A simple model for the prediction of the deflation threshold shear velocity of dry loose particles   总被引：3，自引：0，他引：3  

W. M. Cornelis  D. Gabriels 《Sedimentology》2004,51(1):39-51

A very important parameter in aeolian equations is the deflation threshold shear velocity, which quantifies the instant of particle motion. In this paper, a simple model is presented for the prediction of the threshold shear velocity of dry loose particles. It has the same functional form as the widely used models of Bagnold (1941) and Greeley & Iversen (1985), but differs in its treatment of the so‐called threshold parameter. As the new expression was based on the moment balance equation used by Greeley & Iversen, it includes a function for the aerodynamic forces, including the drag force, the lift force and the aerodynamic moment force, and a function for the interparticle forces. The effect of gravitation is incorporated in both functions. However, rather than using an implicit function for the effect of the aerodynamic forces as in the Greeley & Iversen model, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals' force between two particles, it was also shown that the function for the interparticle cohesion force is inversely proportional to the particle diameter squared. The model was calibrated on data reported by Iversen & White (1982). The new expression compared, at least for terrestrial conditions, very well with the Greeley & Iversen model, although it is much simpler. It was finally validated with data from wind‐tunnel experiments on different fractions of dune sand and sandy loam soil aggregates. The soil aggregates were treated as individual particles with a density equal to their bulk density. The good agreement between observations and predictions means that, when predicting mass transport of particles above a given soil, minimally dispersed particle‐size distributions should be considered rather than the granulometric composition of the soil.  相似文献   

Evolution of the Molopo drainage and its possible disruption by the Morokweng impact event at the Jurassic-Cretaceous boundary     

Cornelis S. Bootsman  Wolf Uwe Remold  Dion Brandt 《Journal of African Earth Sciences》1999,29(4):437

A large, deeply eroded impact structure, known as the Morokweng impact structure, has recently been positively identified in the North West Province of South Africa, centred on the Ganyesa Dome about 80 km northwest of Vryburg. The marked geomorphic effect of the Morokweng impact event at 145 ± 2 Ma is identified through large-scale analysis of the long-term evolution of the Molopo River drainage system. A comparison of the pre- and post-impact drainage patterns shows that a dramatic change in both drainage direction and pattern occurred after the impact event. Prior to the impact, the regional drainage direction of the proto-Molopo had been towards the northwest, where it connected with the major southwest trending drainage line of the Permo-Carboniferous (300 Ma) Kalahari Basin. Reconstruction of the post-impact drainage pattern, as it existed in the Late Cretaceous ( 75 Ma), just before the formation of the rejuvenated Cenozoic Kalahari Basin, shows a generally southward flowing system with annular drainage pattern components. With the filling of the Kalahari Basin since Tertiary times, the affinity between the Molopo drainage and the Morokweng impact structure became less obvious.  相似文献   

Premetamorphic lateritisation in Proterozoic metabasites of Rogaland,SW Norway   总被引：1，自引：0，他引：1  

Luc C. G. M. Bol  Cornelis Maijer  J. Ben H. Jansen 《Contributions to Mineralogy and Petrology》1989,103(3):306-316

Heterogeneous layers of granulite facies metamorphic basites of the Proterozoic supracrustal Faurefjell Formation in Rogaland, S.W. Norway, display an extreme chemical variation. Within a single layer the bulk chemical composition gradually changes from approximately basaltic in basic granulites to alumina-iron-rich in granofelses. Component-ratios and composition-volume relations indicate open-system chemical reactions mainly involving the extraction of silica. Apparent enrichment in Fe, Ti, P, Al, Zr, Ni, Co, Zn, Y, Nb, Hf and REE and variations in resulting metamorphic mineral assemblages are related to premetamorphic progressive lateritisation of a basaltic protolith. The weathering generated a continuous chemical suite from SiO₂=48 wt%, Fe₂O₃=10 wt% and Al₂O₃=19 wt% in the basic granulites to 14 wt%, 40 wt% and 25 wt% in the Fe-Al granofelses. Metasomatism during diagenesis and during (very) high-grade metamorphism (1200–900 Ma) further perturbed the concentrations of relatively mobile elements Ca, Mg, K, Rb, Sr, Ba, Na and Li in the laterites without affecting the transition metal ratios. In particular, the REE did not fractionate differentially during the supracrustal and metasomatic alteration.  相似文献   

