Power-law distribution for solar energetic proton events   总被引：1，自引：0，他引：1  

S. B. Gabriel  J. Feynman 《Solar physics》1996,165(2):337-346

Analyses of the time-integrated fluxes of solar energetic particle events during the period 1965–1990 show that the differential distribution of events with flux F is given by a power law, with indices between 1.2 and 1.4 depending on energy. The power law represents a good fit over three to four orders of magnitude in fluence. Similar power-law distributions have been found for peak proton and electron fluxes, X-ray flares and radio and type III bursts. At fluences greater than 10⁹ cm^–2, the slope of the distribution steepens and beyond 10¹⁰ cm^–2 the power-law index is estimated to be 3.5. At energies greater than 10 MeV, the slope of the distribution was found to be essentially independent of solar cycle, when the active years of solar cycles 20, 21, and 22 were analysed. The results presented are the first for a complete period of 27 years, covering nearly 3 complete solar cycles. Other new aspects of the results include the invariance of the exponent with solar cycle and also with integral energy.  相似文献   

Observations of a post-flare radio burst in X-rays     

Z. Švestka  R. T. Stewart  P. Hoyng  W. Van Tend  L. W. Acton  A. H. Gabriel  C. G. Rapley  A. Boelee  E. C. Bruner  C. De Jager  H. Lafleur  G. Nelson  G. M. Simnett  H. F. Van Beek  W. J. Wagner 《Solar physics》1982,75(1-2):305-329

More than six hours after the two-ribbon flare of 21 May 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which proved to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The X-ray arch extended over 3 or more arc minutes to a projected distance of 95 000 km, and its real altitude was most probably between 110 000 and 180 000 km. The mean electron density in the cloud was close to 10⁹ cm^–3 and its temperature stayed for many hours at a fairly constant value of about 6.5 × 10⁶ K. The bent crystal spectrometer aboard the SMM confirms that the arch emission was basically thermal. Variations in brightness and energy spectrum at one of the supposed footpoints of the arch seem to correlate in time with radio brightness suggesting that suprathermal particles from the radio noise regions dumped in variable quantities into the low corona and transition layer; these particles may have contributed to the population of the arch, after being trapped and thermalized. The arch extended along the H = 0 line thus apparently hindering any upward movement of the upper loops reconnected in the flare process. There is evidence from Culgoora that this obstacle may have been present above the flare since 15–30 min after its onset.  相似文献   

Differential responses of hillslope and channel elements to rainfall events in a semi-arid area     

Juan Puigdefabregas  Gabriel del Barrio  Matthias M. Boer  Leonardo Gutirrez  Albert Sol 《Geomorphology》1998,23(2-4)

The degree of hydrological connectivity of hillslope elements in a semi-arid climate was studied at the season and event timescales. Field data were obtained in Rambla Honda, a Medalus project field site situated in SE Spain, on micaschist bedrock and with 300 mm annual rainfall. The season timescale was assessed using correlation analysis between soil moisture and topographic indices. The event timescale was studied by a quasi-continuous monitoring of rainfall, soil moisture, runoff and piezometric levels. Results show that widespread transfers of water along the hillslope are unusual because potential conditions for producing overland flow or throughflow are spatially discontinuous and extremely short-lived. During extreme events, runoff coefficients may be locally high (ca. 40% on slope lengths of 10 m), but decrease dramatically at the hillslope scale (<10% on slope lengths of 50 m). Two mechanisms of overland flow generation have been identified: infiltration excess, and local subsurface saturation from upper layers. The former occurs during the initial stages of the event while the latter, which is quantitatively more important, takes place later and requires a certain time structure of rainfall intensities that allow saturation of the topsoil and the subsequent production of runoff. Hillslopes and alluvial fans function as runoff sources and sinks respectively. Permanent aquifers are lacking in Rambla Honda. Variable proportions of hillslope areas may contribute to flash floods in the main channel, but their contribution to the formation of saturated layers within the sediment fill is very limited.  相似文献   

Evaluating relative sensitivity of SWAT‐simulated nitrogen discharge to projected climate and land cover changes for two watersheds in North Carolina,USA          下载免费PDF全文

Mark Gabriel  Christopher Knightes  Ellen Cooter  Robin Dennis 《水文研究》2016,30(9):1403-1418

