Infrared spectrophotometry of Comet IRAS-Araki-Alcock (1983d): a bare nucleus revealed?

Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8–13 μm on 11 May and 2–4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ～ 10⁵ g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ～ 10^?5 g/cm²/sec.     

Investigating the mechanisms leading to the deglaciation of past continental northern hemisphere ice sheets with the CLIMBER–GREMLINS coupled model

A coupling procedure between a climate model of intermediate complexity (CLIMBER-2.3) and a 3-dimensional thermo-mechanical ice-sheet model (GREMLINS) has been elaborated. The resulting coupled model describes the evolution of atmosphere, ocean, biosphere, cryosphere and their mutual interactions. It is used to perform several simulations of the Last Deglaciation period to identify the physical mechanisms at the origin of the deglaciation process. Our baseline experiment, forced by insolation and atmospheric CO₂, produces almost complete deglaciation of past northern hemisphere continental ice sheets, although ice remains over the Cordilleran region at the end of the simulation and also in Alaska and Eastern Siberia. Results clearly demonstrate that, in this study, the melting of the North American ice sheet is critically dependent on the deglaciation of Fennoscandia through processes involving switches of the thermohaline circulation from a glacial mode to a modern one and associated warming of the northern hemisphere. A set of sensitivity experiments has been carried out to test the relative importance of both forcing factors and internal processes in the deglaciation mechanism. It appears that the deglaciation is primarily driven by insolation. However, the atmospheric CO₂ modulates the timing of the melting of the Fennoscandian ice sheet, and results relative to Laurentide illustrate the existence of threshold CO₂ values, that can be translated in terms of critical temperature, below which the deglaciation is impeded. Finally, we show that the beginning of the deglaciation process of the Laurentide ice sheet may be influenced by the time at which the shift of the thermohaline circulation from one mode to the other occurs.     

Sediment Trend Analysis through the variation of granulometric parameters: A review of theories and applications

Spatial variations in grain-size parameters (i.e. grain-size trends) contain information on sediment transport patterns. Analytical procedures have been proposed using the grain-size trend to determine net sediment transport pathways. In the first part of this study, the fundamentals of the theory are presented through methods for analysing 1D and 2D variations. The methods used are critically discussed, while pointing out some severe problems. So far, these methods suffer from limitations leading to serious interpretational errors, making it necessary to take account of two kinds of uncertainties. Inputs uncertainties are linked to the physical sediment properties as well as procedures of sampling and analysis. Model uncertainties are then discussed for each step of the grain-size trend analysis. The validity of Sediment Trend Analysis under natural conditions is tested against published field studies to determine the most appropriate variation trend to use in a specific environment. Proposals are given for each step of the procedure for optimal use of the method using a Quality Assurance (QA) approach. Further developments are proposed, such as integration into a Geographic Information System.     

Sediment transport pattern at the Barra Nova inlet, south Portugal: a conceptual model

The Barra Nova inlet, in south Portugal, is known to migrate progressively southeastwards under wave action. The morphodynamics of this system during a representative year suggests that this long-term evolution is dependent on a seasonal behavior of the tidal inlet which can be described through a three-stage model of post-storm, transition and extended fair-weather conditions. Processes involved in this evolution indicate that the historical migration of the Barra Nova is not dependent on the longshore drift forcing constricting the channel on the updrift coast, but rather on the adjustment of the system to a major erosion of the downdrift coast during short storm events.     

GIScience Journals Ranking and Evaluation: An International Delphi Study

Researchers’ fame in most scientific fields is closely linked to their publishing capacity, both in terms of quantity and quality. In GIScience, as in other fields, this situation demands that the researcher evaluate and to be very familiar with the scientific journals in which they could publish. Some specialized journals (e.g. Journal of Citation Reports or JCR) are devoted to ranking these reviews according to various methods and criteria. Compared to other scientific communities, GIScience is relatively new and constantly evolving. Therefore, the journals of this field do not benefit from any real formal ranking yet. The objective of this paper is to present the process and results of a study aimed at addressing this gap. More specifically, the challenge is to elaborate an importance ranking of the scientific journals in the field of GIScience. To do so, both a qualitative (Delphi study carried out with 40 international experts) and a quantitative (JCR impact factor) approach has been implemented. This triangulation method leads to an early global ranking of the journals of this field.     

Ca X line ratios in solar flares

Theoretical Ca X electron temperature sensitive emission line ratios, derived using electron excitation rates interpolated from accurateR-matrix calculations, are presented forR ₁ =I(419.74 )/I(574.02 ,),R ₂ =I(411.65 )/I(574.02 ),R ₃ =I(419.74 )/I(557.75 ), andR ₄ =I(411.65 )/I(557.75 ). A comparison of these with observational data for three solar flares, obtained by the Naval Research Laboratory's S082A slitless spectrograph on boardSkylab, reveals good agreement between theory and observation forR ₁ andR ₃ in one event, which provides limited support for the accuracy of the atomic data adopted in the analysis. However, in the other flares the observed values ofR ₁ –R ₄ are much larger than the theoretical high-temperature limits, which is probably due to blending of the 419.74 line with Civ 419.71 , and 411.65 with possibly Ciii 411.70 .     

Inviscid behaviour of fines-rich pyroclastic flows inferred from experiments on gas-particle mixtures

Experiments were carried out on granular flows generated by instantaneous release of gas-fluidised, bidisperse mixtures and propagating into a horizontal channel. The mixture consists of fine (< 100 μm) and coarse (> 100 μm) particles of same density, with corresponding grain size ratios of ∼ 2 to 9. Initial fluidisation of the mixture destroys the interparticle frictional contacts, and the flow behaviour then depends on the initial bed packing and on the timescale required to re-establish strong frictional contacts. At a fines mass fraction (α) below that of optimal packing (∼ 40%), the initial mixtures consist of a continuous network of coarse particles with fines in interstitial voids. Strong frictional contacts between the coarse particles are probably rapidly re-established and the flows steadily decelerate. Some internal friction reduction appears to occur as α and the grain size ratio increases, possibly due to particle rolling and the lower roughness of internal shear surfaces. Segregation only occurs at large grain size ratio due to dynamical sieving with fines concentrated at the flow base. In contrast, at α above that for optimal packing, the initial mixtures consist of coarse particles embedded in a matrix of fines. Flow velocities and run-outs are similar to that of the monodisperse fine end-member, thus showing that the coarse particles are transported passively within the matrix whatever their amount and grain size are. These flows propagate at constant height and velocity as inviscid fluid gravity currents, thus suggesting negligible interparticle friction. We have determined a Froude number of 2.61 ± 0.08 consistent with the dam-break model for fluid flows, and with no significant variation as a function of α, the grain size ratio, and the initial bed expansion. Very little segregation occurs, which suggests low intensity particle interactions during flow propagation and that active fluidisation is not taking place. Strong frictional contacts are only re-established in the final stages of emplacement and stop the flow motion. We infer that fines-rich (i.e. matrix-supported) pyroclastic flows propagate as inviscid fluid gravity currents for most of their emplacement, and this is consistent with some field data.