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11.
The isotopic ratio of 3He/4He, which is routinely measured in the solar wind, on meteorites and in different astrophysical environments, is confined to several times 10-4. However, in impulsive solar flares this ratio reaches often values larger than unity. The evolution of this ratio from the primordial nucleosynthesis to the present solar conditions is sketched and the resonant plasma effects which enhance spectacularly the abundance of 3He in the impulsive solar flares are described. 相似文献
12.
A. S. G. ROTH H. BAUR V. S. HEBER E. REUSSER R. WIELER 《Meteoritics & planetary science》2011,46(7):989-1006
Abstract– We analyzed cosmogenic He and Ne in more than 60 individual chondrules separated from small chips from the carbonaceous chondrites Allende and Murchison. The goal of this work is to search for evidence of an exposure of chondrules to energetic particles—either solar or galactic—prior to final compaction of their host chondrites and prior to the exposure of the meteoroids to galactic cosmic rays (GCR) on their way to Earth. Production rates of GCR‐produced He and Ne are calculated for each chondrule based on major element composition and a physical model of cosmogenic nuclide production in carbonaceous chondrites ( Leya and Masarik 2009 ). All studied chondrules in Allende show nominal exposure ages identical to each other within uncertainties of a few hundred thousand years. Allende chondrules therefore show no signs of a precompaction exposure. The majority of the Murchison chondrules (the “normal” chondrules) also have nominal exposure ages identical within a few hundred thousand years. However, roughly 20% of the studied Murchison chondrules (the “pre‐exposed” chondrules) contain considerably or even much higher concentrations of cosmogenic noble gases than the normal chondrules, equivalent to exposure ages to GCR at present‐day fluxes in a 4π irradiation of up to about 30 Myr. The data do not allow to firmly conclude whether these excesses were acquired by an exposure of the pre‐exposed chondrules to an early intense flux of solar energetic particles (solar cosmic rays) or rather by an exposure to GCR in the regolith of the Murchison parent asteroid. However, we prefer the latter explanation. Two major reasons are the GCR‐like isotopic composition of the excess Ne and the distribution of solar flare tracks in Murchison samples. 相似文献
13.
In-seam seismic surveys with channel waves have been widely used in the United Kingdom and elsewhere to map coal-seams and to detect anomalous features such as dirt bands, seam thinning and thickening, and particularly in-seam faulting. Although the presence of cleat-induced anisotropy has been recognized in the past, almost all previous analyses have assumed homogeneous isotropic or transversely isotropic coal-seams. Channel waves, however, exhibit properties which cannot be fully explained without introducing anisotropy into the coal-seam. In particular, Love-type channel waves are observed for recording geometries where, in a homogeneous isotropic or transversely isotropic structure, the source would not be expected to excite transverse motion. Similarly, modes of channel-wave propagation display the coupled three-component motion of generalized modes in anisotropic substrates, which would not be expected for Rayleigh and Love wave motion in isotropy or in transversely isotropic media with azimuthal isotropy. We model the observed in-seam seismic channel waves with synthetic seismograms to gain an understanding of the effects of cleat-induced anisotropy on the behaviour of channel waves. The results show a reasonable good match with the observations in traveltime, relative amplitudes, dispersion characteristics and particle motions. We demonstrate that anisotropy in the surrounding country rocks contributes significantly to the coupling of channel wave particle motion, although its effect is not as strong as the anisotropy in the coal-seam. We conclude that the effects of cleat- and stress-induced anisotropy are observed and can be modelled with synthetic seismograms, and that anisotropy must be taken into account for the detailed interpretation of channel waves. 相似文献
14.
