The Effect of Diffuse Light on Crops

Light is not evenly distributed in Dutch glass greenhouses, but this can be improved with diffuse light. Modern greenhouse coverings are able to transform most of the light entering the greenhouse into diffuse light. Wageningen UR Greenhouse Horticulture has studied the effect of diffuse light on crops for several years. Modelling and experimental studies showed that crops such as fruit vegetables with a high plant canopy as well as ornamentals with a small plant canopy can utilize diffuse light better than direct light. Diffuse light penetrates the middle layers of a high-grown crop and results in a better horizontal light distribution in the greenhouse. Diffuse light is absorbed to a better degree by the middle leaf layers of cucumber, resulting in a higher photosynthesis. The actual photosynthesis of four pot plant species was found to be increased and crop temperatures were lower during high irradiation. The yield of cucumbers was increased, and the growth rate of several potted plants was increased. These investigations have resulted in a quantitative foundation for the potentials of diffuse light in Dutch horticultural greenhouses and the selection and verification of technological methods to convert direct sunlight into diffuse light

Measuring the Wilson depression of sunspots using the divergence-free condition of the magnetic field vector

Context: The Wilson depression is the difference in geometric height of unit continuum optical depth between the sunspot umbra and the quiet Sun. Measuring the Wilson depression is important for understanding the geometry of sunspots. Current methods suffer from systematic effects or need to make assumptions on the geometry of the magnetic field. This leads to large systematic uncertainties of the derived Wilson depressions. Aims: We aim at developing a robust method for deriving the Wilson depression that only requires the information about the magnetic field that is accessible from spectropolarimetry, and that does not rely on assumptions on the geometry of sunspots or on their magnetic field. Methods: Our method is based on minimizing the divergence of the magnetic field vector derived from spectropolarimetric observations. We focus on large spatial scales only in order to reduce the number of free parameters. Results: We test the performance of our method using synthetic Hinode data derived from two sunspot simulations. We find that the maximum and the umbral averaged Wilson depression for both spots determined with our method typically lies within 100 km of the true value obtained from the simulations. In addition, we apply the method to Hinode observations of a sunspot. The derived Wilson depression (about 600 km) is consistent with results typically obtained from the Wilson effect. We also find that the Wilson depression obtained from using horizontal force balance gives 110 - 180 km smaller Wilson depressions than both, what we find and what we deduce directly from the simulations. This suggests that the magnetic pressure and the magnetic curvature force contribute to the Wilson depression by a similar amount.Comment: 12 pages, 8 figures. Accepted for publication in Astronomy & Astrophysic

Vertical flows and mass flux balance of sunspot umbral dots

A new Stokes inversion technique that greatly reduces the effect of the spatial point spread function of the telescope is used to constrain the physical properties of umbral dots (UDs). The depth-dependent inversion of the Stokes parameters from a sunspot umbra recorded with Hinode SOT/SP revealed significant temperature enhancements and magnetic field weakenings in the core of the UDs in deep photospheric layers. Additionally, we found upflows of around 960 m/s in peripheral UDs (i.e., UDs close to the penumbra) and $\approx$ 600 m/s in central UDs. For the first time, we also detected systematic downflows for distances larger than 200 km from the UD center that balance the upflowing mass flux. In the upper photosphere, we found almost no difference between the UDs and their diffuse umbral background.Comment: Accepted for publication in Astronomy and Astrophysic

Inclinations of small quiet-Sun magnetic features based on a new geometric approach

High levels of horizontal magnetic flux have been reported in the quiet-Sun internetwork, often based on Stokes profile inversions. Here we introduce a new method for deducing the inclination of magnetic elements and use it to test magnetic field inclinations from inversions. We determine accurate positions of a set of small, bright magnetic elements in high spatial resolution images sampling different photospheric heights obtained by the Sunrise balloon-borne solar observatory. Together with estimates of the formation heights of the employed spectral bands, these provide us with the inclinations of the magnetic features. We also compute the magnetic inclination angle of the same magnetic features from the inversion of simultaneously recorded Stokes parameters. Our new, geometric method returns nearly vertical fields (average inclination of around 14 deg with a relatively narrow distribution having a standard deviation of 6 deg). In strong contrast to this, the traditionally used inversions give almost horizontal fields (average inclination of 75+-8 deg) for the same small magnetic features, whose linearly polarised Stokes profiles are adversely affected by noise. The almost vertical field of bright magnetic features from our geometric method is clearly incompatible with the nearly horizontal magnetic fields obtained from the inversions. This indicates that the amount of magnetic flux in horizontal fields deduced from inversions is overestimated in the presence of weak Stokes signals, in particular if Stokes Q and U are close to or under the noise level. By combining the proposed method with inversions we are not just improving the inclination, but also the field strength. This technique allows us to analyse features that are not reliably treated by inversions, thus greatly extending our capability to study the complete magnetic field of the quiet Sun.Comment: 12 pages, 9 figures, 1 table; Accepted for publication in Astronomy & Astrophysic

