Measuring Polar-Angle Errors in Imaged Helioseismology     

J. R. Kennedy  W. E. Williams 《Solar physics》1998,181(2):275-286

A sixteen-day sequence of GONG full-disk Dopplergrams was computer-rotated by plus and minus 1° with respect to the midpoint of the equator to produce two additional data sets simulating errors in the instrumental alignment with the rotation axis. The unrotated and two rotated data sets were then reduced to produce tables of mode linewidths and line-peak powers. The line characteristics of the two rotated sets were compared to the unrotated set for 20 l 120. It was found that the linewidths increased as much as 55% and the line-peak powers decreased as much as 17%, with increasing l. These results are in good agreement with an earlier model (Kennedy, 1997). Differential linewidths studies indicated that the GONG instrument-network rotation-axis alignment was within 0.048° ± 0.040° during the period of the observation.  相似文献   

Interplanetary scintillation observations of interaction regions in the solar wind     

A. R. Breen  P. J. Moran  C. A. Varley  W. P. Wilkinson  P. J. S. Williams  W. A. Coles  A. Lecinski  J. Markkanen 《Annales Geophysicae》1998,16(10):1265-1282

Co-rotating interaction regions (CIRs) between fast and slow streams of plasma are a prominent feature of the solar wind. Measurements of interplanetary scintillation (IPS) using the three widely separated antennas of the EISCAT facility have been used to detect the compression regions at the leading edges of interaction regions and to determine the location and velocity of the structure. Observations show that interaction regions have developed as close to the Sun as 25–30 solar radii, a result supported by theoretical modelling which shows that the conditions needed for CIRs to develop exist inside 30 solar radii.  相似文献   

Post-collisional volcanism in a sinking slab setting—crustal anatectic origin of pyroxene-andesite magma, Caldear Volcanic Group, Neogene Alborán volcanic province, southeastern Spain     

H. P. Zeck  A. B. Kristensen  I. S. Williams 《Lithos》1998,45(1-4):499-522

Caldear Volcanic Group (CVG), a stratigraphically well defined, calc-alkaline rock complex within S^a de Gata in the eastern part of the Alpine Betic mountain chain, S Spain, consists of three distinct formations: Hernández pyroxene andesites, Bujo hornblende-bearing pyroxene andesites and Viuda hornblende-bearing pyroxene dacites–rhyolites. The letter rock formation may have developed through crystal fractionation of mainly plagioclase and pyroxenes, however there is no direct relation between two formations. CVG has a domainal structure with a northeastern domain where Hernández formation is overlain by Bujo formation while Viuda formation is absent, and a southwestern domain where Viuda formation forms the only fractionate after Hernández formation. Hernández parent magma is thought generated through crustal anatexis by dehydration melting of a predominantly amphibolitic source rock complex which was formed by metamorphism from c. 500 Ma volcano-sedimentary parent material. The domainal structure of CVG is explained by compositional variation within this protogenetic complex. Single crystal U–Pb ages of c. 500 Ma to 1800 Ma for inherited zircon support the presence of clastic material of Proterozoic derivation within the original volcano-sedimentary complex. Regional study of syn-collisional rock formations (Alpine nappe complexes) indicate that the collisional tectonic stage in the Betic-Rif orogenic belt took place rather early (25–30 Ma?) and was followed by a stage of rapid regional rock uplift, fast cooling (c. 500°C/my) and extensional tectonics in the period 22–17 Ma. This later tectonic stage was set into motion by slab break-off which set the stage for a high temperature regime in the overlying lithosphere, providing the framework for the crustal melting and magma production responsible for the calc-alkaline rocks of Alborán volcanic province. Miocene zircon with ages ranging from c. 17 to 11 Ma indicate a rather protracted magmatic development prior to eruption at c. 11 Ma. Post-collisional character of Caldear Volcanic Group thus seems well established.  相似文献   

Precambrian tidal and glacial clastic deposits: implications for Precambrian Earth–Moon dynamics and palaeoclimate     

George E. Williams 《Sedimentary Geology》1998,120(1-4):55-74

Over the past decade the study of Precambrian clastic tidal rhythmites — stacked laminae of sandstone, siltstone and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation — has provided accurate palaeotidal and palaeorotational data. Palaeotidal records obtained from tidal rhythmites may be systematically abbreviated, however, and derived periods and frequencies can be misleading. The validity of such values, including past length of day, can be assessed by testing for internal self-consistency through application of the laws of celestial mechanics. Such a test supports the estimated length of day of h derived from the late Neoproterozoic (620 Ma) Elatina–Reynella rhythmites in South Australia, and the indicated mean rate of lunar retreat of cm/year since 620 Ma. The validity of estimated lengths of day obtained from other Precambrian tidal rhythmites remain unverified because the data sets contain only one primary value directly determined from the rhythmites. The Elatina–Reynella data militate against significant Earth expansion at least since 620 Ma, and suggest that the free nutation or ‘tipping' of the Earth's fluid core has undergone a resonance with the Earth's annual forced nutation since the Neoproterozoic. Glaciogenic deposits are readily distinguishable from ejecta resulting from impacts with Earth-crossing bodies. Palaeomagnetic data, based on the geocentric axial dipole model for the geomagnetic field, indicate that Neoproterozoic and Palaeoproterozoic glaciation and cold climate near sea level occurred in low palaeolatitudes. This enigmatic finding may imply global glaciation or an increased obliquity of the ecliptic, and is relevant to modelling the effect of ice sheet formation on the Earth's obliquity history by obliquity–oblateness feedback mechanisms. Through multidisciplinary studies, clastic sedimentology and geophysics together can make substantial contributions to understanding Precambrian Earth–Moon dynamics and global palaeoenvironments.  相似文献   

