Very Long Baseline Interferometry: Dependencies on Frequency Stability

Very Long Baseline Interferometry (VLBI) is a differential technique observing radiation of compact extra-galactic radio sources with pairs of radio telescopes. For these observations, the frequency standards at the telescopes need to have very high stability. In this article we discuss why this is, and we investigate exactly how precise the frequency standards need to be. Four areas where good clock performance is needed are considered: coherence, geodetic parameter estimation, correlator synchronization, and UT1 determination. We show that in order to ensure the highest accuracy of VLBI, stability similar to that of a hydrogen maser is needed for time-scales up to a few hours. In the article, we are considering both traditional VLBI where extra-galactic radio sources are observed, as well as observation of man-made artificial radio sources emitted by satellites or spacecrafts.     

The organic matter of comet Halley as inferred by joint gas phase and solid phase analyses

During encounters with comet Halley, the experiment PICCA onboard GIOTTO measured the gas phase organic ion composition of the coma and the experiment PUMA onboard VEGA-1 measured the dust composition. Joining both results, we obtain a consistent picture of the parent organic matter from which dust and gas is produced. One recognizes a complex unsaturated polycondensate, which splits during coma-formation into the more refractory C=C,C-N-containing dust part, and the more volatile C=C,C-O-containing gas part. The responsible exothermal chemical reactions, which are triggered by the sunlight, may play a major role in the dynamics of coma formation.This paper is a shortened and upgraded version of Krueger, F.R., Korth, A., and Kissel, J.: 1989, in S. Chang (ed.) Proc. of the ROSETTA Conf., Milpitas CA, January 1989, submitted.     

Fringe fitting and group delay determination for geodetic VLBI observations of DOR tones

Extracting the group and phase delays of interferometric observations produced in the Very Long Baseline Interferometry (VLBI) measurement concept requires a special fringe fitting and delay search algorithm for the recorded bandwidth. While fringe fitting is in use routinely for several megahertz wide channels in geodetic and astrometric VLBI with quasar observations, fringe fitting for artificial tones of very small bandwidth of artificial signals for Differential One-way Ranging (DOR) requires a different way of handling. In a project called Observing the Chang’E-3 Lander with VLBI (OCEL), the DOR tones emitted by the Chang’E-3 lander were observed in a standard geodetic VLBI mode with 8 or 4?MHz wide channels to maintain compatibility with the corresponding quasar observations. For these observations, we modified the existing fringe fitting program of the Haystack Observatory Processing Software (HOPS), fourfit, to properly handle narrow band DOR tones. The main motivations are that through this modification, the data of quasars and artificial radio sources can be processed in the existing geodetic analysis pipeline, and that the algorithm can be used for similar projects as well. In this paper, we describe the algorithm and show that the new algorithm produces much more reliable group delay results than using the standard fourfit algorithm. This is done by a simulation test and in particular by processing of real observations. It is shown that in many cases, systematic deviations of several nanoseconds, which are seen with the standard fourfit algorithm, can be avoided. The ultimate benefit of the new procedure is demonstrated by reducing the errors in delay triangle closures by at least a factor of 3, which, in the OCEL case, is from ～300 to ～100?ps.     

Astrophysical Hydromagnetic Turbulence

Recent progress in astrophysical hydromagnetic turbulence is being reviewed. The physical ideas behind the now widely accepted Goldreich–Sridhar model and its extension to compressible magnetohydrodynamic turbulence are introduced. Implications for cosmic ray diffusion and acceleration is being discussed. Dynamo-generated magnetic fields with and without helicity are contrasted against each other. Certain turbulent transport processes are being modified and often suppressed by anisotropy and inhomogeneities of the turbulence, while others are being produced by such properties, which can lead to new large-scale instabilities of the turbulent medium. Applications of various such processes to astrophysical systems are being considered.     

