A current ribbon model for energy storage and release with application to the flare of 7 January 1992     

Longcope  D. W.  Silva  A. V. R. 《Solar physics》1998,179(2):349-377

Observations of the flare on 7 January 1992 are interpreted using a topological model of the magnetic field. The model, developed here, applies a theory of three-dimensional reconnection to the inferred magnetic field configuration for 7 January. In the model field a new bipole ( 10²¹ Mx) emerges amidst pre-existing active region flux. This emergence gives rise to two current ribbons along the boundaries (separators) separating the distinct, new and old, flux systems. Sudden reconnection across these boundary curves transfers 3 ×10²⁰ Mx of flux from the bipole into the surrounding flux. The model also predicts the simultaneous (sympathetic) flaring of the two current ribbons. This explains the complex two-loop structure noted in previous observations of this flare. We subject the model predictions to comparisons with observations of the flare. The locations of current ribbons in the model correspond closely with those of observed soft X-ray loops. In addition the footpoints and apexes of the ribbons correspond with observed sources of microwave and hard X-ray emission. The magnitude of energy stored by the current ribbons compares favorably to the inferred energy content of accelerated electrons in the flare.  相似文献   

Book reviews     

S. I. Rasool  Charles Pereira F.R.S.  Stephen H. Schneider 《Climatic change》1980,3(1):105-110

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A parameterization of radiative fluxes suitable for use in a statistical-dynamical model     

S. H. Franchito  V. Brahmananda Rao  R. Ramos da Silva 《Meteorology and Atmospheric Physics》1998,69(1-2):23-38

Summary A parameterization of shortwave and longwave radiation fluxes derived from detailed radiative transfer models is included in a global primitive equation statistical-dynamical model (SDM) with two bulk atmospheric layers. The model is validated comparing the model simulations with the observed mean annual and seasonal zonally averaged climate. The results show that the simulation of the shortwave and longwave radiation fluxes matches well with the observations. The SDM variables such as surface and 500 hPa temperatures, zonal winds at 250 hPa and 750 hPa, vertical velocity at 500 hPa and precipitation are also in good agreement with the observations. A comparison between the results obtained with the present SDM and those with the previous version of the model indicates that the model results improved when the parameterization of the radiative fluxes based on detailed radiative transfer models are included into the SDM.The SDM is used to investigate its response to the greenhouse effect. Sensitivity experiments regarding the doubling of CO₂ and the changing of the cloud amount and height are performed. In the case 2×CO₂ the model results are consistent with those obtained from GCMs, showing a warming of the climate system. An enhancement of the greenhouse effect is also noted when the cloud layer is higher. However, an increase of the cloud amount in all the latitude belts provokes an increase of the surface temperature near poles and a decrease in all the other regions. This suggests that the greenhouse effect overcomes the albedo effect in the polar latitudes and the opposite occurs in other regions. In all the experiments the changes in the surface temperature are larger near poles, mainly in the Southern Hemisphere.With 8 Figures  相似文献   

Plate Tectonics and mineralization in the Tethyan region   总被引：1，自引：0，他引：1  

C. J. Dixon  J. Pereira 《Mineralium Deposita》1974,9(3):185-198

The association between mineralization and plate movements is becoming evident in the American Cordillera and the Pacific Island Arcs. This paper attempts to show how these ideas apply to the zone along which the Eurasian and Afro-Arabia plates collided during the late Mesozoic, between the Alps and the Himalayas. The study highlights some of the outstanding problems concerned with plate collision and the important question of the origin of ophiolite complexes. Some of the major mineralized districts in Greece, Turkey and Iran can be related to active plate edges, to island arc development and to related tectonism around pre-Mesozoic crystalline massifs. The study reinforces the view that the younger tectonic zones traversing the Middle East will become major mineral-producing areas in the future.  相似文献   

Local disappearance of bivalves in the Azores during the last glaciation     

Sérgio P. Ávila  Patrícia Madeira  C. Marques da Silva  Mário Cachão  Bernard Landau  Rui Quartau  A. M. de Frias Martins 《第四纪科学杂志》2008,23(8):777-785

