Galactic Evolution of the Light Elements: A New Set of B Observations

The boron 2500 spectral region has been observed with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) in a new set of metal-poor stars and analyzed by spectrum synthesis technique, adopting the most recent model atmospheres. By taking into account the Li and Be abundances available from the literature for this same set of objects, the resulting patterns of their light elements abundances cannot be easily justified with the currently known stellar structure scenarios. The finding of real differences in the B content between stars with very similar stellar characteristics suggest that also production effects, rather than depletion and/or mixing only, should be taken into account as a possible and valuable explanation.     

Measurements of radiation exposure in civil aircraft

The radiation exposures on 12 flights of German airlines were measured with an active dosemeter based on two silicon semiconductors. The dependence on the date, altitude and route of the flights was studied. Measured dose rates and preliminary dose equivalent rates of the individual flights are given and compared with model calculations.     

The Kinelite Project: a new powerful motion analyser for Spacelab and space station

The goal of the Kinelite Project is to develop a space qualified motion analysis system to be used in space by the scientific community, mainly to support neuroscience protocols. The measurement principle of the Kinelite is to determine, by triangulation mean, the 3D position of small, lightweight, reflective markers positioned at the different points of interest. The scene is illuminated by Infra Red flashes and the reflected light is acquired by up to 8 precalibrated and synchronized CCD cameras. The main characteristics of the system are: Camera field of view: 45 degrees; Number of cameras: 2 to 8; Acquisition frequency: 25, 50, 100, or 200 Hz; CCD format: 256 x 256; Number of markers: up to 64; 3D accuracy: 2mm; Main dimensions: 45 cm x 45 cm x 30 cm; Mass: 23 kg; Power consumption: less than 200 W. The Kinelite will first fly aboard the NASA Spacelab; it will be used, during the NEUROLAB mission (4/98), to support the "Frames of References and Internal Models" (Principal Investigator: Pr. A. Berthoz, Co Investigators: J. McIntyre, F. Lacquaniti).     

Vertical motions in Thailand after the 2004 Sumatra–Andaman Earthquake from GPS observations and its geophysical modelling

Following previous findings from ongoing GPS research in Thailand since 2004 we continue to exploit the GPS technique to monitor and model land motions induced by the Sumatra–Andaman Earthquake. Our latest results show that up to the end of 2010, Thailand has been co-seismically displaced and is subsequently undergoing a post-seismic horizontal deformation with total displacements (co-seismic plus post-seismic) ranging from 10.5 to 74.7 cm. We observed the largest horizontal displacements in the southern part of Thailand and moderate and small displacements in the central and northern parts. In addition to horizontal displacements throughout Thailand, continuous GPS measurements show that large parts of Thailand are subsiding at rates up to 1 cm/yr. It is the first time that such vertical post-seismic deformations at large distances (650–1500 km away from the Earthquake’s epicentre) have been recorded. We have investigated the physical processes leading to the observed subsidence. While after-slip on the subduction interface induces negligible or even slightly positive vertical motions, relaxation in the asthenosphere is associated with a sizable subsidence. Predictions from a 3D finite element model feature an asthenosphere with an effective viscosity of the order of 3 * 10¹⁸ Pas, fit the horizontal post-seismic data and the observed subsidence well. This model is then used to predict the subsidence over the whole seismic cycle. The subsidence should go on with a diminishing rate through the next two decades and its final magnitude should not exceed 10 cm in the Bangkok area.     

Delay-Doppler Ambiguity Function for Wideband Signals

The conventional ambiguity function is extended to include the Doppler distortions of the modulation function. The distinctive features of the extension are the use of the complex notation for wideband signals, and inclusion of the Doppler effect on the signal amplitude. The result is an ambiguity function from which Woodward's form can be found by inspection. It is shown that the well-known volume constraint also applies, in unchanged form, to the generalized ambiguity function. For the volume to be constant, it is not required that the distortions of the modulation function be neglected. Rather, the volume constancy is related to the sinusoidal fluctuations of a modulated carrier-type signal and thus is strictly a matter of the percentage bandwidth of the signal.     

The Near-Earth Plasma Environment

An overview of the plasma environment near the earth is provided. We describe how the near-earth plasma is formed, including photo-ionization from solar photons and impact ionization at high latitudes from energetic particles. We review the fundamental characteristics of the earth’s plasma environment, with emphasis on the ionosphere and its interactions with the extended neutral atmosphere. Important processes that control ionospheric physics at low, middle, and high latitudes are discussed. The general dynamics and morphology of the ionized gas at mid- and low-latitudes are described including electrodynamic contributions from wind-driven dynamos, tides, and planetary-scale waves. The unique properties of the near-earth plasma and its associated currents at high latitudes are shown to depend on precipitating auroral charged particles and strong electric fields which map earthward from the magnetosphere. The upper atmosphere is shown to have profound effects on the transfer of energy and momentum between the high-latitude plasma and the neutral constituents. The article concludes with a discussion of how the near-earth plasma responds to magnetic storms associated with solar disturbances.     

Planetary Accumulation with a Continuous Supply of Planetesimals

Previous calculations of the accumulation of small (∼10 km) planetesimals at ∼1 AU to form Mars-sized bodies assumed that the initial assemblage of planetesimals were all present at the outset. This is an obviously reasonable assumption in systems in which the time scale for growth time of ∼10²⁶ g planetary bodies is long compared to estimates of the evolutionary time scale of a protosolar disk, as was the case in the pioneering work of Safronov (1969). It is now found that as a result of the preplanetary assemblage being unstable with respect to the runaway growth of the largest bodies, this is unlikely to be the case. The more realistic alternative of adding the initial planetesimals on a ∼10⁵ year time scale is considered here, as well as the consequences of the initial planetesimals being considerably smaller than those assumed previously. It is found that although the time scale for runaway growth is now actually controlled by the availability of planetesimals, for planetesimal production time scales of ∼10⁵ yrs, the final consequences are very similar. These calculations do show, however, that as a consequence of continuous infall during the runaway growth process, the late initial planetesimals are likely to be catastrophically disrupted by mutual collisions. For this reason, a more detailed treatment of the growth of planetesimals into planetary embryos will require a better understanding of the difficult problem of formation of the initial planetesimals themselves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Additional Measurements on Pit Shielding for Communications Satellite Earth Stations

This correspondence reports measurements taken in both the 4- and 6-GHz frequency bands of the protection afforded a pit-shielded Earth-station antenna. The measurement program described is an expansion of a previous one and was designed to determine the effects of artificial site shielding for a range of satellite locations rather than a single position.