Study of radiative energy losses in the moving envelopes of T Tau stars

Crimean Astrophysical Observatory. Translated from Astrofizika, Vol. 32, No. 1, pp. 69–83, January–February, 1990.     

Magnetic Field Strengths and Structures from Radio Observations of Solar Active Regions

Radio observations of some active regions (ARs) obtained with the Nobeyama radioheliograph at λ=1.76cm are used for estimating the magnetic field strength in the upper chromosphere, based on thermal bremsstrahlung. The results are compared with the magnetic field strength in the photosphere from observations with the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station of Beijing Astronomical Observatory. The difference in the magnetic field strength between the two layers seems reasonable. The solar radio maps of active regions obtained with the Nobeyama radioheliograph, both in total intensity (I-map) and in circular polarizations (V-map), are compared with the optical magnetograms obtained with the SMFT. The comparison between the radio map in circular polarization and the longitudinal photospheric magnetogram of a plage region suggest that the radio map in circular polarization is a kind of magnetogram of the upper chromosphere. The comparison of the radio map in total intensity with the photospheric vector magnetogram of an AR shows that the radio map in total intensity gives indications of magnetic loops in the corona, thus we have a method of defining the coronal magnetic structure from the radio I-maps at λ=1.76 cm. Analysing the I-maps, we identified three components: (a) a compact bright source; (b) a narrow elongated structure connecting two main magnetic islands of opposite polarities (observed in both the optical and radio magnetograms); (c) a wide, diffuse, weak component that corresponds to a wide structure in the solar active region which shows in most cases an S or a reversed S contour, which is probably due to the differential rotation of the Sun. The last two components suggest coronal loops on different spatial scales above the neutral line of the longitudinal photospheric magnetic field.     

Sonar acoustic facies and sediment distribution on an area of the deep ocean floor

Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope.     

Geometry and kinematics of recent deformation in the Mondy–Tunka area (south-westernmost Baikal rift zone, Mongolia–Siberia)

We used satellite imagery and field data to investigate the south‐westernmost Baikal rift zone. We focus our study in the Mondy and Ikhe Ukhgun valleys, site of an Mw = 6.9 seismic event in 1950. Surface deformations are observed along the E–W‐trending Mondy strike‐slip fault and along the Ikhe Ukhgun thrust. The Mondy fault system is 80 km long and is composed of four segments 10–15 km long. These segments are characterized by subvertical planes with left‐lateral movements. The Ikhe Ukhgun thrust is 20 km long, dips 40° to the south and shows reverse movement with a left‐lateral component. These observations are consistent with the present‐day regional NNE–SSW compression and with the focal mechanism of the 1950 Mondy earthquake that was recently re‐evaluated. These features, like those observed in the Tunka basin, demonstrate a recent change of regional strain regime from transtension to transpression that we place before the Late Pleistocene.     

Photometry of EX Hya

Photometric observations of EX Hya inB andVfilters are reported. The 67 min modulation of the light curve also is found to be in good agreement with the results of earlier studies. The (B-V) colour variation with respect to the 67 min variation is found to be opposite to those of typical colour variation during hump/superhump activity in other dwarf novae. The model of an intermediate polar is discussed.     

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.     

Regolith Surface Reflectance: A New Attempt to Model

The reflectance coefficient of the regolith layer of celestial bodies has been studied in relation to the physical properties of regolith particles (size, refractive index, and packing density) on the basis of an accurate numerical radiative-transfer algorithm for a semi-infinite flat layer. Using the geometric-optics approximation, we have found that a shape mixture of randomly oriented spheroids can successfully model the single-scattering phase function of independent soil grains. In order to take into account the effect of packing density in a regolith layer, the concept of the so-called static structure factor was used. The main effect of increasing packing density is to suppress the forward-scattering peak of the phase function and to increase the albedo of the reflecting surface. We also investigated the influence of fine dust on the reflected light. An addition of small particles not only increases the surface albedo, but also changes the brightness profile and enhances the backscattering. Although the problem of unique solution, which is inherent in the retrieval of the properties of a medium from the measurements of the intensity of light scattered by this media, cannot be removed in the proposed model, the procedure used here, in contrast to widely used approximations, allows us to fit observational data with a set of real characteristics of the regolith. Semiempirical approaches are able to fit the measurements well with a small number of free parameters, but they do not explicitly contain crucial physical characteristics of the regolith such as grain sizes or the refractive index. We compared the numerical solution of the radiative-transfer equation with the Hapke approximation, which is most often used by investigators. The errors introduced by the Hapke model are small only for near-isotropic scattering by isolated particles. However, independent regolith grains are known to scatter light mainly in the forward direction.     

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.     

Thaw modification of frost-fissure wedges,Richards Island,Pleistocene Mackenzie Delta,Western Arctic Canada

Thaw modification is the general process whereby frost-fissure wedges are modified during thaw, and by which frost-fissure pseudomorphs may develop. Specific processes of thaw modification are inferred from ice-wedge pseudomorphs, composite-wedge pseudomorphs and deformed sand wedges in the Pleistocene Mackenzie Delta: i.e. thermal erosion, collapse, subsidence, refreezing, loading, buoyancy, spreading, folding and shearing. Thaw modification is believed to result in selective preservation of pseudomorphs and wedges. Sand wedges are more likely to be preserved than are ice-wedge pseudomorphs or compositewedge pseudomorphs, because only those sand wedges that penetrate massive ice or icy sediments are prone to thaw modification. Furthermore, whereas ice wedges preferentially develop in ice-rich, fine-grained sediments (thaw-sensitive), their pseudomorphs appear to be selectively preserved in ice-poor, coarse-grained sediments (thaw-stable).