A Yohkoh search for “black-light flares”     

Lidia Van Driel-Gesztelvi  Hugh S. Hudson  Bachtiar Anwar  Eijiro Hiei 《Solar physics》1994,152(1):145-151

Calculations which predict that a phenomenon analogous to stellar negative pre-flares could also exist on the Sun were published by Hénouxet al. (1990), and Aboudarhamet al., (1990), who showed that at the beginning of a solar white-light flare (WLF) event an electron beam can cause a transient darkening before the WLF emission starts, under certain conditions. They named this event a black light flare (BLF). Such a BLF event should appear as diffuse dark patches lasting for about 20 seconds preceding the WLF emission, which would coincide with intense and impulsive hard X-ray bursts. The BLF location would be at (or in the vicinity of) the forthcoming bright patches. Their predicted contrast depends on the position of the flare on the solar disc and on the wavelength band of the observation.TheYohkoh satellite provided white-light data from the aspect camera of the SXT instrument (Tsunetaet al., 1991), at 431 nm and with a typical image interval of 10–12 s. We have studied nine white-light flares observed with this instrument, with X-ray class larger than M6. We have found a few interesting episodes, but no unambiguous example of the predicted BLF event. This study, although the best survey to date, was not ideal from the observational point of view. We therefore encourage further searches. Successful observations of this phenomenon on the Sun would greatly strengthen our knowledge of the lower solar atmosphere and its effects on solar luminosity variations.  相似文献   

Thermodynamic data from redox reactions at high temperatures. II. The MnO-0Mn3O4 oxygen buffer,and implications for the thermodynamic properties of MnO and Mn3O4     

Hugh St. C. O'Neill  Mark I. Pownceby 《Contributions to Mineralogy and Petrology》1993,114(3):315-320

The \(\mu _{O_2 } \) defined by the reaction 6 MnO+O₂ =2 Mn₃O₄ has been determined from 917 to 1,433 K using electrochemical cells (with calcia-stabilized zirconta, CSZ) of the type: Steady emfs were achieved rapidly at all temperatures on both increasing and decreasing temperature, indicating that the MnO-Mn₃O₄ oxygen buffer equilibrates relatively easily. It therefore makes a useful alternative choice in experimental petrology to Fe₂O₃-Fe₃O₄ for buffering oxygen potentials at oxidized values. The results are (in J/mol, temperature in K, reference pressure 1 bar); \(\mu _{O_2 } \) (±200)=-563,241+1,761.758T-220.490T inT+0.101819T ² with an uncertainty of ±200 J/mol. Third law analysis of these data, including a correction for the deviations in stoichiometry of MnO, impliesS _298.15 for Mn₃O₄ of 166.6 J/K · mol, which is 2.5 J/K · mol higher than the calorimetric determination of Robie and Hemingway (1985). The low value of the calorimetric entropy may be due to incomplete ordering of the magnetic spins. The third law value of Δ _r H _298.15 ⁰ is-450.09 kJ/mol, which is significantly different from the calorimetric value of-457.5±3.4 kJ/mol, calculated from Δ _f H _298.15 ⁰ of MnO and Mn₃O₄, implying a small error in one or both of these latter.  相似文献   

Controlled source apparent resistivity tensors and their relationship to the magnetotelluric impedance tensor   总被引：2，自引：0，他引：2  

T. Grant Caldwell  Hugh M. Bibby  Colin Brown 《Geophysical Journal International》2002,151(3):755-770

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Inferring erosional resistance of bedrock units in the east Tennessee mountains from digital elevation data     

Hugh H. Mills   《Geomorphology》2003,55(1-4):263

Measures of local relief, regional relief, and slope were calculated from digital elevation models (DEMs) for 50 bedrock units in the Ridge and Valley and Blue Ridge provinces of Tennessee. Each of these measures was normalized and the three were then averaged to produce the erosional resistance index (ERI). Bedrock units with higher ERI values include coarse clastics, intermediate clastics, and metaplutonics. Units with lower values include shales, limestones, limestones plus dolostones, and carbonates plus fine clastics. Dolostones tend to have intermediate values. The calculated ERI values were compared with subjective ratings by a geologist with decades of field experience in east Tennessee. Generally, the agreement between the two ratings was good, the most glaring exception being several shales with improbably high ERI values. These turned out to be thin units cropping out beneath very hard sandstones, allowing them to stand higher and steeper than would otherwise be possible. A systematic method for detecting such erroneously high ERI values is suggested. Inspection of a drainage map superimposed on the geology map shows that in a given area, streams tend to flow on rock units with the lowest ERI values. In addition, statistical analysis shows that bedrock units with the lowest ERI values are, on average, almost three times closer to the nearest stream and six times as likely to have streams flowing on them than are units with highest values. Further, the effect of ERI on stream location is strongest for streams with drainage areas between 1 and 30 km². Thus, small streams appear to be subject to greater lithologic control than are larger streams.  相似文献   

