Early Cretaceous volcaniclastic and quartzose sandstones from north central Nepal: composition,sedimentology and geotectonic significance

Early Cretaceous sandstones and shales from the Thakkhola region (Chukh and Tangbe Formations) were deposited on the subsiding northern (Tethyan) margin of Gondwana; current directions indicate a source area to the south. Whereas the Berriasian Chukh Formation is dominated by quartz arenites, volcaniclastic sandstones and minor conglomerates dominate the Tangbe Formation of Valanginian to Aptian age. Upsection, the composition of the volcaniclastic components changes from mainly basaltic to dominantly dacitic/rhyolitic. Magmatism was preceded and accompanied by strong uplift in the source area of the Chukh and Tangbe clastics, resulting in the erosion of probably more than several hundred meters of pre-Cretaceous sedimentary, metamorphic and plutonic rocks. A geochemical whole rock analysis of basaltic pebble fragments shows their alkaline character and suggests a within-plate geotectonic setting for the volcanic source. This volcanotectonic event is probably related to rifting between Australia and Greater India, where seafloor spreading began in late Valanginian/Hauterivian times.     

Small-scale clusters of photospheric network bright points and their dependence on the solar cycle phase

The tendency of network bright points (NBPs) to form in their surface distribution non-random, small-scale clusters is studied by a 2-D nearest-neighbours test, based on observational data for the distance between separate NBPs. High-resolution photographs taken from long time series obtained at4308 with the 50-cm refractor of Pic du Midi Observatory are used. Three different epochs of the solar cycle were chosen. The results obtained suggest that the degree of clustering depends on the solar cycle phase. Supposing that the NBPs are good tracers of the kilogauss small-scale magnetic field, we use them to obtain information about the subphotospheric small-scale magnetic field organisation and its dependence on the solar cycle phase.     

Solar and stellar chromospheric contrast

Positions of active regions estimated from observations of the whole solar disk in Caii K _iv during the period 1977–1989 at the Coimbra Astronomical Observatory are compared with the time-dependent latitudinal distribution of background solar magnetic fields and with the latitudinal shifts of boundaries of their polarities. We confirm that the sunspot groups are located near the zonal boundaries between the opposite polarities of the solar background magnetic field during different phases of the two recent consecutive cycles of activity. We demonstrate a probable connection between the increased number of groups and the commencement of poleward migration of zonal boundaries in both hemispheres. But the influence of the dominant convective rolls seems to he still unclear. A new problem of interrelation between the zonal and sector boundaries has also appeared.     

Radiation-hydrodynamic waves and global solar oscillations

We investigate nonadiabatic hydrodynamic waves in a nongrey, radiating, thermally conducting, homogeneous atmosphere in LTE with a finite mean free path of photons. Avoiding the Eddington approximation the remaining simplifications in the basic equations are discussed, the generalized dispersion relation is analysed, and some wave properties in a grey model are studied. The properties of waves in a stratified atmosphere are analysed as well. In connection with the predicted properties of the nonadiabatic waves we discuss observations ofp-modes by measuring brightness fluctuations.     

Application of a global polytropic model to the Jupiter's system of satellites: a numerical treatment

We study the so-called inverse planetary problem (i.e., given the distances from the centre, masses, and radii of, say, three planets of a planetary system, find the optimum polytropic index, mass, and radius of their star, and also other quantities of interest, which depend either explicitly or implicitly on the foregoing ones, e.g., central and mean density, central and mean pressure, central and mean temperature, etc.) for the system of satellites of Jupiter. In particular, Jupiter is considered as star and its satellites as planets of a proper planetary system, which is then treated numerically on the basis of the so-called global polytropic model, developed recently by the first author.     

Electric field in a pulsar's radiation channel

A pulsar's electric field is investigated for the cases of coaxial and skewed rotators. A mutually consistent solution is found for the electric field in the neutron star, in its magnetosphere (the region of closed magnetic field lines), and in the radiation channel (a channel of open magnetic field lines). In the theory of pulsar radio emission, only the longitudinal component of the electric field strength in the radiation channel, , is important, where R is the radius of the star, B_s is the magnetic inductionin it, is its angular rotation rate, and r is distance from its center.Translated from Astrofizika, Vol. 37, No. 1, pp. 97–110, January–February, 1994.     

Subarcsec 10μm imaging of bipolar flow sources

We have started a program of high-resolution (0.4/pixel) 10m imaging of bipolar outflow sources using the 10m camera CAMIRAS. We present recent results obtained at the Canada France Hawaii Telescope which reveal extended emission or IR companions in several luminous objects. The extended emission we detected probably arises from transiently heated very small grains, while the newly discovered companions could contribute significantly to the outflow activity and extended far-IR emission usually attributed to the main optical source.     

