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Type III solar radio bursts are investigated by modelling the propagation of the electron beam and the generation and subsequent propagation of waves to the observer. Predictions from this model are compared in detail with particle, Langmuir wave, and radio data from the ISEE-3 spacecraft and with other observations to clarify the roles of fundamental and harmonic emission in type III radio bursts. Langmuir waves are seen only after the arrival of the beam, in accord with the standard theory. These waves persist after a positive beam slope is last resolved, implying that sporadic positive slopes persist for some time, unresolved but in accord with the predictions of stochastic growth theory. Local electromagnetic emission sets in only after Langmuir waves are seen, in accord with the standard theory, which relies on nonlinear processes involving Langmuir waves. In the events investigated here, fundamental radiation appears to dominate early in the event, followed and/or accompanied by harmonic radiation after the peak, with a long-lived tail of multiply scattered fundamental or harmonic emission extending long afterwards. These results are largely independent of, but generally consistent with, the conclusions of earlier works. 相似文献
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B.J. Kellett R. Bingham R.A. Cairns V. Tsikoudi 《Monthly notices of the Royal Astronomical Society》2002,329(1):102-108
In this paper we review four different types of X-ray and/or radio observations of active late-type stars. We then consider if a single magnetic source configuration – a toroidal dipole magnetic trap – can possibly explain these various different observations. We conclude that, indeed, dipole magnetic confinement (similar to the magnetic configurations of the Earth's radiation belts and the case of Jupiter and the Io torus) can explain all the diverse observational data. We take this to be very strong observational support for this type of magnetic confinement scheme. We also consider that this magnetic configuration is only likely to be established and maintained in the most active stars. 相似文献
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Although microbes have traditionally been used as models for testing ecological theory, research on the organization of these communities has largely been isolated from other areas of community ecology. Early studies indicated that microbial populations can form dynamic, interacting assemblages. Observations of distributional patterns suggest that certain deterministic forces regulate community structure. It appears that protozoans are tolerant of a wide range of environmental conditions although evidence for this is largely based on gross environmental analyses. Laboratory studies have suggested the importance of negative and positive biotic interactions in determining community membership, but little field evidence for this exists. The importance of transport processes in controlling community composition is being increasingly recognized. All three types of processes likely act to regulate the colonization and successional dynamics of these communities. A simple model is presented to promote the devel 相似文献
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An analysis of the spatial and temporal scales of cloud variability and their coupling provided by the results from existing cloud observing systems allows us to reach the following conclusions about the necessary attributes of a cloud monitoring system. (1) Complete global coverage with uniform density is necessary to obtain an unbiased estimate of cloud change and an estimate of the reliability with which that change can be determined. (2) A spatial sampling interval of less than 50 km is required so that cloud cover distributions will generally be homogeneous, or statistically homogeneous, within a sample. (3) A sampling frequency of at least six times a day ensures not only that the diurnal and semi-diurnal cycles are not aliased into long term mean values, but also that changes in them can be monitored. (4) Since estimated climate changes are only evident on a decadal time-scale, unless cloud monitoring is continuous with a record length greater than 10 years and has very high precision ( 1%) instrument calibration with overlapping observations between each pair of instruments, it will not be possible either to detect or to diagnose the effects of cloud changes on the climate. 相似文献
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Effective exploration for polymetallic ore deposits in the Cobar region is hampered by incomplete knowledge of the mineralogical controls on element dispersion in the different regolith-landform settings throughout the area. A detailed mineralogical and geochemical study of regolith profiles over two major mineralised shear zones in a strongly weathered but dominantly erosional setting has delineated the important host minerals for a range of base metal cations. Iron oxides/oxyhydroxides, particularly goethite and to a much lesser extent hematite, are major hosts for Pb, Cu, and Zn as substituted/adsorbed cations and as constituents of associated or intergrown minerals, probably including members of the jarosite–alunite group. Correlations between elements