The Effect of Website Interactivity on Political Involvement:The Moderating Role of Political Cynicism

This study examines the extent to which interactive communication on political websites affects various forms of citizens’ involvement in politics, and the moderating role of political cynicism in this relationship. Based on the outcomes of a laboratory experiment with a single-factor (interactivity: low vs. medium vs. high interactivity) between-subjects design, we found that interactive political websites have a positive effect on citizen involvement, and this effect is particularly present for websites with high levels of interactivity. We also demonstrate that interactivity effects are, to some extent, contingent on citizens’ political cynicism. For higher levels of political cynicism, deviations in the level of interactivity make less of a difference in their impact on political involvement

Causes and Effects of Private Property Rights Security

Private property rights security is currently seen as central to explaining cross-country differences in economic development. Variation in private property rights security itself is perceived to be best explained by differences in the degree to which the political system is able to constrain the despotic power of state executives. I reassess the existing evidence for these two hypotheses and find that: (1) higher levels and significant changes in private property rights security and constraints on the executive are not correlated with higher levels of income and (changes in) growth rates; (2) the commonly used instrument for constraints on the executive and private property rights security - the natural log of European settler mortality - is invalid because it is associated with current levels of income besides its effect through private property rights security and constraints on the executive; and (3) the regularly cited Korean case is in fact evidence against these hypotheses. I provide explanations for these findings and call for a rethinking of which type of institutions and policies are decisive for growth

Screening Mothers: Representations of motherhood in Australian films from 1900 to 1988.

Although the position of mothers has changed considerably since the beginning of the twentieth century, an idealised notion of motherhood persists. The cinema provides a source of information about attitudes towards mothering in Australian society which is not diminished by the fact that mothers are often marginal to the narrative. While the study recognises that cinematic images are not unconditionally authoritative, it rests on the belief that films have some capacity to reflect and influence society. The films are placed in an historical context with regard to social change in Australian society, so that the images can be understood within the context of the time of the making and viewing of the films. The depictions of the mother are scrutinised with regard to her appearance, her attitude, her relationship with others and the expectations, whether explicit or implicit, of her role. Of particular significance is what happens to her during the film and whether she is punished or rewarded for her behaviour. The conclusions reached after analysis are used to challenge those ideas which assume that portrayals of motherhood are unchangeable and timeless. The study examines Australian feature films from 1900 to 1988. To augment its historical focus, it uses sociological, psychoanalytical and feminist theoretical writing with special relevance for motherhood and mothering practice. Looking at areas of importance to mothers, it comprises an exploration of what makes a mother good or bad; the significance of the birth of female and male children; the relationship of mothers to daughters; the mother's sexuality and the metaphor of the missing mother. It shows that images of motherhood on screen are organised according to political, social and economic requirements in the community. Further, films frequently show mothers in traditional roles which are useful for maintaining notions of patriarchal privilege in society. The analysis exposes stereotypical depictions of motherhood which are often inaccurate, unfair and oppressive to women

Digital diplomacy in GCC countries: strategic communication of Western embassies on Twitter

Drawing upon online communication research, this study identifies six effective communication strategies for social media-based diplomacy on Twitter: interactive, personalized, positive, relevant, and transparent communication among a broad network of stakeholders. By using an extensive mix-method design (i.e., combining a manual content and automated network analyses, N = 4438 tweets), this research examines to what extent these communication strategies are adopted on Twitter by Western embassies active in countries from the Gulf Cooperation Council. We found that embassies are not utilizing social media to its full potential. Although embassies are transparent, use positive sentiment in their online communication and post relevant information to their stakeholders, they hardly engage in direct interactive and personal communication, and only reach out to a limited group of stakeholders. We recommend embassies to put more emphasis on two-way interactive communication with a vast variety of stakeholders