The Pleistocene (Eemian) outcrops of Lagoinhas and Prainha, located at Santa Maria Island (Azores), were investigated and their fossil mollusc content reported. These studies revealed that the last glaciation affected two groups of molluscs: the ‘warm‐guest’ gastropods with West African or Caribbean affinities (e.g. Conus spp., Cantharus variegatus, Bulla amygdala, Trachypollia nodulosa) and shallow bivalve species mainly associated with sandy habitats (Ensis minor, Lucinella divaricata, and probably Laevicardium crassum). In this paper we focus on this group of bivalves, which has since locally disappeared from the Azores. We relate the local disappearance of these bivalves in the Azores with the lack of sand in the shelf. The specific characteristics of the Santa Maria shelf combined with the sea‐level drop during the Weichselian prevented deposition of the lowstand deposits and permitted erosion of the previous ones, leaving the shelf without a sediment cover. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Ultraviolet dust attenuation in spiral galaxies: the role of age-dependent extinction and the initial mass function     

P. Panuzzo  G. L. Granato  V. Buat  A. K. Inoue  L. Silva  J. Iglesias-Páramo  A. Bressan 《Monthly notices of the Royal Astronomical Society》2007,375(2):640-648

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Habitat provision of barnacle tests for overcrowded periwinkles          下载免费PDF全文

Ana C. F. Silva  Vanessa Mendonça  Rita Paquete  Nuno Barreiras  Catarina Vinagre 《Marine Ecology》2015,36(3):530-540

In habitats where competition for space is a shaping force of animal distribution such as in the intertidal rocky ecosystem, new habitats are readily taken by colonising species. We examined the importance of empty Chthamalus spp. tests as a habitat for the intertidal common periwinkle Melaraphe neritoides on Portuguese rocky shores. The role played by the space between neighbouring barnacles as a habitat for other species has been largely studied with regard to how an ecosystem functions, whereas the equivalent role of empty barnacle tests remains largely unknown. The small periwinkle is one of the most abundant snails in European rocky shores and is an important prey for key mobile predators. Biological facilitation is common in the rocky intertidal zone, where biological structures often potentiate the abundance of other species. The role played by barnacles as biological facilitators through habitat provision is not fully understood. In this study, the abundance and morphometric features of empty barnacle tests and their occupants were examined across shore levels and shores with differing exposures, as these are important gradients explaining barnacle distribution. The availability of empty barnacle tests was also experimentally manipulated to examine the percentage, time and length of occupation. Empty barnacles were more abundant on the midshore of sheltered shores and barnacle tests were wider on the upper shore but taller on the midshore. The minimum barnacle test occupation rate by the periwinkle was 70%. Barnacle shell height was an important factor determining snail occupation, where taller barnacles harboured significantly more, but not necessarily larger, snails. Snails outside the barnacle tests were significantly larger than those found within, suggesting that this crustacean group has an increased importance as a habitat for juvenile snails, thus potentially influencing the population dynamics of M. neritoides. We found that occupation of experimentally created empty barnacles was extremely high (70%) on the day after, and remained at 100% after 3 months of monitoring. Our study is the first of its kind to focus on the features of barnacle tests that snails occupy and their occupying snail traits. According to our results, it is clear that barnacles have an important role in providing additional habitat for young gastropods. The small periwinkle is an important grazer and prey for intertidal and subtidal foraging predators; hence, the potential refuge role of barnacle tests for juvenile M. neritoides may be important in the dynamics of intertidal communities.  相似文献   

Solar Spectral Irradiance at 782 nm as Measured by the SES Sensor Onboard Picard     

M. Meftah  A. Hauchecorne  A. Irbah  G. Cessateur  S. Bekki  L. Damé  D. Bolsée  N. Pereira 《Solar physics》2016,291(4):1043-1057

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Petrographic and geochemical characterization of the granitic rocks of the Araguainha impact crater,Brazil          下载免费PDF全文

Dailto Silva  Cristiano Lana  Carlos Roberto de Souza Filho 《Meteoritics & planetary science》2016,51(3):443-467