Late Pleistocene and Holocene palaeogeography of the Lower Tagus Valley (Portugal): effects of relative sea level, valley morphology and sediment supply     

Geert-Jan Vis  Cornelis Kasse  Jef Vandenberghe 《Quaternary Science Reviews》2008,27(17-18):1682-1709

Because of the deep glacial incision, the Lower Tagus Valley hosts a sedimentary record since 20 000 cal BP, making this a unique site along the European Atlantic margin with respect to palaeogeographic and sea level changes. Based on nine cross-sections and 55 radiocarbon dates together with a newly created relative sea level curve, we constructed five palaeogeographic maps of the infill of the Lower Tagus Valley since 20 000 cal BP. We illustrate that relative sea level rise and fluvial sediment supply were the prime forcing factors determining the depositional history and palaeogeographic changes. Around 20 000 cal BP a deeply incised braided river existed, which was directly connected to the ocean across the narrow continental shelf. After that (12 000 cal BP) the gradually moister and warmer climate caused a change to a single-channel river. During the following period (12 000–7000 cal BP) relative sea level rise resulted in a transgression in the Lower Tagus Valley and the establishment of extensive tidal environments. After relative sea level rise had ended (7000 cal BP) the valley was progressively filled by a fluvial wedge and tidally influenced bayhead delta. Since 1000 cal BP the valley-fill history was dominated by increased sediment input due to human-induced degradation of catchment slopes. Generally, climate was of subordinate importance during the entire studied period, merely causing a single-channel river resulting from the change from the cold Heinrich event 1 to the temperate Bölling–Allerød interstadial. Despite the tectonic activity in the region, neotectonic uplift or subsidence was limited, as supported by the horizontal relative sea level curve since 7000 cal BP. Neotectonics played a minor role due to the large distance from the Fennoscandian ice sheet and the narrow continental shelf, which prohibited strong glacio- and hydro-isostatic movements.  相似文献   

3-D MHD SIMULATION OF X-TYPE COALESCENCE OF TWO CURRENT-LOOPS     

Sakai  Jun-ichi  de Jager  Cornelis 《Solar physics》1997,173(2):347-358

We present simulation results of X-type coalescence of two current-loops, by using a 3-D resistive MHD code. The results are compared with the limb flare observed by Yohkoh on 2 August, 1993 at 08:30 UT, which is a good example of partial X-type coalescence. It is shown that the maximum temperature enhancement near the cross-point of the two loops, obtained from the simulation, agrees well with the observations when the plasma is 0.08.  相似文献   

New observations of coronal explosions and their interpretation     

André F. P. Lemmens  Cornelis De Jager 《Solar physics》1986,106(2):365-379

We examined five flares, observed by the Hard X-Ray Imaging Spectrometer aboard the Solar Maximum Mission, for the occurrence of coronal explosions and found that these occur only if (a) the flare shows distinct single impulsive hard X-ray bursts and (b) it shows upward (convective) motions during the initial part of the impulsive phase. Coronal explosions are therefore explained as a manifestation of plasma streaming laterally out of the flare kernel(s). There is some evidence that streaming occurs into a number of cylindrical fluxtubes which spread over a larger area, thus supporting the spaghetti-bundle model for the flaring region.  相似文献   

A high-energy solar flare burst complex and the physical properties of its source region     

Cornelis De Jager  Jan Kuijpers  Emilia Correia  Pierre Kaufmann 《Solar physics》1987,110(2):317-326

