868 research outputs found
Conicoid Mirrors
The first order equation relating object and image location for a mirror of
arbitrary conic-sectional shape is derived. It is also shown that the parabolic
reflecting surface is the only one free of aberration and only in the limiting
case of distant sources.Comment: 9 page
Fractal dimension of lightning discharge
International audienceUsing a stochastic model, we simulate the process of dielectric breakdown in the atmosphere and calculate the fractal dimension of 3-dimensional lightning patterns. Finite-size effects have been studied. The projections of our patterns on vertical planes fit the experimental fractal dimension obtained from photographic analysis. This work is inspired by a previous work by A.A. Tsonis
Thermoremanence anomaly in Fe-Zr(B,Cu) Invar metallic glasses: Volume expansion induced ferromagnetism
We report the existence of a thermally induced sharp increase of thermoremanence around the Curie temperature of Invar-like Fe-Zr(B,Cu) soft magnetic glasses. Neutron-diffraction measurements indicate that a true enhancement of the average local magnetic moment, rather than only a change in the domain structure, occurs. Such enhancement has been tentatively attributed to the increasing volume expansion that takes place beyond the Curie temperature and reinforces ferromagnetism in some low-density clusters
Temporal dynamics of soil fungal communities after partial and total clear-cutting in a managed Pinus sylvestris stand
Forest management aimed to maximize timber production might impact soil fungi, especially those symbiotically associated to tree roots. In this study, we analyse the temporal dynamics of soil fungi along five sampling years after tree removal in a managed Pinus sylvestris stand in northern Spain, where timber production is combined with regular mushroom harvesting. Two management methods were tested: total and partial clear-cutting leaving retention trees for seedling regeneration. Undisturbed, uncut plots were also included in the experiment as a control treatment. The whole fungal community (phylotypes and ecological guilds) were analysed by high-throughput Illumina MiSeq sequencing of fungal ITS1 amplicons. We hypothesized that (1) ectomycorrhizal fungal communities will decrease after both clear-cutting treatments with a concurrent increase in the abundance of saprotrophs, (2) the abundance and diversity of the ectomycorrhizal guild will be more preserved in partially clear-cut than in total clear-cut plots, and (3) the overall fungal diversity will decrease in the cut plots leading to major losses of ectomycorrhizal species. Our results show that soil fungal composition changed across the five years after clear-cutting by decreasing ectomycorrhizal fungi and increasing saprotrophs. However, these changes did not significantly affect fungal diversity and there were taxa-specific responses to tree harvest treatments. Boletus edulis, the most abundant ectomycorrhizal species fruiting in the study area and a valuable local non-forest resource, was negatively affected by either clear-cutting treatments. Soil fungal community composition in partially clear-cut areas was not different from that of total clear-cut areas. Our results indicate a strong effect of tree harvest on the relative abundance of ectomycorrhizal fungi along the first years after clear-cutting. However, levels of fungal diversity were comparable to the undisturbed forest, thus suggesting a potential further recovery of ectomycorrhizal fungi through the colonization of the regenerated seedlings.info:eu-repo/semantics/acceptedVersio
The effect of low solubility organic acids on the hygroscopicity of sodium halide aerosols
In order to accurately assess the influence of fatty acids on the
hygroscopic and other physicochemical properties of sea salt aerosols,
hexanoic, octanoic or lauric acid together with sodium halide salts (NaCl,
NaBr and NaI) have been chosen to be investigated in this study. The
hygroscopic properties of sodium halide sub-micrometre particles covered with
organic acids have been examined by Fourier-transform infrared spectroscopy
in an aerosol flow cell. Covered particles were generated by flowing
atomized sodium halide particles (either dry or aqueous) through a heated
oven containing the gaseous acid. The obtained results indicate that gaseous
organic acids easily nucleate onto dry and aqueous sodium halide particles.
On the other hand, scanning electron microscopy (SEM) images indicate that
lauric acid coating on NaCl particles makes them to aggregate in small
clusters. The hygroscopic behaviour of covered sodium halide particles in
deliquescence mode shows different features with the exchange of the halide
ion, whereas the organic surfactant has little effect in NaBr particles,
NaCl and NaI covered particles experience appreciable shifts in their
deliquescence relative humidities, with different trends observed for each
of the acids studied. In efflorescence mode, the overall effect of the
organic covering is to retard the loss of water in the particles. It has
been observed that the presence of gaseous water in heterogeneously
nucleated particles tends to displace the cover of hexanoic acid to
energetically stabilize the system
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