The spatial structure of the dayside ionospheric trough     

S. E. Pryse  L. Kersley  M. J. Williams  I. K. Walker 《Annales Geophysicae》1998,16(10):1169-1179

Tomographic imaging provides a powerful technique for obtaining images of the spatial distribution of ionospheric electron density at polar latitudes. The method, which involves monitoring radio transmissions from the Navy Navigation Satellite System at a meridional chain of ground receivers, has particular potential for complementing temporal measurements by other observing techniques such as the EISCAT incoherent-scatter radar facility. Tomographic reconstructions are presented here from a two-week campaign in November 1995 that show large-scale structuring of the polar ionosphere. Measurements by the EISCAT radar confirm the authenticity of the technique and provide additional information of the plasma electron and ion temperatures. The dayside trough, persistently observed at high latitudes during a geomagnetically quiet period but migrating to lower latitudes with increasing activity, is discussed in relationship to the pattern of the polarcap convection.  相似文献   

Measurements of the direction of the solar wind using interplanetary scintillation     

P. J. Moran  A. R. Breen  C. A. Varley  P. J. S. Williams  W. P. Wilkinson  J. Markkanen 《Annales Geophysicae》1998,16(10):1259-1264

EISCAT observations of the interplanetary scintillation of a single source were made over an extended period of time, during which the orientation of the baselines between the two observing sites changed significantly. Assuming that maximum correlation between the scintillations observed at the two sites occurs when the projected baseline is parallel to the direction of plasma flow, this technique can be used to make a unique determination of the direction of the solar wind. In the past it has usually been assumed that the plasma flow is radial, but measurements of eleven sources using this technique have indicated conclusively that in at least six cases observed at mid or high heliocentric latitude there is a significant non-radial component directed in four cases towards the heliocentric equator and in two cases towards the pole.  相似文献   

Velocity of auroral arcs drifting equatorward from the polar cap     

P. J. S. Williams  C. F. del Pozo  I. Hiscock  R. Fallows 《Annales Geophysicae》1998,16(10):1322-1331

The drift velocity of an auroral arc is compared with the component of F-region plasma velocity in the same direction for ten cases where the arc is seen to move steadily equatorward for several minutes without any major change in appearance or orientation. In most cases the two velocities are close, but on two occasions the drift velocity of the arc is much higher than the plasma velocity. From the cases studied it appears that during the growth and recovery phase of the substorm cycle the arc moves with a velocity close to the convection velocity, but during the expansion phase this is not the case.  相似文献   

Recruitment of juvenile marine fishes to seagrass habitat in a temperature Australian estuary     

J. C. Hannan  R. J. Williams 《Estuaries and Coasts》1998,21(1):29-51

We investigated the relationship between distance from the ocean and the recruitment of ocean-spawned juvenile fish to seagrass shallows within marine dominated Lake Macquarie, a coastal barrier lagoon in Southeast Australia. Samples were taken by seine net every 6 wk between June 1986 and June 1987, at 20 sites established at various distances from the entrance channel. The fish fauna was diverse: 80 species from 39 families were caught, with the Gobiidae, Monacanthidae, Syngnathidae, Tetraodontidae, Mugilidae, Atherinidae, Clupeidae, Mullidae, Sparidae, and Blenniidae being well represented. Ambassis jacksoniensis, Atherinomorus ogilbyi, and Gerres subfasciatus accounted for 46% of all individuals. Thirty-two species were classified as ocean spawners, 38 as lagoon spawners, and 10 as unknown in terms of spawning area. Newly settled juveniles of ocean spawners were concentrated near the lagoon's entrance, where most recruited in spring. This pattern occurred in the absence of a salinity gradient: distance from the ocean coupled with weak internal water circulation appears to limit larval distribution and hence juvenile recruitment. Small juveniles of Rhabdosargus sarba were sufficiently abundant for their subsequent dispersal to be directly traced. On the basis of results for this species, and indirect evidence of dispersal for several others, it is suggested that ocean-spawned juveniles, after settling near the entrance, gradually disperse as they grow and change their habitat. Thus, further from the entrance, their recruitment to seagrass shallows is later, at larger sizes, and in smaller numbers. Newly settled juveniles of lagoon spawners, however, were widely distributed within the lagoon. The optimal recruitment of ocean-spawned juveniles to similar coastal lagoons may depend on suitable habitat being available near the entrance. *** DIRECT SUPPORT *** A01BY081 00003  相似文献   

Book review – D.L. Clements and I. Perez-Fournon (eds.), Quasar Hosts     

Robin Williams 《Astrophysics and Space Science》1998,260(4):551-552

  相似文献   

The Mars oxidant experiment (MOx) for Mars '96     

McKay CP  Grunthaner FJ  Lane AL  Herring M  Bartman RK  Ksendzov A  Manning CM  Lamb JL  Williams RM  Ricco AJ  Butler MA  Murray BC  Quinn RC  Zent AP  Klein HP  Levin GV 《Planetary and Space Science》1998,46(6-7):769-777

The MOx instrument was developed to characterize the reactive nature of the martian soil. The objectives of MOx were: (1) to measure the rate of degradation of organics in the martian environment; (2) to determine if the reactions seen by the Viking biology experiments were caused by a soil oxidant and measure the reactivity of the soil and atmosphere: (3) to monitor the degradation, when exposed to the martian environment, of materials of potential use in future missions; and, finally, (4) to develop technologies and approaches that can be part of future soil analysis instrumentation. The basic approach taken in the MOx instrument was to place a variety of materials composed as thin films in contact with the soil and monitor the physical and chemical changes that result. The optical reflectance of the thin films was the primary sensing-mode. Thin films of organic materials, metals, and semiconductors were prepared. Laboratory simulations demonstrated the response of thin films to active oxidants.  相似文献