Variable stars,photoelectric photometry,science education,and the A.A.V.S.O.

The role of amateurs and small-scale equipment in astronomical photometry is reviewed. The value of suitably selected projects for both research and educational purposes is stressed. Recent technical developments are noted, and a brief extrapolation to possible future arrangements included.Keynote address at the conference     

A simulation tool for high data-rate acoustic communication in ashallow-water, time-varying channel

The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array     

OSCR wave measurements-some preliminary results

OSCR is an HF radar system that has been developed for high spatial resolution coastal surface current measurement. This paper describes preliminary results that demonstrate that wave measurement can be successfully obtained from suitably processed OSCR data. Comparisons with data from a WAVEC directional buoy are presented and show encouraging agreement. Some of the limitations to the measurement process are discussed and indicate a maximum range of about 20 km. Surface current variability on short time scales presents the most serious obstacle to wave measurement. This appears to be more of a problem when the mean currents are large, in that in these circumstances the data fail initial quality control criteria. However, in lower mean currents, the effect is often still present and leads to errors in long wave measurement     

Application of an improved self-starter to geoacoustic inversion

The self-starter is improved using the operator of the split-step Pade solution. In addition to providing greater stability and being applicable closer to the source, the improved self-starter is an efficient forward model for geoacoustic inversion. It is necessary to solve only O(10) tridiagonal systems of equations to obtain the acoustic field on a vertical array located O(10) wavelengths from a source. This experimental configuration is effective for geoacoustic inverse problems involving unknown parameters deep in the ocean bottom. For problems involving depth-dependent acoustic parameters, the improved self-starter can be used to solve nonlinear inverse problems involving O(10) unknown sediment parameters in less than a minute on the current generation of workstations     

The variability of Mercury's exosphere by particle and radiation induced surface release processes

Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere (P_surface<1×10⁻⁸ Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surface-collisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere (N?10¹⁴ cm⁻²) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na⁺ ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H⁺ are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4×10⁸ cm⁻² s⁻¹ calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8-1.0×10⁷ cm⁻² s⁻¹ and for Na approximately 1.3-1.6×10⁵ cm⁻² s⁻¹ depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0-1.4×10⁴ cm⁻² s⁻¹. By taking an average open magnetic field line area of about 2.8×10¹⁶ cm² modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2×10²¹ s⁻¹ and for O of about 2.5×10²³ s⁻¹. The particle sputter rate for K atoms is about 3.0×10²⁰ s⁻¹ assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0×10⁶ cm⁻² s⁻¹. The average source rates for Na particles released by PSD are about 1×10²⁴ s⁻¹. By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873-5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 10⁴ cm⁻² s⁻¹ along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same order as particles produced by micrometeoroid vaporization. We also discuss the capability of in situ measurements of Mercury's highly variable particle environment by the proposed NPA-SERENA instrument package on board ESA's BepiColombo Mercury Planetary Orbiter (MPO).     

Variable interstellar absorption lines: a brief review

I review the recent discoveries of variable interstellar absorption lines, widely taken to imply the existence of very small scale(10–100 AU) structure in the interstellar medium. Possible origins of this structure are discussed, and attention is drawn to the fact that most known examples seem to be associated with interstellar shells and bubbles of various kinds. On the other hand, in at least one case (κ Velorum),the variation appears to occur in the supposedly quiescent cold neutral medium, consistent with earlier studies of small scale structure in atomic hydrogen probed at radio wavelengths. Further work is urgently required to determine the prevalence, mode of formation, and the physical and chemical state of these enigmatic structures. Only then will their implications for our wider understanding of the interstellar medium become apparent. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the Moment Method for the Modeling of Cloud Microphysics in Rarefied Turbulent Atmospheres: II. Stochastic Coagulation

The previously proposed (Rodin, 2002) method for calculating the microphysical properties of spatially inhomogeneous rarefied aerosol media with mixing using the lowest-order moments of the size distribution is generalized to particle coagulation. We show that when the problem is formulated in terms of moments, all of the solutions admitted by the stochastic coagulation equation lie within a narrow range whose boundaries can be determined by means of quadratic programming. We discuss the choice of an optimal solution within this range and compare the moment method with the results of our computations by the classical finite-difference method using a model of photochemical aerosols in Titan's atmosphere as an example. The moment method allows the efficiency of microphysical computations to be significantly increased by using precomputed low-dimension interpolation tables. It can be used to construct self-consistent models for the globular circulation of planetary atmospheres.