SVEN BLOMQVIST 《Sedimentology》1985,32(4):605-612
The performances of four light-weight, open sampling devices intended for use in soft sediments, the Axelsson-Håkanson gravity corer, the Kajak gravity corer, the Jenkin bottom sampler and the Ekman grab (box corer), were examined in situ by direct observation, measurement and photographic documentation by a SCUBA diver. Restrictions on the reliability of the sediment samples obtained with these devices and sediment coring instruments in general are evaluated. Separate studies of core shortening show: (1) a positive linear relationship between sediment penetration depth at which shortening of cores starts and coring tube inner diameter, (2) a tube size related shift of curve pattern in the regressions of the core shortening versus sediment depth, and (3) a negative non-linear relationship of shortening intensity versus increasing coring tube inner diameter. These findings show the great risk, when sampling soft sediments, of obtaining a sample quantitatively unrepresentative of the in situ stratification. An accurate correction factor for the degree of core shortening requires a knowledge of: (1) the sediment depth at which core shortening commences, and (2) the curve describing the relationship of shortening to depth of penetration. 相似文献
15.
Conceptual box models of the interhemispheric thermohaline circulation are studied with respect to bifurcations. Freshwater fluxes are the main control parameters of the system: they determine the stable states and transitions between stable states of the large-scale thermohaline circulation. In this study of interhemispheric box models both numerical and analytical methods are used to investigate transition mechanisms of the thermohaline circulation. The box model examined first is an interhemispheric four-box model. It is shown that the two bifurcations where the present THC can become unstable, the saddle-node and the Hopf bifurcation, depend in a different way on hemispheric freshwater fluxes. A reduction of the model variables leads to the conclusion that two fixed freshwater fluxes between three surface boxes are the model feature responsible for the bifurcation behaviour found. The significance of the Hopf bifurcation for the stability of the thermohaline circulation is discussed. 相似文献
16.
One of the main objectives of seismic digital processing is the improvement of the signal-to-noise ratio in the recorded data. Wiener filters have been successfully applied in this capacity, but alternate filtering devices also merit our attention. Two such systems are the matched filter and the output energy filter. The former is better known to geophysicists as the crosscorrelation filter, and has seen widespread use for the processing of vibratory source data, while the latter is. much less familiar in seismic work. The matched filter is designed such that ideally the presence of a given signal is indicated by a single large deflection in the output. The output energy filter ideally reveals the presence of such a signal by producing a longer burst of energy in the time interval where the signal occurs. The received seismic trace is assumed to be an additive mixture of signal and noise. The shape of the signal must be known in order to design the matched filter, but only the autocorrelation function of this signal need be known to obtain the output energy filter. The derivation of these filters differs according to whether the noise is white or colored. In the former case the noise autocorrelation function consists of only a single spike at lag zero, while in the latter the shape of this noise autocorrelation function is arbitrary. We propose a novel version of the matched filter. Its memory function is given by the minimum-delay wavelet whose autocorrelation function is computed from selected gates of an actual seismic trace. For this reason explicit knowledge of the signal shape is not required for its design; nevertheless, its performance level is not much below that achievable with ordinary matched filters. We call this new filter the “mini-matched” filter. With digital computation in mind, the design criteria are formulated and optimized with time as a discrete variable. We illustrate the techniques with simple numerical examples, and discuss many of the interesting properties that these filters exhibit. 相似文献
17.
SVEN ILLERIS M.Sc. 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):58-64
Sven Illeris: Location of Service Activities in Denmark, with Special Regard to Rural Areas. Geografisk Tidsskrift 75: 58–64. København, juni 1, 1976. The paper describes the purposes and methods of a study of the present and future location of service activities. Some of the most important findings are dealt with, e.g. estimates and plans for the 1988 distribution of service between size-classes of towns. 相似文献
18.
