Dust and gas emission from cometary nuclei: the case of comet 67P/Churyumov-Gerasimenko

Comets display with decreasing solar distance an increased emission of gas and dust particles, leading to the formation of the coma and tail. Spacecraft missions provide insight in the temporal and spatial variations of the dust and gas sources located on the cometary nucleus. For the case of comet 67P/Churyumov-Gerasimenko (67P/C-G), the long-term observations from the Rosetta mission point to a homogeneous dust emission across the entire illuminated surface. Despite the homogeneous initial distribution, a collimation in jet-like structures becomes visible. We propose that this observation is linked directly to the complex shape of the nucleus and projects concave topographical features into the dust coma. To test this hypothesis, we put forward a gas-dust description of 67P/C-G, where gravitational and gas forces are accurately determined from the surface mesh and the rotation of the nucleus is fully incorporated. The emerging jet-like structures persist for a wide range of gas-dust interactions and show a dust velocity dependent bending.Comment: 17 pages, with 7 figures. To appear in Advances in Physics X (2018

Magnetic excitations and amplitude fluctuations in insulating cuprates

We present results from light scattering experiments on three insulating antiferromagnetic cuprates, YBa$_2$Cu$_3$O$_{6.05}$, Bi$_2$Sr$_2$YCu$_2$O$_{8+\delta}$, and La$_2$CuO$_4$ as a function of polarization and excitation energy {using samples of the latest generation. From the raw data we derive symmetry-resolved spectra.} The spectral shape in $B_{1g}$ symmetry is found to be nearly universal and independent of the excitation energy. The spectra agree quantitatively with predictions by field theory [\onlinecite{Weidinger:2015}] facilitating the precise extraction of the Heisenberg coupling $J$. {In addition, the asymmetric line shape on the high-energy side is found to be related to amplitude fluctuations of the magnetization. In La$_2$CuO$_4$ alone minor contributions from resonance effects may be identified.} The spectra in the other symmetries are not universal. The variations may be traced back to weak resonance effects and extrinsic contributions. For all three compounds we find support for the existence of chiral excitations appearing as a continuum in $A_{2g}$ symmetry having an onset slightly below $3J$. In La$_2$CuO$_4$ an additional isolated excitation appears on top of the $A_{2g}$ continuum.Comment: 8 pages, 7 figure

Signal-mediated export of proteins from the malaria parasite to the host erythrocyte

Intracellular parasites from the genus Plasmodium reside and multiply in a variety of cells during their development. After invasion of human erythrocytes, asexual stages from the most virulent malaria parasite, P. falciparum, drastically change their host cell and export remodelling and virulence proteins. Recent data demonstrate that a specific NH2-terminal signal conserved across the genus Plasmodium plays a central role in this export process

Prevalence and Longitudinal Trends of Early Internationalisation Patterns among Canadian SMEs

Purpose: Recently, studies call for a more nuanced perspective on different internationalization patterns pursued by early internationalizers. These studies argue that most Born Global firms turn out to be Born Regional and that the proportion of true Born Global firms would be overestimated. Moreover, literature claims that the proportion of Born Global firms increases over time due to macroeconomic trends. We investigate these assumptions by providing a dynamic perspective on the prevalence of different types of internationalization patterns among Canadian small and medium-sized exporters (SMEs). Design/methodology/approach: To empirically examine the ideas above, we constructed a unique large-scale longitudinal (1997–2004) dataset. A multinomial logit model is employed to estimate a firm’s predicted probability, ceteris paribus, of choosing different internationalization patterns: Born Global, Born Regional, and Gradual Internationalization. Findings: We find that Born Global firms indeed account for a smaller proportion than Born Regional firms (16% vs. 27%). However, we find evidence that Born Globals and Born Regionals are increasingly established over time and that macroeconomic factors seem to account for this development at least partially. Originality/value Combining a rigorous empirical analysis with a unique large scale longitudinal dataset, we address two fundamental research questions in the international entrepreneurship (IE) literature a) which internationalization pattern prevails and b) if the Born Global pattern is increasingly established over time. We therewith theoretically contribute by comparing the predictive value of different internationalization frameworks international new venture (INV) framework, stage-models and regionalization hypothesis), toward which there is considerable current debate

Prevalence and Longitudinal Trends of Early Internationalisation Patterns among Canadian SMEs

Purpose: Recently, studies call for a more nuanced perspective on different internationalization patterns pursued by early internationalizers. These studies argue that most Born Global firms turn out to be Born Regional and that the proportion of true Born Global firms would be overestimated. Moreover, literature claims that the proportion of Born Global firms increases over time due to macroeconomic trends. We investigate these assumptions by providing a dynamic perspective on the prevalence of different types of internationalization patterns among Canadian small and medium-sized exporters (SMEs). Design/methodology/approach: To empirically examine the ideas above, we constructed a unique large-scale longitudinal (1997–2004) dataset. A multinomial logit model is employed to estimate a firm’s predicted probability, ceteris paribus, of choosing different internationalization patterns: Born Global, Born Regional, and Gradual Internationalization. Findings: We find that Born Global firms indeed account for a smaller proportion than Born Regional firms (16% vs. 27%). However, we find evidence that Born Globals and Born Regionals are increasingly established over time and that macroeconomic factors seem to account for this development at least partially. Originality/value Combining a rigorous empirical analysis with a unique large scale longitudinal dataset, we address two fundamental research questions in the international entrepreneurship (IE) literature a) which internationalization pattern prevails and b) if the Born Global pattern is increasingly established over time. We therewith theoretically contribute by comparing the predictive value of different internationalization frameworks international new venture (INV) framework, stage-models and regionalization hypothesis), toward which there is considerable current debate

The twistor spinors of generic 2- and 3-distributions

Generic distributions on 5- and 6-manifolds give rise to conformal structures that were discovered by P. Nurowski resp. R. Bryant. We describe both as Fefferman-type constructions and show that for orientable distributions one obtains conformal spin structures. The resulting conformal spin geometries are then characterized by their conformal holonomy and equivalently by the existence of a twistor spinor which satisfies a genericity condition. Moreover, we show that given such a twistor spinor we can decompose a conformal Killing field of the structure. We obtain explicit formulas relating conformal Killing fields, almost Einstein structures and twistor spinors.Comment: 26 page

How to adequately represent biological processes in modeling multifunctionality of arable soils

Essential soil functions such as plant productivity, C storage, nutrient cycling and the storage and purification of water all depend on soil biological processes. Given this insight, it is remarkable that in modeling of these soil functions, the various biological actors usually do not play an explicit role. In this review and perspective paper we analyze the state of the art in modeling these soil functions and how biological processes could more adequately be accounted for. We do this for six different biologically driven processes clusters that are key for understanding soil functions, namely i) turnover of soil organic matter, ii) N cycling, iii) P dynamics, iv) biodegradation of contaminants v) plant disease control and vi) soil structure formation. A major conclusion is that the development of models to predict changes in soil functions at the scale of soil profiles (i.e. pedons) should be better rooted in the underlying biological processes that are known to a large extent. This is prerequisite to arrive at the predictive models that we urgently need under current conditions of Global Change