We discuss a solar flare microwave burst complex, which included a major structure consisting of some 13 spikes of 60 ms FWHM each, observed 21 May, 1984 at 90 GHz (3 mm). It was associated with a simultaneous very hard X-ray burst complex. We suggest that the individual spikes of both bursts were caused by the same electron population: the X-bursts by their bremsstrahlung, and the microwave bursts by their gyrosynchrotron emission. This latter conclusion is based on the evidence that the radio turnover frequency was 150 GHz. It follows that the emission sources were characterized by an electron density of about 10¹¹ cm^–3, a temperature of 5 × 10⁸ K and a magnetic field of about 1400–2000 G. They had a size of about 350 km; if the energy release is caused by reconnection the sources of primary instability could have been smaller and in the form of thin sheets with reconnection speed at a fraction of the Alfvén velocity and burst-like energy injections of 10²⁷ erg during about 50 ms each. The energized plasma knots lost their injection energy by saturated convective flux (collisionless conduction) in about 30 ms.  相似文献   

Acceleration through X-type coalescence in a small energetic flare     

Jun-Ichi Sakai  Cornelis de Jager 《Solar physics》1989,123(2):389-392

We studied the acceleration conditions in the small but fairly energetic flare of May 21, 1984 at 1326 UT. The most pronounced aspect of this flare was a series of 13 microwave/X-ray spikes, each lasting for about 0.1 s. A previous study has shown that each of these was due to a series of successive sudden formations of small plasma knots of high-energy particles. Each of these knots lost its energy in about 50 ms. In the present study we show that these knots can originate by the process of X-type (3-D) flux tube coalescence. The predicted rise time (30 to 50 ms) and energy are in good agreement with the observationally derived parameters.  相似文献   

Kernel heating and ablation in the impulsive phase of two solar flares     

De Jager  Cornelis 《Solar physics》1985,98(2):267-280

At the very start of the impulsive phase of two solar flares the temperature derived from medium-energy ( 16 keV) X-ray countrates was observed to rise abruptly, by several times 10⁷ K above the temperature derived from low-energy X-ray ( 7 keV) countrates. The difference between the two temperatures relaxed to zero thereafter, quasi-exponentially, with a characteristic time of 1.5 min. This differential temperature variation appears to mimique the differences between the ionic kinetic and the electron temperatures derived from spectral observations (Figures 1 and 2).These observations are explained in a quantitatively supported model of the flare kernel (Figure 4) in which the kernel is heated by electron beams from above. The low-energy electrons are stopped above the kernel and only the medium and high energy electrons penetrate down to the top of the chromosphere, causing heating of the chromospheric gas to about 50 MK, and ablation (evaporation), leading to the abrupt formation of a superhot flare kernel and a likely superhot dome above it (Figure 4), through which gas rises up and spreads out convectively, while cooling down in approximately the same time (45 s). The heating process lasts only for a few minutes. The difference between the Doppler temperature and the electron temperature derived from line intensity ratios or from low energy countrate ratios is ascribed to truncation of the tail of the electron energy distribution in the kernel. The kernel is about 2500 km deep; H emission is radiated by a thin layer at its basis.  相似文献   

Coronal explosions     

Cornelis De Jager 《Solar physics》1985,96(1):143-156

We searched for a new phenomenon, called coronal explosions, in three solar flares, and found them in all three. A coronal explosion is the propagation of a density wave through the flaring area. The wave emerges from one or two small areas (the sources) which are close to, but not identical with the sources of hard X-ray burst emission. In all three cases the explosion starts at the end of the impulsive phase, during or after the last hard ( 20 keV) X-ray burst. The velocities of propagation range between 1800 and a few tens of km s^-1, and tend to decrease with time. We suggest that the bursts are magneto-hydrodynamical (shock) waves moving downward into denser regions.  相似文献