The ponderomotive force (PMF) is a ubiquitous nonlinear wave effect arising in plasma physics when applied wave fields or plasma parameters have significant spatial gradients. Some basic properties of the PMF are described as they relate to ULF hydromagnetic waves in the magnetosphere. Examples are given of recent results obtained using both analytical and numerical techniques for waves from the lowest frequencies (determined by the dimensions of the magnetosphere) up to the vicinity of the ion cyclotron frequency. These results include the possibilities that the PMF may transport plasma over large distances in the magnetosphere, and that the PMF may energize magnetospheric ions significantly. In particular the PMF may play a role in transporting and energizing O+ ions from the ionosphere into the body of the magnetosphere. The PMF can also generate nonlinear coupling between the slow magnetosonic mode and the other hydromagnetic modes. This should lead to limitation of density enhancements and, notably in the case of standing Alfvén waves, to spatial harmonic generation, secularly growing frequency shifts, and saturation of driven wave fields. Some implications of these results for the magnetosphere are discussed.Based on an invited review given in IAGA Symposium 3.07, Nonlinear and Kinetic Effects in ULF Waves, at the IUGG XXI General Assembly, Boulder, Colorado, July 2-14, 1995. 相似文献
Ground-based visible differential absorption spectrometry during twilight has been used for NO2 total column observations at the Antarctica Peninsula, Marambio Base (64S, 56W), during the austral spring of 1989 (9 September to 25 November).Results show moderate NO2 vertical column levels of 1.5 to 2.5×1015 molec cm-2 in the morning and 2 to 3×1015 molec cm-2 in the evening until middle October, highly modulated by planetary wave activity. From that date until the end of the period, a steady increase occurs which is associated with the rising of lower stratosphere temperature as the vortex weakens, reaching values of 5×1015 molec cm-2 in late November, with small a.m.-p.m. differences. NO2 is found to be positively correlated to both total ozone and 50 hPa temperature during the entire spring. However, when analyzing the departures from linear trends, a highly negative correlation has been observed from day 301 onwards. 相似文献
Seismic investigations to determine the crustal structure in the southwestern part of the Iberian Peninsula have been initiated in 1970. First experiments were carried out during July 1970, when a series of ten shots was fired off Cabo de Sines (Portugal) in shallow water and recorded up to distances of 185 km along a SE-profile towards Huelva (Spain). The profile was reversed in December 1970, when a series of twelve shots was fired off the south coast near Fuzeta (east of Faro) and recorded up to distances of about 260 km along a NW-profile towards Cabo da Roca west of Lisboa. A considerable increase in the seismic efficiency of the explosions could be achieved by generating standing waves in the water.
The structure deduced exhibits some peculiar features. Below the Palaeozoic sediments a fairly high velocity of 6.4 km/sec is found for the dome-shaped basement in that area. The lower crust, which is separated from the upper crust by a distinct velocity inversion (with a minimum velocity of about 5.3–5.6 km/sec), is characterized by a velocity of 7.1 km/sec. From the geological evidence and the sequence of seismic velocities it must be concluded that the upper crustal block in the southwestern part of the Iberian Peninsula has been uplifted by about 2–5 km since Permo-Triassic time, thus emphasizing the significance of vertical movement in tectonic activity.
