Optimizing computational efficiency and user convenience in plasma simulation codes

It is usually important to write plasma simulation codes in such a way that they execute efficiently and are convenient to use. I discuss here practical techniques to achieve this goal. Numerical algorithms must be well formulated and advantage taken of machine architecture in casting the algorithm into a high level language such as Fortran. The advantages of writing critical routines in Assembler are discussed. For large simulation codes, disks must often be used as a temporary store for working data. Efficient methods for doing this are presented. Codes must not only be well organized for ease of implementation and maintenance, but also for ease of use. Ways are suggested for packaging codes such that setup, batch production, restarting and diagnostic postprocessing is facilitated. Particular emphasis is placed on graphics postprocessors, since they must be used in real time with graphics terminals as well as with hardcopy devices.     

Modulation of dayside ion and neutral distributions at Venus: Evidence of direct and indirect solar energy inputs

In-situ measurements of ion and neutral composition and temperature across the dayside of Venus during 1979–1980 exhibit long and short-term changes attributed to solar variations. Following solar maximum, dayside concentrations of CO⁺ and the neutral gas temperature are relatively smoothly modulated with a 28-day cycle reasonably matching that of the solar F_10.7 and EUV fluxes. Measurements some 6–8 months earlier show less pronounced and more irregular modulation, and short-term day-to-day fluctuations in the ions and neutrals are relatively more conspicuous than in the later period. During the earlier period, the solar wind at Venu exhibits relatively large velocity enhancements, which appear to be consistent with differences in solar coronal behavior during the two periods. It is suggested that through the solar wind variations and associated changes in the draping of the interplanetary magnetic field about the dayside, fluctuating patterns of joule heating may occur, producing the observed short term ion and neutral variations. This indirect energy effect, if verified, presents a complication for quantitatively analyzing the modulation in neutral temperature and ion concentration produced by changes in direct EUV radiation.     

Studies of optical and biological properties of landcover employing shuttle flights of a multispectral linear array radiometer

Studies to characterize optical and biological properties of land cover as observed from space are planned using a six channel, imaging spectroradiometer employing newly developed multispectral linear array (MLA) detector technology. These studies are to take place by mounting the radiometer on the Shuttle and observing areas with dynamic and diverse types of land cover condition. The radiometer will have 15 meter spatial resolution for four, 20 nanometer bands in the visible and near infrared and 30 meter resolution for similarily narrow bands in the shortwave infrared bands. The instrument will scan ± 45 degrees along the Shuttle orbital path. The principle objective of this experiment is to obtain observations that augment knowledge of the distribution of basic land cover types in regions that are known to be key to questions of biogeochemical cycles, energy balance and climatic change. Another key objective is to quantify the bidirectional reflectance of key land cover conditions in major portions of the visible, near infrared and shortwave infrared as they are observed from space. The initial execution of this experiment is presently scheduled for late 1987.     

Stable Member Equations of Motion for a Three-Axis Gyro Stabilized Platform

This paper presents a detailed development of the equations of motion for the stable member of a three-axis platform. Kinematic relations are presented for an x, z, y Euler sequence. This development includes the effects of friction, inertia, and torque motors. A set of six first-order differential equations of state are presented representing the equations of motion of the system members. These equations may be integrated and the resulting Euler angles and rates used to describe the stable member motions. When combined with the servo loops, the motion of the stable member represents the angular motion environment of the stable member mounted instruments.     

Tides and the evolution of planetary habitability

Tides raised on a planet by the gravity of its host star can reduce the planet's orbital semi-major axis and eccentricity. This effect is only relevant for planets orbiting very close to their host stars. The habitable zones of low-mass stars are also close in, and tides can alter the orbits of planets in these locations. We calculate the tidal evolution of hypothetical terrestrial planets around low-mass stars and show that tides can evolve planets past the inner edge of the habitable zone, sometimes in less than 1 billion years. This migration requires large eccentricities (>0.5) and low-mass stars ( less or similar to 0.35 M(circle)). Such migration may have important implications for the evolution of the atmosphere, internal heating, and the Gaia hypothesis. Similarly, a planet that is detected interior to the habitable zone could have been habitable in the past. We consider the past habitability of the recently discovered, approximately 5 M(circle) planet, Gliese 581 c. We find that it could have been habitable for reasonable choices of orbital and physical properties as recently as 2 Gyr ago. However, when constraints derived from the additional companions are included, most parameter choices that indicate past habitability require the two inner planets of the system to have crossed their mutual 3:1 mean motion resonance. As this crossing would likely have resulted in resonance capture, which is not observed, we conclude that Gl 581 c was probably never habitable.     

Non-Gaussian clutter characterization applied to OTHR using a mixture of two Rayleigh probability density functions

A practical technique for characterizing non-Gaussian radar clutter is specified and demonstrated using Over The Horizon Radar (OTHR) data, as an example. The technique employs maximum likelihood to fit the probability density of the clutter amplitude returns to a mixture of two Rayleigh probability densities instead of the single Rayleigh density typically used for Gaussian clutter. This model for non-Gaussian clutter is fully specified for any set of clutter amplitudes by a log likelihood, two Rayleigh parameters, and a mixing coefficient. A 3D plot of these values yields an easily-visualized clutter characterization, as is illustrated using OTHR data. This technique is a demonstration of clutter characterization using OTHR data, but the method can be applied to characterize other types of clutter data.     

Continuing challenges in lithium battery development

In these days of emphasizing standardization, Acquisition Reform, Non-Developed Items (NDI) and Commercial-Off-The-Shelf (COTS) technologies, we are facing new challenges associated with these trends. Program managers are pressured to use a standard or COTS battery, while simultaneously, the new systems being developed have increasingly complex and demanding power requirements. Hardware must be developed with shorter schedules, and policies of Acquisition Reform limit the amount of control the government has over the development of a given item. In this paper, we review battery development efforts that have resulted in unexpected problems. Relevant data from both current and past test programs are presented. Recommendations are provided concerning how to best avoid duplication of effort, while ensuring that the final product will have the best chances of succeeding     

In situ results on the variation of neutral atmospheric hydrogen at venus

The concentrations of neutral hydrogen within the atmosphere of Venus are investigated for the period 1979–1980. During this period, the planet made nearly three orbits about the Sun, so that nearly three complete diurnal cycles were observed from the Pioneer Venus Orbiter (PVO). Values of n(H) are derived from in-situ ion and neutral composition measurements from the Orbiter Ion Mass Spectrometer (OIMS) and the Orbiter Neutral Mass Spectrometer (ONMS) using a charge exchange relationship involving O⁺, H⁺, O and CO₂. The dawn bulge in the diurnal distribution of n(H), reported from the first diurnal cycle by Brinton et al., is found to persist with n(H) peaking at levels near 2 - 5 × 10⁷/cm³ at altitudes below 165 km. At peak levels, the bulge exhibits a concentration ratio up to 400/1 relative to dayside values. Large day to day variations of up to a factor of five in n(H) are frequently encountered, and are attributed to perturbations induced by the solar wind interaction. These short term variations, plus a suggestion of some local time variation in the bulk location, make precise assessment of interannual variations in the n(H) difficult. Between the first diurnal cycle in early 1979 and the third in mid 1980, the decline in solar euv flux was of the order of 10% or less. Allowing for uncertainties due to short term variations, no clear evidence is found for an interannual variation in the hydrogen concentrations.