Petrographic and geochemical data obtained on the Araguainha impact crater (Goiás/Mato Grosso States, Brazil) indicate the existence of several molten products that originated during impact‐induced congruent melting of an alkali‐granite exposed in the inner part of the central uplift of the structure. Although previous studies have described these melts to some extent, there is no detailed discussion on the petrographic and geochemical variability in the granite and its impactogenic derivatives, and therefore, little is known about the geochemical behavior and mobility of trace elements during its fusion in the central part of the Araguainha crater. This paper demonstrates that the preserved granitoid exposed in the core of the structure is a magnesium‐rich granite, similar to postcollisional, A‐type granites, also found in terrains outside the Araguainha crater, in the Brasília orogenic belt. The molten products are texturally distinct and different from the original rock, but have very similar geochemical composition, making it difficult to separate these lithotypes based on concentrations of major and minor elements. This also applies for trace and rare earth elements (REE), thus indicating a high degree of homogenization during impact‐induced congruent melting under high pressure and postshock temperature conditions. Petrographic observations, along with geochemical data, indicate that melting occurs selectively, where some of the elements are transported with the melt. Simultaneously, there is an effective dissolution of the rock (granite), which leads to entrainment of the most resistant solid phases (intact or partially molten minerals) into the melt. Minerals more resistant to melting, such as quartz and oxides, contribute substantially to a chemical balance between the preserved granite and the fusion products generated during the meteoritic impact.  相似文献   

Yardangs in terrestrial ignimbrites: Synergistic remote and field observations on Earth with applications to Mars   总被引：1，自引：0，他引：1  

S.L. de Silva  J.E. Bailey 《Planetary and Space Science》2010,58(4):459-482

The conditions of formation and the form of yardangs in ignimbrites in the Central Andes of Chile, Bolivia, and Argentina may be the most convincing terrestrial analog to the processes and lithology that produce the extensive yardangs of the Medusae Fossae Formation (MFF) of Mars. Through remote and field study of yardang morphologies in the Central Andes we highlight the role that variable material properties of the host lithology plays in their final form. Here, ignimbrites typically show two main facies: an indurated and jointed facies, and a weakly to poorly indurated, ash- and pumice-rich facies. Both facies are vertically arranged in large (erupted volume >100's of km³) ignimbrites resulting in a resistant capping layer, while smaller (10's of km³) ignimbrites are made predominantly of the weakly indurated facies. The two facies have quite different mechanical properties; the indurated facies behaves as strong rock, fails by block collapse and supports steep/vertical cliffs, while the non-indurated facies is more easily eroded and forms gentle slopes and manifests as more subdued erosional forms. In response to aeolian action, the presence of an upper indurated facies results in large, elongate, high aspect ratio (1:20-1:40) megayardangs that form tall (100 m), thin ridges with steep to vertical walls. These are built on a broad apron of the weakly indurated facies with abundant fallen blocks from the upper indurated facies. These terrestrial megayardangs appear to be analogous to megayardangs with associated block fields seen on Mars. Smaller-volume, weakly indurated ignimbrites are sculpted into smaller, stubbier forms with aspect ratios of 1:5-1:10 and heights rarely exceeding 10 m. Excavation of a windward basal moat suggests an erosional progression like that seen in incipient yardangs on Mars. Excavation rates of 0.007-0.003 cm/year are calculated for the weakly indurated ignimbrites. While a persistent strong unidirectional wind is the dominant parameter controlling yardang formation and orientation, a role for flow separation and vorticity is also suggested by our observations at both yardang types. While the indurated facies is commonly pervasively jointed, jointing is of secondary importance in controlling yardang orientation. Serrated margins, a common feature on Mars, result from oblique intersections of jointing with yardang flanks or scarps of ignimbrite. The processes of yardang formation we describe from ignimbrites from the Central Andes are not necessarily specific to ignimbrites, but do connote that degree and distribution of induration is a major control in yardang formation and this has implications for the lithology of the MFF on Mars.  相似文献