours), revealing that when the daily precipitation exceeds 144 mm, the risk of landslides on Himalayan mountain slopes is high. Normalized rainfall intensity–duration relationships and landslide initiation thresholds were established from the data after normalizing rainfall-intensity data with respect to mean annual precipitation (MAP) as an index in which NI = 1.10D− 0.59 (NI = normalized intensity in h− 1). Finally, the role of antecedent rainfall in causing landslides was also investigated by considering daily rainfall during failure and the cumulative rainfall to discover at what point antecedent rainfall plays an important role in Himalayan landslide processes. Rainfall thresholds presented in this paper are generalized so they can be used in landslide warning systems in the Nepal Himalaya. 相似文献
136.
Predictive modelling of rainfall-induced landslide hazard in the Lesser Himalaya of Nepal based on weights-of-evidence 总被引:12,自引:0,他引:12
Ranjan Kumar Dahal Shuichi Hasegawa Atsuko Nonomura Minoru Yamanaka Santosh Dhakal Pradeep Paudyal 《Geomorphology》2008,102(3-4):496-510
Landslide hazard mapping is a fundamental tool for disaster management activities in mountainous terrains. The main purpose of this study is to evaluate the predictive power of weights-of-evidence modelling in landslide hazard assessment in the Lesser Himalaya of Nepal. The modelling was performed within a geographical information system (GIS), to derive a landslide hazard map of the south-western marginal hills of the Kathmandu Valley. Thematic maps representing various factors (e.g., slope, aspect, relief, flow accumulation, distance to drainage, soil depth, engineering soil type, landuse, geology, distance to road and extreme one-day rainfall) that are related to landslide activity were generated, using field data and GIS techniques, at a scale of 1:10,000. Landslide events of the 1970s, 1980s, and 1990s were used to assess the Bayesian probability of landslides in each cell unit with respect to the causative factors. To assess the accuracy of the resulting landslide hazard map, it was correlated with a map of landslides triggered by the 2002 extreme rainfall events. The accuracy of the map was evaluated by various techniques, including the area under the curve, success rate and prediction rate. The resulting landslide hazard value calculated from the old landslide data showed a prediction accuracy of > 80%. The analysis suggests that geomorphological and human-related factors play significant roles in determining the probability value, while geological factors play only minor roles. Finally, after the rectification of the landslide hazard values of the new landslides using those of the old landslides, a landslide hazard map with > 88% prediction accuracy was prepared. The methodology appears to have extensive applicability to the Lesser Himalaya of Nepal, with the limitation that the model's performance is contingent on the availability of data from past landslides. 相似文献
137.
Two-dimensional coda Q structure beneath Tohoku, NE Japan 总被引:1,自引:1,他引:0
138.
Ichiro Hasegawa 《Earth, Moon, and Planets》1995,71(3):311-316
Chinese and Japanese historical records of daytime fireballs, and world-wide daytime meteorite falls in the catalogue have been investigated. Among them, there are 253 and 104 records of great daytime fireballs in China and in Japan respectively, and 506 meteorite falls in the daytime are recorded in the Catalogue of Meteorites (1985).The same trends of seasonal and daily variations in the flux of daytime fireballs are clearly seen in both Chinese and Japanese records, and then the distributions of the daytime fireballs seem to suggest the association with meteorites and near-earth minor planets rather than with comets.Possible relations with minor planets, such as (1566) Icarus, (3671) Dionysius, (4450) Pan, (4486) Mithra and others are suggested. 相似文献
139.
Ichiro Hasegawa 《Celestial Mechanics and Dynamical Astronomy》1992,54(1-3):129-142
Numbers of meteors recorded in Chinese and Japanese histories are counted. Two distinct maxima in Chinese records are found in the 11th and 15th centuries, and the latter is also recorded in Japan. Of those records, numbers of bright meteors with sound and great fireballs that appeared in the daytime are also investigated.Correlations between the meteor numbers and the apparitions of naked-eye comet likely to be found, and seasonal variations in the meteor flux recorded during nineteen centuries show two maxima in July–August and October–November, the latter may be related to the Taurid complex. 相似文献
140.