A model of impulsive loop flares revisited

A critical examination of the components of the recent impulsive loop flare model of Takakura is made. It is found that his analysis of the stability of the electron distribution resulting from anomalous heat conduction is in error and electron plasma waves would not be excited. Rather, in the regions where the electron/proton temperature ratioT _e/T _i 10, electrostatic ion-cyclotron waves would be excited and in the regions whereT _e 10, ion-acoustic waves would be excited. Ratios ofT _e/T _i 10 occur only in the late time development behind the conduction fronts. Since the anomalous resistivity due to electrostatic ion-cyclotron waves is fortuitously about 70% of the one used by Takakura, the general development will follow closely the one calculated by him. Because the anomalous resistivity due to ion-acoustic waves is about 95 times the one used by Takakura, the development in the parts of the loop whereT _e/T _i 10 for late times would be altered considerably.Also Guest Worker at NOAA Space Environment Laboratory, Boulder, Colorado, U.S.A.     

Integrability of the Yang-Mills Hamiltonian system

This paper considers the integrability of generalized Yang-Mills system with the HamiltonianH _a (p, q)=1/2(p ₁ ² +p ₂ ² +a ₁ q ₁ ² +a ₂ q ₂ ² )+1/4q ₁ ⁴ +1/4a ₃ q ₂ ⁴ + 1/2a ₄ q ₁ ² q ₂ ² . We prove that the system is integrable for the cases: (A)a ₁=a ₂,a ₃=a ₄=1; (b)a ₁=a ₂,a ₃=1,a ₄=3; (C)a ₁=a ₂/4,a ₃=16,a ₄=6. Our main result is the presentation of these integrals. Only for cases A and B does the Yang-Mills Hamiltonian possess the Painlevé property. Therefore the Painlevé test does not take account of the integrability for the case C.     

Cryovolcanism on the icy satellites

Evidence of past cryovolcanism is widespread and extremely varied on the icy satellites. Some cryovolcanic landscapes, notably on Triton, are similar to many silicate volcanic terrains, including what appear to be volcanic rifts, calderas and solidified lava lakes, flow fields, breached cinder cones or stratovolcanoes, viscous lava domes, and sinuous rilles. Most other satellites have terrains that are different in the important respect that no obvious volcanoes are present. The preserved record of cryovolcanism generally is believed to have formed by eruptions of aqueous solutions and slurries. Even Triton's volcanic crust, which is covered by nitrogen-rich frost, is probably dominated by water ice. Nonpolar and weakly polar molecular liquids (mainly N₂, CH₄, CO, CO₂, and Ar), may originate by decomposition of gas-clathrate hydrates and may have been erupted on some icy satellites, but without water these substances do not form rigid solids that are stable against sublimation or melting over geologic time. Triton's plumes, active at the time of Voyager 2's flyby, may consist of multicomponent nonpolar gas mixtures. The plumes may be volcanogenic fumaroles or geyserlike emissions powered by deep internal heating, and, thus, the plumes may be indicating an interior that is still cryomagmatically active; or Triton's plumes may be powered by solar heating of translucent ices very near the surface. The Uranian and Neptunian satellites Miranda, Ariel, and Triton have flow deposits that are hundreds to thousands of meters thick (implying highly viscous lavas); by contrast, the Jovian and Saturnian satellites generally have plains-forming deposits composed of relatively thin flows whose thicknesses have not been resolved in Voyager images (thus implying relatively low-viscosity lavas). One possible explanation for this inferred rheological distinction involves a difference in volatile composition of the Uranian and Neptunian satellites on one hand and of the Jovian and Saturnian satellites on the other hand. Perhaps the Jovian and Saturnian satellites tend to have relatively "clean" compositions with water ice as the main volatile (ammonia and water-soluble salts may also be present). The Uranian and Neptunian satellites may possess large amounts of a chemically unequilibrated comet-like volatile assemblage, including methanol, formaldehyde, and a host of other highly water- and ammonia-water-soluble constituents and gas clathrate hydrates. These two volatile mixtures would produce melts that differ enormously in viscosity The geomorphologic similarity in the products of volcanism on Earth and Triton may arise partly from a rheological similarity of the ammonia-water-methanol series of liquids and the silicate series ranging from basalt to dacite. An abundance of gas clathrate hydrates hypothesized to be contained by the satellites of Uranus and Neptune could contribute to evidence of explosive volcanism on those objects.