Tensor time domain electromagnetic resistivity measurements at Ngatamariki geothermal field, New Zealand     

George F. Risk  T. Grant Caldwell  Hugh M. Bibby 《Journal of Volcanology and Geothermal Research》2003,127(1-2):33-54

Experimental measurements in the Ngatamariki geothermal field, North Island, New Zealand were made to test the applicability of the time domain electromagnetic method for detailed investigation of the resistivity structure within a geothermal field. Low-frequency square wave signals were transmitted through three grounded bipole current sources sited about 8 km from the measurement lines. Despite high levels of electrical noise, transient electric field vectors could be determined reliably for times between 0.02 and 3.3 s after each step in the source current. Instantaneous apparent resistivity tensors were then calculated. Apparent resistivity pseudosections along the two measurement lines show smooth variations of resistivity from site to site. Over most of the field the images consistently show a three-layer resistivity structure with a conductive middle layer (3–10 Ωm) representing the conductive upper part of the thermal reservoir. A deep-seated region of low resistivity in the northwest of the field may indicate a conductive structure at about 1 km associated with a deeper diorite intrusion. Measurements sited closer than about 100 m to drillholes appear to have been disturbed by metallic casing in the holes. A change in resistivity structure in the east of the field may indicate a major geological or hydrothermal boundary.  相似文献   

Papua New Guinea Highlands: palaeomagnetic constraints on terrane tectonics     

Chris Klootwijk  John Giddings  Chris Pigram  Charles Loxton  Hugh Davies  Rick Rogerson  David Falvey 《Tectonophysics》2003,362(1-4):239-272

Oblique convergence since the Early Cenozoic between the northward-moving Australian plate, westward-moving Pacific plate and almost stationary Eurasian plate has created a world-ranking tectonic zone in the eastern Indonesia–New Guinea–Southwest Pacific region (Tonga–Sulawesi megashear) that is notorious for its complex mix of tectonic styles and terrane juxtapositions. Unlike an ancient analog—the Mesozoic–Cenozoic Cordillera of North America—palaeomagnetic constraints on terrane motions in the zone are few. To improve the framework of quantitative control on such motions and therefore our understanding of the development of the zone, results of a palaeomagnetic study in the Highlands region of Papua New Guinea (PNG), in the southern part of the New Guinea Orogen, are reported. The study yields new insights into terrane tectonics along the Australian craton's active northern margin and confirms the complexity of block rotations to be expected at the local scale in tectonically intricate zones. The study is based on more than 500 samples (21 localities) collected from an interior and an exterior zone of New Guinea's central cordillera. The two zones are separated by the Tahin and Stolle–Lagaip–Kaugel Fault zones and collectively represent the para-autochthonous northern margin of the Australian craton. Samples from the interior zone, which in the study area comprises a cratonic spur of uncertain—probably displaced—origin, come from Triassic to Miocene sediments and subordinate volcanics of the Kubor Anticline, Jimi Terrane, and Yaveufa Syncline (16 localities) in the central and eastern Highlands. Samples from the exterior zone, which represent a basement-involved, Pliocene foreland fold-and-thrust belt, come from Middle Eocene to Middle Miocene carbonates and clastics (five localities) in the southern Highlands of the Papuan Fold Belt. Results permit us to constrain the tectonic evolution of the two zones palaeomagnetically. Using mainly thermal demagnetization techniques, three main magnetic components have been identified in the collection: (1) a recent field overprint of both normal and reverse polarity; (2) a pervasive overprint of mainly normal polarity that originated during extensive Middle to Late Miocene intrusive activity in the central cordillera; and (3) a primary component which has been identified in only 7 of the 21 localities (5 of 11 stratigraphic units represented in the collection). All components show patterns of rotation that are consistent within the zones, but differ between them. In the interior zone (central and eastern Highlands), large-scale counterclockwise rotations of between 30°+ and 100°+ have been established throughout the Kubor Anticline and Jimi Terrane, with some clockwise rotation present in the southern part of the Yaveufa Syncline. In contrast, in the Mendi area of the exterior zone (southern Highlands), clockwise rotations of between 30°+ and 50°+ can be recognized. These contrasting rotation patterns across the Tahin and Stolle–Lagaip–Kaugel Fault zones indicate decoupling of the two tectonic zones, probably along basement-involved faults. The clockwise rotations in the southern Highlands of the Papuan Fold Belt are to be expected from its structural grain, and are probably governed by regional basement faults and transverse lineaments. In contrast, the pattern of counterclockwise rotations in the Kubor Anticline–Jimi Terrane cratonic spur of the central and eastern Highlands was unexpected. The pattern is interpreted to result from non-rigid rotation of continental terranes as they were transported westward across the northeastern margin of the Australian craton. This margin became reorganised after the Middle Miocene, when the steadily northward-advancing Australian craton impinged into the westward-moving Pacific plate/buffer-plate system. Transpressional reorganisation under the influence of the sinistral Tonga–Sulawesi megashear became enhanced with Mio-Pliocene docking, and subsequent southward overthrusting, of the Finisterre Terrane onto the northeastern margin of the Australian craton.  相似文献   

