VIDEO AND PHOTOGRAPHIC SPECTROSCOPY OF 1998 AND 2001 LEONID PERSISTENT TRAINS FROM 300 TO 930 nm

Spectra of persistent meteor trains were observed at wavelength between 300 and 930 nm. Two obtained train spectra during the 1998 and 2001 Leonid meteor showers are reported here. During the 1998 Leonids, one train was detected by a photographic camera with a spectrograph covering 370–640 nm region. On the other hand, during the 2001 Leonids, video observations were carried out using image intensified cameras in ultraviolet (UV), visible and near infrared (near-IR) wavelengths. Temperatures in persistent trains have been measured by atmospheric O₂ A(0,1) band at the wavelength near 864.5 nm. From a video spectrum obtained just 7 s after parent fireball’s flare, a rotational temperature of 250 K at altitude of 88.0±0.5 km was estimated. We can say that the cooling time scale of train strongly depends on the initial mass of its fireball at least for Leonids. Based on cooling constant calculated from our results, we estimated a temperature of ∼ ∼130 K as a final exothermic temperature at early stage of persistent trains.     

Causes of catastrophic failure of Tam Pokhari moraine dam in the Mt. Everest region

The moraine dam of the Tam Pokhari glacial lake breached on 3 September 1998 and caused a catastrophic flood in the downstream areas. To learn from the event, a field survey was conducted. The survey team found that a landslide, which is considered to be responsible for the outburst flood, occurred in the northeast-facing slope of the moraine dam. The dam internal structure played a crucial role in forming a landslide that triggered the excess overflow and finally the breach of the dam. The internal structure of the dam was made of alternating layers of finer and coarser sediments inclining at 30° downstream and layers are truncated in the upslope direction by a huge pile of unconsolidated and structureless moraine materials. Since the upstream slope angle of the dam i.e., 40° is larger than the angle of repose i.e. 35° of sediments, the increased pore water pressure in the dam triggered a landslide. The rainfall and seismological activities of that particular day, which hit the record high, were crucial in triggering the failure. It is estimated that the dam’s north and northeast-facing slopes completely slid involving about 30,000 m³ of sediment mass of unconsolidated moraine materials above the shear plane. A slope stability analysis was also performed. The calculated safety factor was 0.85, and the calculated slip circle agreed with the shear plane marked in the dam. About 18 million cubic metres of water was swiftly released due to the sudden breach of the moraine dam.     

Activity Profile of the Sextantid Meteor Shower

The detailed activity profile of the Sextandids - one of the day-time meteor showers - is poorly known and still unclear. Using the forward-scatter radio technique we have successfully been able to obtain further detailed overall activity profile of the Sextantids for seven consecutive years: 1991–1997. Analysis confirmed the Sextantid activity duration in solar longitude (J2000) of at least 184–193° and the maximum solar longitude at 188.35 ± 0.10° with a full width at half maximum (FWHM) of 2.0 ± 0.2°. Performing the numerical integrations, we also substantiated a possibility of the association between Apollo-type asteroid (3200) Phaethon and the Sextantids. Furthermore, we roughly estimated relative maximum flux rate of Sextantids : Geminids as 1 : 3 amplitude ratio. Depending upon the flux rates and the time lags of the orbital evolution with Phaethon, we conclude that the Sextantids are at a more progressive stage of orbital evolution than the Geminids if both meteor streams are really associated with Phaethon. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Influence Of Urban Canopy Configuration On Urban Albedo

We propose a calculation method for shortwave radiation flux and longwave radiation flux within the urban canopy and investigate the influence of urban canopy configuration on net radiation flux. In the assumed urban configuration, buildings of equal size are arranged in a regular lattice within the urban canopy. The net shortwave radiation flux and longwave radiation flux within the urban canopy were calculated by the photon tracking method based on the Monte Carlo method. The albedo value obtained by this method shows close agreement with experimental data, and the average sky view factor shows almost perfect agreement with the theoretical value. Moreover, we calculated the urban albedo for the urban canopy configuration including roads and building height distribution.%Moreover, we calculated net radiation within the urban canopy in %consideration of roads and building height distribution.We found that the sky view factor of the ground surface is high when building coverage is low, building height is low, open space by roads exists, and building height is non-uniform. Moreover, we found that the albedo value is high when building height is small, open space by roads is wide, and building height is uniform. The albedo value was found to vary in a complicated manner with change in building coverage.