The top of the upper mantle (8.15 km/sec) was detected at a depth of 30 km close to the Atlantic coast in the west, while near the Algarve coast in the south the depth to the M-discontinuity is about 34–35 km. This result in conjunction with studies of earthquake focal mechanisms confirms the suggestion that the Iberian block is being underthrust under the African plate. 相似文献
Paleoecology of Laguna Babícora, Chihuahua, Mexico was reconstructed using ostracode faunal assemblages and shell chemistry. The paleolimnological record is used to show the magnitude of paleoclimatic changes in the area from 25,000 years to the present.Faunal assemblages consist of four species of the genus Limnocythere: L. sappaensis, L. ceriotuberosa, L. bradburyi and L. platyforma, all associated with Candona caudata, Candona patzcuaro and Cypridopsis vidua. A paleosalinity index developed from these assemblages indicates that the lake's salinity fluctuated frequently from oligo- to meso-haline conditions during the last 25,000 years. This pattern and low salinity range are in good agreement with modern TDS (here used as an indicator of salinity) values recorded from 26 wells and one spring from the area (258–975 mg l–1). To estimate paleotemperature we examined the trace element content (Mg/Ca ratios) from individual valves of L. ceriotuberosa and L. platyforma, the two species most commonly recorded in Laguna Babícora.Shell Mg/Ca ratios of 204 specimens of these two species were used to estimate water temperature (Mg/Ca) by means of experimental standard coefficients. Our data show that paleowater temperature ranged from 5.6–21.3 °C (with 2 values ranging from 0.2–4.8 °C), which suggest a close correlation with atmospheric temperatures around the lake. These results are in good agreement with a modern mean winter temperature (3.5 °C) and mean summer temperature (20 °C) recorded in the area between 1970 and 1980. 相似文献
Guanabara Bay, in Rio de Janeiro state, is impacted by organic matter, oil and heavy metals. The present study evaluated the total mercury (THg) and methylmercury (MeHg) concentrations and the MeHg to THg ratio (%MeHg) in water samples from different points of the bay and in 245 organisms of three different trophic levels sampled between 1990 and 2000. Dissolved mercury concentration in estuarine water samples ranged from 0.72 to 5.23 ngl(-1). THg and MeHg in mussel, ranging from 11.6 to 53.5 microg THg kg(-1) wet wt. and 4.5-21.0 microg MeHg kg(-1) wet wt., varied according to sampling point and water quality. Planktivorous fish and mussel presented similar MeHg concentrations, meanwhile THg in planktivorous fish were lower than in mussel. Carnivorous fish showed higher THg and MeHg concentrations (199.5 +/- 119.3, 194.7 +/- 112.7 microg kg(-1) wet wt. respectively) than organisms from other feeding habits and lower trophic levels. There was a significant difference in the %MeHg among trophic levels: mussel presented lower MeHg percentage (33%) than planktivorous fish (54%) and carnivorous fish (98%). 相似文献
The notion of remote element synthesis has recently been modified to explain the presence of nucleogenetic isotopic anomalies
and decay products of short-lived nuclides by injection of a small amount ofexotic nucleogenetic material. Even with this modification, remote element synthesis seems inconsistent with the following observations:
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Evidence of coupled variations in the chemical and isotopic compositions of the source material for meteorites.
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Residual coupling of chemical and isotopic heterogeneities across planetary distances in the solar system today.
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The mass-fractionation relationship seen across isotopes of elements in the planetary system, in the solar wind, and in solar
flares.
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Linkage of short-lived radioactivities with isotopic anomalies and with physical properties of their host grains, as expected
for early condensate of fresh stellar debris.
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Temporal and spatial distributions of short-live nuclides and their decay products.
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Mirror-image (+ and −) isotopic anomalies in meteorite grains that sum to “normal” isotopic ratios, as expected of unmixed
products of the same nuclear reactions that produced our bulk elements.
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The lac supporting evidence for “presolar” grains or nearby stars that injectedexotic material into the early solar nebula.
The ability of California's water supply system to adapt to long-term climatic and demographic changes is examined. Two climate warming and a historical climate scenario are examined with population and land use estimates for the year 2100 using a statewide economic-engineering optimization model of water supply management. Methodologically, the results of this analysis indicate that for long-term climate change studies of complex systems, there is considerable value in including other major changes expected during a long-term time-frame (such as population changes), allowing the system to adapt to changes in conditions (a common feature of human societies), and representing the system in sufficient hydrologic and operational detail and breadth to allow significant adaptation. While the policy results of this study are preliminary, they point to a considerable engineering and economic ability of complex, diverse, and inter-tied systems to adapt to significant changes in climate and population. More specifically, California's water supply system appears physically capable of adapting to significant changes in climate and population, albeit at a significant cost. Such adaptation would entail large changes in the operation of California's large groundwater storage capacity, significant transfers of water among water users, and some adoption of new technologies. 相似文献