SVEN LUKAS 《Boreas: An International Journal of Quaternary Research》2005,34(3):287-307
The melt-out of material contained within englacial thrust planes has been proposed to result in the formation of stacked moraine sequences with characteristic proximal rectilinear slopes. This model has been applied to explain the formation of Scottish Younger Dryas ice-marginal ('hummocky') moraines on the basis of these morphological characteristics. However, no sedimentological data exist to support this proposal. This article reviews hitherto proposed models of 'hummocky' moraine formation and presents detailed geomorphological and sedimentological results from the NW Scottish Highlands with the aims of reconstructing the dynamics of Younger Dryas glaciers and of testing the applicability of the englacial thrusting model. Exposures demonstrate that moraines represent terrestrial ice-contact fans throughout, with a variety of postdepositional deformation structures being identified in most cases, indicating that glacier retreat was incremental and oscillatory; proximal rectilinear slopes are interpreted as ice-contact faces formed after ice support was withdrawn during retreat. This evidence strongly suggests a temperate glacier regime and short glacier response times similar to those in present-day SW Norway or Iceland. It contradicts the thrusting model and the proposal that Svalbard might form a suitable analogue for Younger Dryas moraines in Scotland. 相似文献
19.
Middle Pleistocene to Holocene sediment variations observed in a 26 metre long core taken during a cruise of the RV Marion Dufresne are presented. Core MD992202 was retrieved from the northern slope of Little Bahama Bank and provides an excellent example for sedimentation processes in a mid‐slope depositional environment. The sediment composition indicates sea‐level related deposition processes for the past 375 000 years (marine isotope stages 1 to 11). The sediments consist of: (i) periplatform ooze (fine‐grained particles of shallow‐water and pelagic origin) with moderate variations in carbonate content, carbonate mineralogy and grain‐size; and (ii) coarser intervals with cemented debris consisting of massive, poorly sorted, mud‐supported or clast‐supported deposits with an increased high‐magnesium calcite content. During interglacial stages (marine isotope stages 1, 5, 7, 9 and 11) periplatform oozes (i) are characterized by higher aragonite contents, finer grain‐size and higher organic contents, whereas during glacial stages (marine isotope stages 2 to 4, 6, 8 and 10), increased low‐magnesium and high‐magnesium calcite values, coarser grain‐size and lower organic contents are recorded. These glacial to interglacial differences in mineralogy, grain‐size distribution and organic content clearly show the impact of climatically controlled sea‐level fluctuations on the sedimentation patterns of the northern slope of Little Bahama Bank. The coarser deposits (ii) occur mainly at the transitions from glacial to interglacial and interglacial to glacial stages, and are interpreted as redeposition events, indicating a direct link between sediment properties (changes in mineralogy, grain‐size distribution, variations in organic contents) and sea‐level fluctuations. Changes in hydrostatic pressure and the wave base position during sea‐level changes are proposed to have triggered these large‐scale sediment redepositions. 相似文献
20.
T. NEWTON T.R. OKE C.S.B. GRIMMOND M. ROTH 《Geografiska Annaler: Series A, Physical Geography》2007,89(4):331-347
Summertime measurements of local scale energy balance of a suburban site in west Miami, Florida, in 1995 are presented. All of the radiation and turbulent heat fluxes were measured directly. In addition several derived surface characteristics are calculated including the albedo, radiation temperature, emissivity, aerodynamic and canopy resistances, Bowen's ratio, Priestley‐Taylor and McNaughton‐Jarvis coefficients. The results are used to test parameterizations to calculate several heat fluxes. Most radiation fluxes can be calculated with simple schemes in cloudless conditions but the spatial and temporal variability of cloud degrades results greatly. This highlights the value of observing incoming solar radiation since it can form an excellent surrogate for daytime net all‐wave radiation in all sky conditions. The heat flux results for this warm, wet subtropical site demonstrate similarities with those from similarly developed locations in temperate climates. Interestingly this finding includes the fraction of energy used in evaporation. It is thought that this may be related to the relatively large heat storage in Miami which may reflect the presence of wet soils and free‐standing water, the persistently low vapor pressure deficits (7‐14 hPa) typical of this humid climate and the relatively poor coupling between the surface and the whole planetary boundary layer due to relatively low surface roughness. 相似文献