Carbonaceous cherts in the Barberton greenstone belt and their significance for the study of early life in the Archean record

The 3.5-3.2 Ga old volcano-sedimentary succession of the Barberton greenstone belt (South Africa) is characterized by lithological units that are repeated in a regular manner. Komatiitic, basaltic, and dacitic volcanic and volcaniclastic sequences are capped by zones of silica enrichment, followed by bedded carbonaceous cherts. Stratiform and crosscutting carbonaceous chert veins are common in silica alteration zones and bedded cherts. A detailed field study of several chert horizons and chert veins that range in age from 3.47 to 3.30 Ga revealed the importance of syndepositional hydrothermal activity for their origin. Bedded cherts consist of silicified detrital and tuffaceous sediments that were deposited on the seafloor. Silicification took place at the sediment-water interface as a result of diffuse upflow of low-temperature hydrothermal fluids, which gave rise to the formation of impermeable chert caps. Fluid overpressure resulted in the breaching of the cap rocks at times. Chert veins contain angular host rock fragments, replace wall rocks, and show evidence of multiple vein fillings and in situ brecciation of earlier generations of vein fillings. They represent hydraulic fractures that were initiated by overpressuring of the hydrothermal system. The vein networks were infilled, partly by hydrothermal chert precipitates, and partly by still unconsolidated (not yet silicified) sedimentary material derived from overlying sedimentary horizons. Field, petrographic, isotopic, and trace element evidence indicate that most carbonaceous matter represents sedimentary material that originated by biogenic processes in the Archean oceans and not by hydrothermal processes in the subsurface.     

Current Status of Turbulent Dynamo Theory

Several recent advances in turbulent dynamo theory are reviewed. High resolution simulations of small-scale and large-scale dynamo action in periodic domains are compared with each other and contrasted with similar results at low magnetic Prandtl numbers. It is argued that all the different cases show similarities at intermediate length scales. On the other hand, in the presence of helicity of the turbulence, power develops on large scales, which is not present in non-helical small-scale turbulent dynamos. At small length scales, differences occur in connection with the dissipation cutoff scales associated with the respective value of the magnetic Prandtl number. These differences are found to be independent of whether or not there is large-scale dynamo action. However, large-scale dynamos in homogeneous systems are shown to suffer from resistive slow-down even at intermediate length scales. The results from simulations are connected to mean field theory and its applications. Recent work on magnetic helicity fluxes to alleviate large-scale dynamo quenching, shear dynamos, nonlocal effects and magnetic structures from strong density stratification are highlighted. Several insights which arise from analytic considerations of small-scale dynamos are discussed.     

Microphysics of Cosmic Ray Driven Plasma Instabilities

Energetic nonthermal particles (cosmic rays, CRs) are accelerated in supernova remnants, relativistic jets and other astrophysical objects. The CR energy density is typically comparable with that of the thermal components and magnetic fields. In this review we discuss mechanisms of magnetic field amplification due to instabilities induced by CRs. We derive CR kinetic and magnetohydrodynamic equations that govern cosmic plasma systems comprising the thermal background plasma, comic rays and fluctuating magnetic fields to study CR-driven instabilities. Both resonant and non-resonant instabilities are reviewed, including the Bell short-wavelength instability, and the firehose instability. Special attention is paid to the longwavelength instabilities driven by the CR current and pressure gradient. The helicity production by the CR current-driven instabilities is discussed in connection with the dynamo mechanisms of cosmic magnetic field amplification.     

Is Route Learning More Than Serial Learning?

Abstract

The nature of route learning in terms of the memorizing of landmarks was investigated. In Experiment 1, participants memorized landmarks while being guided through a computer-simulated hallway (dynamic, with spatial context), or while viewing the landmarks one by one in front of a black background (static, without context). Two more conditions completed the 2 × 2 design. One condition preserved the dynamic landmark viewing properties (observers approached each object, passed it, turned to the next object, and so on), but the background was black (dynamic, without context). In the other condition the observer saw a stationary display of each object within a hallway, but did not approach the object (static, with context). Serial recall was much better after viewing the landmarks in the dynamic presentation format with spatial context than in the other conditions. Experiment 2 showed that the superior performance in the dynamic condition with context was abolished when all hallway segments were equally long. This implies that metric information is a component of route knowledge at a very early stage, which is incompatible with the dominant framework, but is compatible with the alternative framework for spatial microgenesis.