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  • 1
    Electronic Resource
    Electronic Resource
    Ionospheric measurements of relative coronal brightness during the total solar eclipses of 11 August, 1999 and 9 July, 1945 (2000)
    Springer
    Annales geophysicae 18 (2000), S. 182-190 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (solar radiation and cosmic ray effects) ; Radio science (ionospheric physics) ; Solar physics, astrophysics, and astronomy (corona and transition region)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Swept-frequency (1/10 MHz) ionosonde measurements were made at Helston, Cornwall (50°06′N, 5°18′W) during the total solar eclipse on August 11, 1999. Soundings were made every three minutes. We present a method for estimating the percentage of the ionising solar radiation which remains unobscured at any time during the eclipse by comparing the variation of the ionospheric E-layer with the behaviour of the layer during a control day. Application to the ionosonde date for 11 August, 1999, shows that the flux of solar ionising radiation fell to a minimum of 25±2% of the value before and after the eclipse. For comparison, the same technique was also applied to measurements made during the total solar eclipse of 9 July, 1945, at Sörmjöle (63°68′N, 20°20′E) and yielded a corresponding minimum of 16 ± 2%. Therefore the method can detect variations in the fraction of solar emissions that originate from the unobscured corona and chromosphere. We discuss the differences between these two eclipses in terms of the nature of the eclipse, short-term fluctuations, the sunspot cycle and the recently-discovered long-term change in the coronal magnetic field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-0182-z
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  • 2
    Electronic Resource
    Electronic Resource
    The correct application of Poynting’s theorem to the time-dependent magnetosphere: reply to Heikkila (1999)
    Springer
    Annales geophysicae 17 (1999), S. 178-181 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-999-0178-2
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  • 3
    Electronic Resource
    Electronic Resource
    Ultrastructure of the gonad and gametogenesis in the eastern oyster,Crassostrea virginica. I. Ovary and oogenesis (1996)
    Springer
    Marine biology 127 (1996), S. 79-87 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The ultrastructural features of the ovary and oogenesis are described in the eastern oyster,Crassostrea virginica (Gmelin, 1791). The ovary is a diffuse organ consisting of highly branching acini in which oocytes develop. The acini are surrounded by a matrix of vesicular connective tissue (VCT tells) which serves a nutrient storage function. Each acinus is bathed by fluid within a surrounding connective tissue compartment, the hemocoel, which likely serves as a means of transporting nutrients to the oocytes. Oocytes begin growth while positioned near to the inner acinus wall. As differentiation proceeds and they enter the late stages of vitellogenesis, they become stalle-shaped and project into the acinus lumen. Follicle tells are closely associated with oocytes during the early and middle stages of vitellogenesis but they are largely confined to the basal, stalked region of late-stage oocytes. Vitellogenesis occurs through a process of autosynthesis, involving the combined activity of the Golgi complex and rough endoplasmic reticulum, and heterosynthesis in which extraovarian precursors are incorporated into oocytes via receptor-mediated endocytosis involving the basal surface of the oocytes. It is suggested that the follicle tells play some important role during oogenesis but probably are not the major source of yolk precursors. The VCT celas are probably the main source of nutrients for vitellogenesis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00993647
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  • 4
    Electronic Resource
    Electronic Resource
    Ultrastructure of the gonad and gametogenesis in the eastern oyster,Crassostrea virginica. II. Testis and spermatogenesis (1996)
    Springer
    Marine biology 127 (1996), S. 89-96 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The ultrastructural features of the testis and spermatogenesis have been described from the eastern oyster,Crassostrea virginica (Gmelin, 1791). The testis is a diffuse organ consisting of branching acini containing differentiating sperm in a variety of stages. Spermatogonia are located nearest the outer wall of the acinus, while spermatocytes and spermatids are positioned nearer the lumen. Mature spermatozoa are all confined to the central region of the acinus. The acinus is surrounded by an intermittent layer of myoepithelial cells and is bathed by a $uid-filled hemocoel. Vesicular connective tissue (VCT cells) fills the region between adjacent acini, and the cells contain glycogen granules and lipid droplets. Each acinus is divided radially into subcompartments that are partially separated by pleomorphic accessory cells which remain in close contact with sperm until late stages of development. Sperm are similar to those described in other oysters, except that five midpiece mitochondria were observed in some sperm rather than the usual four, and the acrosomal vesicle lacked the “whorled” substructure described in some other oyster sperm. We suggest that the neutral term “accessory cells” be applied to bivalve testicular somatic cells until more detailed studies are available to justify the use of “Sertoli cell” and other descriptive terms which have previously been adopted from other taxa only distantly related to bivalves.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00993648