CURRENT GEOSCIENCE LABORATORIES GEOANALYTICAL PROGRAMS AND THEIR QUALITY ASSURANCE UNDERPINNINGS     

Jean M. RICHARDSON  Peter C. LIGHTFOOT  Hugh de  SOUZA 《Geostandards and Geoanalytical Research》1996,20(1):141-156

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Litho-asthenospheric structures of northern Italy as inferred from teleseismic P-wave tomography     

S. Solarino  D. Spallarossa  S. Parolai  M. Cattaneo  C. Eva 《Tectonophysics》1996,260(4):271-289

Regional three-dimensional inversions of teleseismic P-wave travel time residuals recorded by high-frequency regional and local seismic networks operating along the Western Alps and surrounding regions were carried out and lithosphere and upper mantle P-wave velocity models down to 300 km were obtained.

Residuals of more than 500 teleseismic events, recorded by 98 fixed and temporary seismic stations, have been inverted.

The comparison between real residuals and the ones obtained from tomographic model indicates that the method is able to solve the feature of the regional heterogeneities.

Where the resolution is good, coherent lithospheric and upper mantle structures are imaged. In the shallower layers, high- and low-velocity anomalies follow the structural behaviour of the Alpine-Apenninic chains showing the existence of very strong velocity contrasts. In the deepest layers, velocity contrast decreases however two deep-seated high-velocity structures are observed. The most extended in depth and approximately trending NE-SW has been interpreted as a wreck of the oldest subduction responsible of the Alpine orogenesis. The second one, connected to the northwestern sector of the Apenninic chain, appears to vanish at depths greater than 180 km and is probably due to still active Apenninic roots.

Cross-sections depict the spatial trend of perturbations and in particular outline the sub-vertical character of the Alpine and Apenninic anomalies. Under the Ligurian Sea, the 3-D inversion confirms the uplift of the asthenosphere in agreement with the tectonic evolution of the basin.  相似文献   

The reaction MgCr2O4 + SiO2 = Cr2O3 + MgSiO3 and the free energy of formation of magnesiochromite (MgCr2O4)     

Stephan Klemme  Hugh S. C. O'Neill 《Contributions to Mineralogy and Petrology》1997,130(1):59-65

Low-temperature heat capacity measurements for MgCr₂O₄ have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K⁻¹ mol⁻¹) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Néel temperature, at which magnetic ordering of the Cr³⁺ ions in MgCr₂O₄ occurs, is at ∼15 K. Hence a substantial contribution to the entropy of MgCr₂O₄ has been missed. We have determined the position of the near-univariant reaction MgCr₂O₄+SiO₂=MgSiO₃+Cr₂O₃. The reaction, which has a small positive slope in P-T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish, and lengthy run times with a flux (H₂O, BaO-B₂O₃ or K₂O-B₂O₃) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr₂O₃, MgSiO₃ and SiO₂, give the entropy and enthalpy of formation of MgCr₂O₄ spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data. We obtain Δ _f H ^∘ ₂₉₈=−1759.2±1.5 kJ mol⁻¹ and S ^∘ ₂₉₈=122.1±1.0 J K⁻¹ mol⁻¹ for MgCr₂O₄. This entropy is some 16 J K⁻¹ mol⁻¹ more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr₂O₄ consistent with an effective spin quantum number (S') for Cr³⁺ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching. Received: 9 May 1997 / Accepted: 28 July 1997  相似文献