We investigated how projected changes in land cover and climate affected simulated nitrate (NO₃^?) and organic nitrogen discharge for two watersheds within the Neuse River Basin, North Carolina, USA, for years 2010–2070. We applied the Soil and Water Assessment Tool watershed model to predict nitrogen discharge using (1) atmospheric carbon dioxide (CO₂) concentrations predicted by the Intergovernmental Panel on Climate Change, (2) land cover change predicted by the Integrated Climate and Land Use Change project and (3) precipitation and temperature simulated by two statistically downscaled and bias‐corrected Global Circulation Models. We determined the sensitivity of simulated nitrogen discharge to separate changes in each treatment [(1) CO₂, (2) land cover and (3) precipitation and temperature (PT)] by comparing each treatment to a reference condition. Results showed that nitrogen discharges were most sensitive to changes in PT over the 60‐year simulation. Nitrogen discharges had similar sensitivities to the CO₂ and land cover treatments, which were only one‐tenth the influence of the PT treatment. Under the CO₂ treatment, nitrogen discharges increased with increasing ambient CO₂. NO₃^? discharge decreased with increased urbanization; however, organic nitrogen had a varied response. Under the PT treatment, there was high spatial variability in nitrogen discharges. In a single year, certain sub‐basins showed an 80% increase in nitrogen discharge relative to reference, while others showed a 400% decrease. With nitrogen discharge showing high sensitivity to PT change, we suggest that more emphasis should be placed on investigating impacts of PT on nutrient transport in the Neuse River Basin. Published 2015. This article is a U.S. Government work and is in the public domain in the USA  相似文献   

The response of a gravel‐bed river planform configuration to flow variations and bed reworking: a modelling study          下载免费PDF全文

Gabriel Kaless  Luca Mao  Johnny Moretto  Lorenzo Picco  Mario A. Lenzi 《水文研究》2015,29(17):3812-3828

A 2D depth‐averaged model has been developed for simulating water flow, sediment transport and morphological changes in gravel‐bed rivers. The model was validated with a series of laboratory experiments and then applied to the Nove reach of the Brenta River (Northern Italy) to assess its bed material transport, interpret channel response to a series of intensive flood events (R.I. ≈ 10 years) and provide a possible evolutionary scenario for the medium term. The study reach is 1400 m long with a mean slope of 0.0039 m m^?1. High‐resolution digital terrain models were produced combining LiDAR data with colour bathymetry techniques. Extensive field sedimentological surveys were also conducted for surface and subsurface material. Data were uploaded in the model and the passage of two consecutive high intensity floods was simulated. The model was run under several hypotheses of sediment supply: one considering substantial equilibrium between sediment input and transport capacity, and the others reducing the sediment supply. The sediment supply was then calibrated comparing channel morphological changes as observed in the field and calculated by the model. Annual bed material transport was assessed and compared with other techniques. Low‐frequency floods (R.I. ≈ 1.5 years) are expected to produce negligible changes in the channel while high floods may erode banks rather than further incising the channel bed. Location and distribution of erosion and deposition areas within the Nove reach were predicted with acceptable biases stemming from imperfections of the model and the specified initial, boundary and forcing conditions. A medium‐term evolutionary scenario simulation underlined the different response to and impact of a consecutive sequence of floods. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

The spectra of point measurements of currents: Their features and their interpretation     

Gabriel Godin 《大气与海洋》2013,51(3):263-284

Abstract

Differences in the speed or direction of a harmonic component of the current between ebb and flood, cause a transfer of spectral energy from one tidal band to the others. Cyclic changes in the speed or direction of a component between large and small tides, induce a leakage to neighbouring frequencies. Transient fluctuations, even in a perfectly isotropic flow, have the same effects. As a consequence, the spectral estimates of point measurements of current are not independent.  相似文献   

Impact area determination of pyroclastic deposits in Lascar volcano, eruptive process in the year 1993     

Luis Fernández  Gabriel Álvarez  Renato Salinas 《Natural Hazards》2012,60(2):747-760

By means of multi-temporal analysis of satellite images and statistical algorithms, the amount of pyroclastic material deposited on Lascar volcano walls after the gravitational collapse of the eruptive column caused by the eruption that occurred on April the 19th and the 20th in 1993 was determined and quantified. For this analysis, scenes corresponding to Landsat 4 TM in 1989 and Landsat 7 ETM+ in 2001 were used. By careful selection methods such as combination of bands, unsupervised classification, and the Karhunen–Loève transform, detailed analysis of zones of change that correspond to pyroclastic deposits were made. This analysis was complemented with in situ data in order to correct and calibrate the satellite images to identify zones of 1993s eruption pyroclastic deposits. Using Kittler’s and other thresholding algorithms, a search was performed for a proper threshold to binarize the images to determine the surface area covered by the eruptive process. Matlab™ software was used both for general programming and for digital image processing.  相似文献   

“The ESA XMM-Newton Science Operations Centre: Making Basic Space Science Available to the Whole Scientific World”     

Carlos Gabriel  Matteo Guainazzi  Leo Metcalfe 《Astrophysics and Space Science》2006,305(3):315-320

XMM-Newton is a major X-ray observatory of the European Space Agency (ESA). Its observing time is open to astronomers from the whole scientific community on a peer reviewed competitive basis. The Science Operations Centre, located at ESA’s premises in Villafranca del Castillo, Spain, is responsible for the instrument operations, as well as for all the tasks related to facilitating the scientific exploitation of the data which the mission has been producing since its launch in December 1999. Among them, one may list:

