Thermal properties of heterogeneous grains

Cometary dust is not spherical nor homogeneous, yet these are the assumptions used to model its thermal, optical, and dynamical properties. To better understand the effects of heterogeneity on the thermal and optical properties of dust grains, the effective dielectric constant for an admixture of magnetite and a silicate were calculated using two different effective medium theories: the Maxwell-Garnett theory and the Bruggeman theory. In concept, the MG theory describes the effective dielectric constant of a matrix material into which is embedded a large number of very small inclusions of a second material. The Bruggeman theory describes the dielectric constant of a well mixed aggregate of two or more types of materials. Both theories assume that the individual particles are much smaller than the wavelength of the incident radiation. The refractivity for a heterogeneous grain using the MG theory is very similar to the refractivity of the matrix material, even for large volume fractions of the inclusion. The equilibrium grain temperature for spherical particles sized from .001 to 100 microns in radius at 1 astronomical unit from the sun was calculated. Further explanation is given

Numerical simulations of cometary dust

Most observations of comets are done photometrically or spectrophotometrically. The interpretation of the aperture-averaged flux is relatively simple for an isotropic, radially expanding coma of infinite extent - the canonical model. However, the interpretation of the observations is not so clear when the motion of the dust is affected by radiation pressure, or when the emission is time-varying and anisotropic. For example, in a sample of CCD images of 10 comets, Jewitt and Meech (1987, Ap.J. 317, 992) found that the photometric profiles of only three comets were consistent, within the observational errors, with the profiles predicted form the canonical model. Photometric observations with large apertures, however, seem to suggest that the canonical model may be quite adequate (c.f. Osip, Schleicher, and Millis, 1992, Icarus 98, 115). The dust itself is characterized by a size distribution, with size dependencies on the expansion velocity, the scattered and thermal radiation, the response to radiation pressure, and probably the density. How good then are the approximations normally used in determining the production rates of the dust when these effects are present? As part of a program to better understand the dynamics of cometary dust and gas, a computer program has been developed which numerically simulates the emission of both dust and gas from a tilted rotating nucleus

Risk and economic sustainability of crop farming systems

Environmental, social and economic attributes are important for the sustainability of a farming system. Comparing farming systems by considering only expected profitability ignores differences in both sustainability and in the riskiness of system returns. Further, in choosing between farming systems, the ability to survive various risks and shocks and con-tinue in the future is important, i.e., system resilience and persistence are important aspects of sustainabil-ity. Yet resilience and persistence have seldom been directly considered in evaluations of economic sus-tainability. A whole-farm stochastic simulation model over a six-year planning horizon was used to compare organic and conventional cropping systems for a representative farm situation in Eastern Norway. The relative sustainability of alternative systems under changing assumptions about future technology and price regimes was examined in terms of terminal financial position. The risk efficiency of the same alternatives was also compared. The results illustrate possible conflicts between pursuit of risk efficiency versus sustainability. The model used could be useful in supporting farmers’ choice between farming sys-tems as well for policy makers to develop more sharply targeted policies

The impact of business-cycle fluctuations on private-label share.

This study investigates the cyclical dependence of private-label success in four countries. The results show that private-label share behaves countercyclically. Moreover, asymmetries are present in both the extent and speed of up- and down-ward movements in private-label share over the business cycle. Finally, part of private-labels' share gain during contractions is found to be permanent.Business; Country; Dependence; Studies; Success;

Model for Anisotropic Directed Percolation

We propose a simulation model to study the properties of directed percolation in two-dimensional (2D) anisotropic random media. The degree of anisotropy in the model is given by the ratio $\mu$ between the axes of a semi-ellipse enclosing the bonds that promote percolation in one direction. At percolation, this simple model shows that the average number of bonds per site in 2D is an invariant equal to 2.8 independently of $\mu$. This result suggests that Sinai's theorem proposed originally for isotropic percolation is also valid for anisotropic directed percolation problems. The new invariant also yields a constant fractal dimension $D_{f} \sim 1.71$ for all $\mu$, which is the same value found in isotropic directed percolation (i.e., $\mu = 1$).Comment: RevTeX, 9 pages, 3 figures. To appear in Phys.Rev.

Optimal Length of Leys in an Area with Winter Damage Problems – Optimal Economic Lengths of Leys

The optimal economic life cycle of grass leys with winter damage problems in northern Norway and the threshold of winter damage before it is profitable to re-seed grasses are investigated. The loss in profit of a sub-optimal strategy compared to an optimal strategy is briefly discussed. An infinite horizon stochastic dynamic programming model including a normally distributed yield process with possibilities for discrete downward jumps is developed. The jump process reflects the sudden drop in production after winter damage. Normally the yield of ley increases the first few years after a year with downward jump, and this dynamic is included in the model. The transition probabilities used in the model are estimated with Monte Carlo simulation. Our result show that, in the case of winter damage of 50% or more compared to fields without winter damage, it is optimal to replace the ley immediately. If the winter damage is limited to 20-40% of the yield of fields without winter damage, the replacement decision depends on the age of the ley and the current yield level. It does not always pay to replace immediately mildly winter damaged fields that are still producing high yields. It does not pay to replace a ley if there has been no winter damage since establishment and if the relative yield level is ‘satisfactory’ at least until the first episode of winter damage occurs.Farm Management,

DETERMINANTS OF PART-TIME FARMING AND ITS EFFECT ON FARM PRODUCTIVITY AND EFFICIENCY

Little attention has been given in the agricultural economics literature to the impact of off-farm work on farm productivity and efficiency. More knowledge about what determines part-time farming and whether farm productivity and efficiency are affected by part-time farming could help policy makers introduce better targeted rural development policies. This paper aims to fill the above-mentioned gaps by first analysing factors that influence the choice of off-farm work; and then examining how off-farm work influences productivity and technical efficiency at the farm level. An unbalanced panel data set from 1991 to 2005 from Norwegian grain farms is used for this purpose. The results show that the likelihood of off-farm work and the share of time allocated to it increase with increasing age (up to 39 years), and with low relative yields (compared to others farms in the surrounding area/region). The level of support payments is not significantly associated with the extent of off-farm work. Large-scale farms and single farmers tend to have a lower likelihood of off-farm work. Average technical efficiency was found to be 79%. Farmers with low variability in farm revenue were found to be more technically efficient than farmers with high revenue variability. We did not find any evidence of off-farm work share affecting farm productivity − the predicted off-farm work share was not statistically significant. In other words, we did not find any systematic difference in farm productivity and technical efficiency between part-time and full-time farmers.off-farm work, productivity, efficiency, unobserved heterogeneity, panel data, Farm Management,