and major regolith minerals suggest that goethite is also a host phase for As, Bi and Sb. Minor manganese minerals, including lithiophorite and cryptomelane group minerals, also host base metals in appreciable amounts. No clear association was found between gold and any particular secondary mineral. It is likely that gold is present largely as elemental gold particles associated with a range of minerals.Sampling strategies for geochemical exploration in variably leached and stripped regolith in the Cobar area should take into account the relative abundance of goethite and manganese oxides/oxyhydroxides within the profiles and overlying lag. Goethite would appear to be the preferred sampling medium for base metals. Highly ferruginous lag has a high proportion of hematite with variable maghemite and very low manganese oxide contents. Most of the base metal content in this surface material is strongly bound to the crystalline oxides/oxyhydroxides. More work is required to understand the effects of surface transformation of goethite to hematite and maghemite on the mobility and distribution of base metal cations in soil and ferruginous lags. 相似文献
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Poitras St&#;phane P. Pearson D. Graham Hardman Matthew F. Stachel Thomas Nowell Geoff M. Cairns Scott 《Mineralogy and Petrology》2018,112(2):719-736
The Central Mackenzie Valley (CMV) area of Northwest Territories is underlain by Precambrian basement belonging to the North American Craton. The potential of this area to host kimberlitic diamond deposits is relatively high judging from the seismologically-defined lithospheric thickness, age of basement rocks (2.2–1.7 Ga) and presence of kimberlite indicator minerals (KIMs) in Quaternary sediments. This study presents data for a large collection of KIMs recovered from stream sediments and till samples from two study areas in the CMV, the Horn Plateau and Trout Lake. In the processed samples, peridotitic garnets dominate the KIM grain count for both regions (> 25% each) while eclogitic garnet is almost absent in both regions (< 1% each). KIM chemistry for the Horn Plateau indicates significant diamond potential, with a strong similarity to KIM systematics from the Central and Western Slave Craton. The most significant issue to resolve in assessing the local diamond potential is the degree to which KIM chemistry reflects local and/or distal kimberlite bodies. Radiogenic isotope analysis of detrital kimberlite-related CMV ilmenite and rutile grains requires at least two broad age groups for eroded source kimberlites. Statistical analysis of the data suggests that it is probable that some of these KIMs were derived from primary and/or secondary sources within the CMV area, while others may have been transported to the area from the east-northeast by Pleistocene glacial and/or glaciofluvial systems. At this stage, KIM chemistry does not allow the exact location of the kimberlitic source(s) to be constrained.
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The low frequency array (LOFAR) radiotelescope will be a powerful instrument for answering fundamental, unresolved scientific questions concerning solar system radio phenomena and related emissions from nearby stellar systems. This paper reviews the phenomena, emission mechanisms, open scientific questions, and LOFAR's capabilities. LOFAR will detect metric solar radio bursts in the corona and interplanetary medium, out to distances of order 10 solar radii, as well as Jovian radio emissions. Arguments are given that LOFAR may be sufficiently sensitive to detect stellar analoges of solar type II and III bursts, and may detect cyclotron-maser emissions from extra-solar planets. LOFAR may also aid space weather research, by passively detecting coronal mass ejections (CMEs) via scintillation and Faraday rotation effects, or by detecting radar signals bounced off CMEs and coronal density structures if a suitable solar radar is developed. 相似文献
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A treeline can respond to climate change by shifting position, infilling, increasing recruitment, and increasing radial growth. More studies from understudied areas and their associated species are needed to understand treeline structure and dynamics. We established two transects of 20 m width and 120 m length (100 m above and 20 m below the forestline) in the Betula utilis subalpine forest of the Dhorpatan Hunting Reserve in Western Nepal. All individuals of B. utilis within the transects were classified into three height classes – trees (>2 m), saplings (0.5–2 m), and seedlings (<0.5 m) – and measured for morphometric features. Tree-ring cores were collected for age structure analysis. B. utilis forms an abrupt treeline in the region, and the mean forestline and treeline elevations were 3893 and 3898 m, respectively. The average age (57 yr) of trees at the treeline ecotone suggests a young stand. Poor regeneration was observed above the forestline in both transects when compared to below the forestline. Low regeneration at the treeline ecotone suggests site-specific biotic and abiotic controlling factors. Seedling and sapling establishment above the forestline is limited by a lack of moisture, absence of suitable microsites, and presence of herbivores. 相似文献