•	distribution of scientific data in different formats, from raw telemetry, up to processed and calibrated high-level science products, such as images, spectra, source lists, etc;
•	development and distribution of dedicated science analysis software, as well as of continuously updated instrument calibration;
•	regular organisation of training workshops (free of cost), for potential users of XMM-Newton data, where the procedures and techniques to successfully reduce and analyze XMM-Newton data are introduced;
•	access to the data through state-of-the-art, in-house-developed archival facilities, either through the Internet or via CD-ROM;
•	continuously updated documentation on all aspects of spacecraft and instrument operations, data reduction and analysis;
•	maintenance of a comprehensive set of project web pages;
•	a competent and responsive HelpDesk, providing dedicated support to individual XMM-Newton users.

Everyone can be an XMM-Newton observer. So far, astronomers from 36 countries submitted observing programs. Public data can be accessed by every scientist in the world through the XMM-Newton Science Archive (XSA).Despite all these efforts, one can’t help noticing an asymmetric level of scientific exploitation in the realm of X-ray astronomy between developing and developed countries. The latter have traditionally enjoyed the comparative advantage of deeper know-how, deriving from direct experience in hardware and mission development. The XMM-Newton Science Operations Centre’s efforts act to alleviate this situation through, for example, increasing the usage of the web for data and information dissemination, as well as by supporting actively such initiatives as the COSPAR Capacity-Building Workshops, specifically designed to create long-lasting bridges between researchers in developing and developed countries.  相似文献   

A 3-D coastal tidal rectification process: observations, theory and experiments     

Denis Aelbrecht  Gabriel Chabert d'Hires  Dominique Renouard 《Continental Shelf Research》1999,19(15-16)

In homogeneous rotating fluid, when there is an oscillating forcing in the interior fluid with a period long enough for an Ekman layer to develop, there is an interaction between the oscillatory Ekman layer and the vertical wall, since the latter imposes an alternating adjustment flow confined near the wall. As a result, this coastal rectification process leads to a Lagrangian transport along the coast. The Ekman number, the Rossby number and the temporal Rossby number of the forcing flow are the governing parameters of that mechanism which can be described by a simplified analytical model taking into account both the vertical time-dependent structure of the current and the presence of the wall. The model shows that the residual (rectified) current flowing with the coast to its right results from the strong nonlinear interaction between along- and cross-shore tidal currents leading to asymmetrical momentum exchanges between the Ekman bottom layer and the coastal boundary layer. The model provides simple scaling laws for the maximum intensity and width of the residual current. The latter is significantly larger than the friction (Stokes) lateral boundary layer of the forcing flow. A comprehensive set of experiments is performed in the 13 m diameter rotating tank by oscillating an 8 m×2 m horizontal plate and vertical wall in a homogeneous fluid at rest in solid-body rotation and measuring the two horizontal components of the current at several locations and depths above the central part of the plate. The predicted and experimentally measured maximum intensity and width of the residual current are in very good agreement, within the range of validity of the model, i.e. when the Ekman number is sufficiently small. However experiments also show that the residual current still occurs when the Ekman layer thickness is of the same order as the fluid depth, but it is then confined to a narrower band along the vertical wall. The flow structure found experimentally is also correctly described by a numerical model developed by Zhang et al. (1994). Current measurements in the Eastern part of the English Channel near the French coast reveal a significant coastal residual current flowing Northward and the coastal rectification process described here may account for part of it.  相似文献   

Analytical methods that use natural heat as a tracer to quantify surface water–groundwater exchange, evaluated using field temperature records     

Gabriel C. Rau  Martin S. Andersen  Andrew M. McCallum  Richard Ian Acworth 《Hydrogeology Journal》2010,18(5):1093-1110

Two methods applying natural heat as a tracer to quantify surface water–groundwater exchange were evaluated using field data. Arrays capable of monitoring and recording the streambed response to diurnal temperature variations in the surface water were deployed for a 2-month period in three locations in perennial pools at Maules Creek, New South Wales, Australia. Multi-level array design, field deployment and parameter estimation are discussed. The applicability of analytical solutions derived from the heat transport equation to the streambed environments was analysed using the recorded temperature time series. The stream was found to lose water to the aquifer, which was supported by simultaneously recorded hydraulic gradients. However, the one-dimensional (1D) analytical solutions did not adequately describe the observed streambed thermal response at two locations. The resulting artefacts in the estimated flow velocities are discussed. It was hypothesised that the artefacts originate from model limitation due to streambed heterogeneity and application of 1D solutions to multi-dimensional and dynamic streambed flow. This consequently imposes limitations on the field applicability of the methods. Nevertheless, in combination with time series of surface water and streambed water levels, the use of heat as a tracer provided a powerful tool for better understanding the shallow hydrogeological system.  相似文献