Graded-Density Reservoirs for Accessing High Stress Low Temperature Material States

In recently developed laser-driven shockless compression experiments an ablatively driven shock in a primary target is transformed into a ramp compression wave in a secondary target via unloading followed by stagnation across an intermediate vacuum gap. Current limitations on the achievable peak longitudinal stresses are limited by the ability of shaping the temporal profile of the ramp compression pulse. We report on new techniques using graded density reservoirs for shaping the loading profile and extending these techniques to high peak pressures.     

New 1982 — 1990 photometry of λ Andromedae and its 11-year cycle

We present photoelectric photometry of λ And never before published, obtained between February 1982 and December 1990 at 29 different observatories. Then we combine it with all other photometry available to us (previously published, contained in the I.A.U. Commission 27 Archives, and obtained with the Vanderbilt 16-inch automatic telescope but not yet published), to yield a 14.8-year data base. Analysis reveals a long-term cycle in mean brightness, with a full range of 0^m.15 and a period of 11.4 ± 0.4 years. Because most of our new photometry was concentrated in the 1983-84 observing season, we analyze that one well-defined light curve with a two-spot model. Spot A keeps a 0^m.04 amplitude throughout four rotation cycles whereas the amplitude of spot B diminishes from 0^m.09 down almost to 0^m.03. The spot rotation periods were 55^d.9 ± 0^d.6 and 520^d.8 ± 1^d.0, respectively.     

Sputtering and high altitude meteors

Conventional ablation theory assumes that a meteoroid undergoes intensive heating during atmospheric flight and surface atoms are liberated through thermal processes. Our research has indicated that physical sputtering could play a significant role in meteoroid mass loss. Using a 4th order Runge-Kutta numerical integration technique, we tabulated the mass loss due to the two ablation mechanisms and computed the fraction of total mass lost due to sputtering. We modeled cometary structure meteoroids with masses ranging from 10⁻¹³ to 10⁻³ kg and velocities ranging from 11.2 to 71 km s⁻¹. Our results indicate that a significant fraction of the mass loss for small, fast meteors is due to sputtering, particularly in the early portion of the light curve. In the past 6 years evidence has emerged for meteor luminosity at heights greater than can be explained by conventional ablation theory. We have applied our sputtering model and find excellent agreement with these observations, and therefore suggest that sputtered material accounts for the new type of radiation found at great heights.     

Collective effects in diffuse matter-antimatter plasma: II

It is shown that even very slightly ionized clouds of matter and anti-matter can interpenetrate only a little on collision. Initial interpenetration produces fast electrons and positrons from annihilation. These, in turn, produce strong magneto-hydrodynamic shocks which give the small ionized component enough energy to ionize the neutral fraction and produce a Leidenfrost layer in about ten years after which interpenetration stops.     

Proterozoic East Gondwana： supercontinent assembly and breakup

The editors of this volume assembled a remarkable group of contributors to discuss both the geology of East Gondwana and also many related issues. The 19 papers in this book provide a wealth of information on East Gondwana, the assembly of Gondwana as a supercontinent, the configuration of Rodinia,and general problems of the origin of continental crust and movement of continental plates. They also illustrate some of the controversies, with most papers having at least some points of difference in both fact and interpretation from other oaoers in the book.     

Mass loss due to sputtering and thermal processes in meteoroid ablation

Conventional meteoroid theory assumes that the dominant mode of ablation (which we will refer to as thermal ablation) is by evaporation following intense heating during atmospheric flight. Light production results from excitation of ablated meteoroid atoms following collisions with atmospheric constituents. In this paper, we consider the question of whether sputtering may provide an alternative disintegration process of some importance. For meteoroids in the mass range from 10^-3 to and covering a meteor velocity range from 11 to , we numerically modeled both thermal ablation and sputtering ablation during atmospheric flight. We considered three meteoroid models believed to be representative of asteroidal ( mass density), cometary () and porous cometary () meteoroid structures. Atmospheric profiles which considered the molecular compositions at different heights were use in the sputtering calculations. We find that while in many cases (particularly at low velocities and for relatively large meteoroid masses) sputtering contributes only a small amount of mass loss during atmospheric flight, in some cases sputtering is very important. For example, a porous meteoroid at will lose nearly 51% of its mass by sputtering, while a asteroidal meteoroid at will lose nearly 83% of its mass by sputtering. We argue that sputtering may explain the light production observed at very great heights in some Leonid meteors. We discuss methods to observationally test the predictions of these computations. A search for early gradual tails on meteor light curves prior to the commencement of intense thermal ablation possibly represents the most promising approach. The impact of this work will be most dramatic for very small meteoroids such as those observed with large aperture radars. The heights of ablation and decelerations observed using these systems may provide evidence for the importance of sputtering.     

REMINISCENCES OF A ROVING PHOTOGRAMMETRIST

The author reminisces upon a career of almost 40 years as a practising surveyor photogrammetrist, both as a Royal Engineer officer and as a director of a commercial survey organisation. The reminiscences include service with the Survey of India, war time operations in the Middle East and South East Asia Command, and commercial surveys of all types in developing countries. Professional service with international surveying and photogrammetric organisations is also mentioned, with particular reference to the fraternal aspects of international photogrammetry.     

Measuring Vertical Heat Flux with RASS

Summary A wind‐profiling Doppler radar equipped with a radio acoustic sounding system (RASS) may be used to estimate the vertical profile of the vertical flux of heat in the atmosphere. Simultaneous measurements of the time‐varying temperature and vertical air velocity are combined to give the convective heat flux using the eddy‐correlation method. The accuracy of the estimates depends on the fundamental accuracy of the temperature and vertical velocity measurements. Also, in common with all eddy‐correlation methods, uncertainties are introduced by the need to define a suitable averaging time and to remove trends. A problem unique to RASS is the possible presence of ground and intermittent clutter at close ranges, which can cause errors in the vertical air velocity measurements. These considerations are discussed with particular reference to observations using a UHF radar wind profiler situated in an urban environment, where clutter is a serious problem. A Rank‐Order Signal Processing Algorithm (ROSPA) for recognizing and eliminating outliers in the vertical velocity, is introduced. It is explained how ROSPA uses both a minimum filter and a median filter on the velocity data. It is shown, using a comparison with nearly clutter free data from a rural site, that the filtering substantially improves the quality of the noisy urban data. The paper then compares RASS‐measured urban and rural heat flux profiles, along with the heat flux profile measured by an instrumented airplane. It is concluded that the main obstacles to RASS heat flux measurements are the effects of winds and turbulence in the boundary layer, rather than clutter. Received September 24, 1998 Revised January 27, 1999