The New Multichannel Radiospectrograph ARTEMIS-IV/HECATE, of the University of Athens

We present the new solar radiospectrograph of the University of Athensoperating at the Thermopylae Station since 1996. Observations cover thefrequency range from 110 to 688 MHz. The radiospectrograph has a 7-meterparabolic antenna and two receivers operating in parallel. One is a sweepfrequency receiver and the other a multichannel acousto-optical receiver.The data acquisition system consists of a front-end VME based subsystem anda Sun Sparc-5 workstation connected through Ethernet. The two subsystems areoperated using the VxWorks real-time package. The daily operation is fullyautomated: pointing of the antenna to the sun, starting and stopping theobservations at pre-set times, data acquisition, data compression by`silence suppression', and archiving on DAT tapes. The instrument can beused either by itself to study the onset and evolution of solar radio bursts or in conjunction with other instruments including theNançay Decametric Array and the WIND/WAVES RAD1 and RAD2 low frequencyreceivers to study associated interplanetary phenomena.     

The Expansion of the Hamiltonian of the Planetary Problem into the Poisson Series in All Keplerian Elements (Theory)

The study of the evolution of planetary systems, primarily of the Solar System, is one of the basic problems of celestial mechanics. The stability of motion of giant planets on cosmogonic time scales was established by numerical and analytical methods, but the question about the evolution of orbits of terrestrial planets and arbitrary solar-type planetary systems remained open. This work initiates a series of papers allowing one to advance in solving the problem of the evolution of the solar-type planetary systems on cosmogonic time scales by using powerful analytical tools. In the first paper of this series, we choose the optimum reference system and obtain the Poisson series expansion of the Hamiltonian of the problem in all Keplerian elements. We propose to use the integral representation of the corresponding coefficients or the Poisson processor means instead of conventionally addressing any possible special functions. This approach extremely simplifies the algorithm. The next paper of this series deals with the calculation of the expansion coefficients.     

Cokriging nonstationary data

Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science.     

Pulsar reflections within the Crab Nebula

Between 1997 August and October, the radio pulses from the Crab pulsar were followed by discrete moving echoes, which appear to be reflections from part of an ionized shell in the outer part of the Crab Nebula, crossing the line of sight to pulsar. Similar events have now been recognized in recordings from the past 30 yr, and it seems that the Nebula must contain a large number of ionized shell-like surfaces on a much finer scale than recognized hitherto.     

On the mathematical description of fine structure under the conditions of weak variability

The idea of the weak variability of the fields is introduced, which means that when they are recorded by some measuring instrument (MI), the output signal obtains increments of the order of one/several quanta of sensitivity at the discreteness interval. At small scales (microstructure), this field is described in terms of random discrete values which reflect the properties of both the field measured and the MI. Theoretical relationships which define the probabilities of increments at the compound intervals in terms of the probabilities of increments at the sampling intervals, are derived for the case of independent random increments at the sampling intervals of discreteness. The validity of this model is illustrated by respective computations using data of microstructure measurements of the temperature profile in the main thermocline in the Sargasso Sea.Translated by Mikhail M. Trufanov.     

The interannual leading modes of the extratropical variability in the Southern Hemisphere simulated by the ECHAM-4 atmospheric model

 An ensemble of twenty-three 14-year experiments conducted with the ECHAM-4 GCM has been examined to test the model's capability to simulate the principal modes of interannual variability. The integrations were performed under specified monthly SST between 1979–1993. The analysis was focused on the Southern Hemisphere (SH) extratropics. Empirical orthogonal functions analysis (EOF) using seasonal anomaly fields has been performed to isolate the principal modes that dominate the southern extratropical variability at the interannual time scale. Leading patterns of 500 hPa geopotential height (z500) have been compared with those estimated from the ECMWF re-analysis dataset. The model is able to adequately reproduce the spatial pattern of the annular mode, but it represents the temporal variations of the oscillation less satisfactorily. The model simulation of the Pacific South American (PSA) pattern is better, both in the shape of the pattern and in the temporal evolution. To verify if the capability of the model to adequately simulate the temporal oscillation of the propagating patterns is related to the increased influence of the tropical external forcing, covarying SST-atmospheric modes have been identified by singular value decomposition (SVD). In winter (July-August-September, JAS) the tropical SST variability is highly correlated with the ENSO mode. In summer (January-February-March, JFM) the strength of the teleconnections is related to strong westerly anomalies, disrupted by a meridional out of phase relation near to South America. The large size of the ensemble was exploited by comparing the time-varying model spread and degrees of freedom of the simulated extratropical circulation. Results show that when the extratropical circulation has a few degrees of freedom, the reproducibility is relatively low and the ensemble is governed by a fairly robust zonally symmetric structure of dispersion. Received: 9 May 2000 / Accepted: 30 January 2001     

H II regions and radio visibility of quasar nuclei

The core (injector) and the jet (relativistic plasma outflow) of AGN objects are surrounded by an ionized medium, an H II region observed in emission lines. The synchrotron radiation from the core and the jet is observed through a thin screen that cocoons the structure under consideration. The screen transparency depends on wavelength and distance from the injector. We consider the objects 3C 345 and 1803+784 whose core emission at decimeter wavelengths is absorption by more than 25 dB. The visible bright compact component is the bright nearby portion of the jet that extends outside the dense part of the screen. We explore the possibility of measuring the screen transparency from absorption in Hα recombination lines with different quantum numbers at centimeter wavelengths.     

Challenging a Paradigm: Do We Need Active and Inactive Areas to Account for Near-Nuclear Jet Activity?

We briefly describe an advanced 3D gas dynamical model developed for the simulation of theenvironment of active cometary nuclei. The model canhandle realistic nucleus shapes and alternative physical models for the gas and dust production mechanism.The inner gas coma structure is computed by solving self-consistently(a) near to the surface the Boltzman Equation(b) outside of it, Euler or Navier-Stokes equations.The dust distribution is computed from multifluid ``zero-temperature' Euler equations,extrapolated with the help of a Keplerian fountain model.The evolution of the coma during the nucleus orbital and spin motion,is computed as a succession of quasi-steady solutions. Earlier versions of the model using simple,``paedagogic' nuclei have demonstrated that the surface orographyand the surface inhomogeneity contribute similarly to structuring the near-nucleusgas and dust coma,casting a shadow on the automatic attribution of such structures to ``active areas'.The model was recently applied to comet P/Halley, for whichthe nucleus shape is available. In the companion paper of this volume,we show that most near-nucleus dust structuresobserved during the 1986 Halley flybys are reproduced, assuming that the nucleus is strictly homogeneous. Here, we investigate the effect of shape perturbations and homogeneityperturbations. We show that the near nucleus gas coma structure is robust vis-a-vissuch effects. In particular, a random distribution of active and inactive areaswould not affect considerably this structure, suggesting that such areas,even if present, could not be easily identified on images of the coma.     

Wave Beams in an Inhomogeneous Plasma in a Transverse Magnetic Field

The propagation of axisymmetric magnetohydrodynamic waves near the equatorial plane of the crust of a neutron star in a transverse magnetic field is considered. The magnetic field is perpendicular to the equatorial plane. The magnetic fields and electric currents excited by this wave beam at the stellar surface are determined.