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  • 5
    Electronic Resource
    Electronic Resource
    Marine diatoms grown in chemostats under silicate or ammonium limitation. III. Cellular chemical composition and morphology of Chaetoceros debilis, Skeletonema costatum, and Thalassiosira gravida (1977)
    Springer
    Marine biology 43 (1977), S. 19-31 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Three marine diatoms, Skeletonema costatum, Chaetoceros debilis, and Thalassiosira gravida were grown under no limitation and ammonium or silicate limitation or starvation. Changes in cell morphology were documented with photomicrographs of ammonium and silicate-limited and non-limited cells, and correlated with observed changes in chemical composition. Cultures grown under silicate starvation or limitation showed an increase in particulate carbon, nitrogen and phosporus and chlorophyll a per unit cell volume compared to non-limited cells; particulate silica per cell volume decreased. Si-starved cells were different from Si-limited cells in that the former contained more particulate carbon and silica per cell volume. The most sensitive indicator of silicate limitation or starvation was the ratio C:Si, being 3 to 5 times higher than the values for non-limited cells. The ratios Si:chlorophyll a and S:P were lower and N:Si was higher than non-limited cells by a factor of 2 to 3. The other ratios, C:N, C:P, C:chlorophyll a, N:chlorophyll a, P:chlorophyll a and N:P were considered not to be sensitive indicators of silicate limitation or starvation. Chlorophyll a, and particulate nitrogen per unit cell volume decreased under ammonium limitation and starvation. NH4-starved cells contained more chlorophyll a, carbon, nitrogen, silica, and phosphorus per cell volume than NH4-limited cells. N:Si was the most sensitive ratio to ammonium limitation or starvation, being 2 to 3 times lower than non-limited cells. Si:chlorophyll a, P:chlorophyll a and N:P were less sensitive, while the ratios C:N, C:chlorophyll a, N:chlorophyll a, C:Si, C:P and Si:P were the least sensitive. Limited cells had less of the limiting nutrient per unit cell volume than starved cells and more of the non-limiting nutrients (i.e., silica and phosphorus for NH4-limited cells). This suggests that nutrient-limited cells rather than nutrient-starved cells should be used along with non-limited cells to measure the full range of potential change in cellular chemical composition for one species under nutrient limitation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00392568
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  • 6
    Electronic Resource
    Electronic Resource
    Marine diatoms grown in chemostats under silicate or ammonium limitation. II. Transient response of Skeletonema costatum to a single addition of the limiting nutrient (1976)
    Springer
    Marine biology 35 (1976), S. 187-199 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Skeletonema costatum was grown at different steady-state growth rates in ammonium or silicate-limited chemostats. The culture was perturbed from its steady-state condition by a single addition of the limiting nutrients ammonium or silicate. The transient response was followed by measuring nutrient disappearance of the liliting perturbation experiment indicate that three distinct modes of uptake of the limiting nutrient can be distinguished; surge uptake (V s ), internally controlled uptake (V i ), and externally controlled uptake (V e ). An interpretation of these three modes of uptake is given and their relation to control of uptake of the limiting nutrient is discussed. The uptake rates of the non-limiting nutrients were shown to be depressed during the surge of the uptake of the limiting nutrient. Kinetic uptake parameters, K s and V max, were obtained from data acquired during the externally controlled uptake segment, V e . The same V max value of 0. 12 h-1, was obtained under either silicate or ammonium limitation. Estimates of K s were 0.4 μg-at NH4-N l-1 and 0.7 μg-at Si l-1. Short-term 15N uptake-rate measurements conducted on nitrogen-limited cultures appear to be a combination of V s or V i , or at lower substrate concentrations V s and V e . It is difficult to separate these different uptake modes in batch or tracer experiments, and ensuing problems in interpretation are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00390940
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  • 7
    Electronic Resource
    Electronic Resource
    Predatory control of copepod seasonal cycles on Georges Bank (1984)
    Springer
    Marine biology 82 (1984), S. 31-40 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract A stimulation model of copepod population dynamics (development rate, fecundity, and mortality) was used to compute the predatory consumption necessary to control population growth in three dominant copepod species (Pseudocalanus sp., Paracalanus parvus, and Calanus finmarchicus) on Georges Bank, given observed seasonal cycles of copepod and predator populations. The model also calculated secondary production of each species. Copepod development rate and fecundity were functions of temperature while mortality was a function of predator abundance and consumption rate. Daily inputs of temperature and predator abundance (chaetognaths, ctenophores, and Centropages spp.) were derived from equations fit to field data. Model runs were made with various consumption rates until the model output matched observed copepod seasonal cycles. Computed consumption rates were low compared with published values from field and laboratory studies indicating that, even at conservative estimates of consumption, predators are able to control these copepod populations. Combined annual secondary production by the small copepod species, Pseudocalanus sp. and P. parvus, was nearly twice that of the larger C. finmarchicus with P. parvus having the highest total annual production.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00392761
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  • 8
    Electronic Resource
    Electronic Resource
    Food concentrations on Georges Bank: non-limiting effect on development and survival of laboratory reared Pseudocalanus sp. and Paracalanus parvus (Copepoda: Calanoida) (1984)
    Springer
    Marine biology 82 (1984), S. 41-46 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Laboratory and field studies were used to show that development rate and survival of the copepods Pseudocalanus sp. and Paracalanus parvus are generally not food limited on Georges Bank. Copepods were reared in the laboratory at a series of phytoplankton concentrations (25 to 200 μg carbon l-1) spanning the range of particulate carbon and nitrogen measured on Georges Bank in October, 1978 and February, 1979. Pseudocalanus sp. and P. parvus were reared at 5° and 12°C, respectively. Development rate and survival in both species were not affected at Georges Bank food concentrations but were reduced at lower food levels. Development times from hatching to adulthood was 60 to 70 d for Pseudocalanus sp. and 25 to 30 d for P. parvus.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00392762
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  • 9
    Electronic Resource
    Electronic Resource
    An optimised method for calculating the O+-O collision parameter from aeronomical measurements (1995)
    Springer
    Annales geophysicae 13 (1995), S. 541-550 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A study has been made of the interaction between the thermosphere and the ionosphere at high latitudes, with particular regard to the value of the O+-O collision parameter. The European incoherent scatter radar (EISCAT) was used to make tristatic measurements of plasma parameters at F-region altitudes while simultaneous measurements of the neutral wind were made by a Fabry-Perot interferometer (FPI). The radar data were used to derive the meridional neutral winds in a way similar to that used by previous authors. The accuracy of this technique at high latitudes is reduced by the dynamic nature of the auroral ionosphere and the presence of significant vertical winds. The derived winds were compared with the meridional winds measured by the FPI. For each night, the value of the O+-O collision parameter which produced the best agreement between the two data sets was found. The precision of the collision frequency found in this way depends on the accuracy of the data. The statistical method was critically examined in an attempt to account for the variability in the data sets. This study revealed that systematic errors in the data, if unaccounted for by the analysis, have a tendency to increase the value of the derived collision frequency. Previous analyses did not weight each data set in order to account for the quality of the data; an improved method of analysis is suggested.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-995-0541-x
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  • 10
    Electronic Resource
    Electronic Resource
    An analysis of the accuracy of magnetopause reconnection rate variations deduced from cusp ion dispersion characteristics (1996)
    Springer
    Annales geophysicae 14 (1996), S. 149-161 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present an analysis of the accuracy of the method introduced by Lockwood et al. (1994) for the determination of the magnetopause reconnection rate from the dispersion of precipitating ions in the ionospheric cusp region. Tests are made by applying the method to synthesised data. The simulated cusp ion precipitation data are produced by an analytic model of the evolution of newly-opened field lines, along which magnetosheath ions are firstly injected across the magnetopause and then dispersed as they propagate into the ionosphere. The rate at which these newly opened field lines are generated by reconnection can be varied. The derived reconnection rate estimates are then compared with the input variation to the model and the accuracy of the method assessed. Results are presented for steady-state reconnection, for continuous reconnection showing a sine-wave variation in rate and for reconnection which only occurs in square wave pulses. It is found that the method always yields the total flux reconnected (per unit length of the open-closed field-line boundary) to within an accuracy of better than 5%, but that pulses tend to be smoothed so that the peak reconnection rate within the pulse is underestimated and the pulse length is overestimated. This smoothing is reduced if the separation between energy channels of the instrument is reduced; however this also acts to increase the experimental uncertainty in the estimates, an effect which can be countered by improving the time resolution of the observations. The limited time resolution of the data is shown to set a minimum reconnection rate below which the method gives spurious short-period oscillations about the true value. Various examples of reconnection rate variations derived from cusp observations are discussed in the light of this analysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-0149-9
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