Separating the scales in a compressible interstellar medium

We apply Gaussian smoothing to obtain mean density, velocity, magnetic and energy density fields in simulations of the interstellar medium based on three-dimensional magnetohydrodynamic equations in a shearing box $1\times1\times2 \, \rm{kpc}$ in size. Unlike alternative averaging procedures, such as horizontal averaging, Gaussian smoothing retains the three-dimensional structure of the mean fields. Although Gaussian smoothing does not obey the Reynolds rules of averaging, physically meaningful central statistical moments are defined as suggested by Germano (1992). We discuss methods to identify an optimal smoothing scale $\ell$ and the effects of this choice on the results. From spectral analysis of the magnetic, density and velocity fields, we find a suitable smoothing length for all three fields, of $\ell \approx 75 \, \rm{pc}$. We discuss the properties of third-order statistical moments in fluctuations of kinetic energy density in compressible flows and suggest their physical interpretation. The mean magnetic field, amplified by a mean-field dynamo, significantly alters the distribution of kinetic energy in space and between scales, reducing the magnitude of kinetic energy at intermediate scales. This intermediate-scale kinetic energy is a useful diagnostic of the importance of SN-driven outflows

Cosmic rays and random magnetic traps

The spatial distribution of cosmic ray (CR) particles in the interstellar medium (ISM) is of major importance in radio astronomy, where its knowledge is essential for the interpretation of observations, and in theoretical astrophysics, where CR contribute to the structure and dynamics of the ISM. Local inhomogeneities in interstellar magnetic field strength and structure can affect the local diffusivity and ensemble dynamics of the cosmic ray particles. Magnetic traps (regions between magnetic mirrors located on the same magnetic line) can lead to especially strong and persistent features in the CR spatial distribution. Using test particle simulations, we study the spatial distribution of an ensemble of CR particles (both protons and electrons) in various magnetic field configurations, from an idealized axisymmetric trap to those that emerge in intermittent (dynamo-generated) random magnetic fields. We demonstrate that both the inhomogeneity in the CR sources and the energy losses by the CR particles can lead to persistent local inhomogeneities in the CR distribution and that the protons and electrons have different spatial distributions. Our results can have profound implications for the interpretation of the synchrotron emission from astronomical objects, and in particular its random fluctuations.Comment: 19 pages, 18 figure

Productivity of Pre-Modern Agriculture in the Cucuteni-Trylillia Area

We present palaeoeconomy reconstructions for pre-modern agriculture; we select, wherever required, features and parameter values specific for the Cucuteni–Trypillia Cultural unity (CTU: 5,400–2,700 BC, mostly the territory of modern Ukraine, Moldova and Romania). We verify theself-consistency and viability of the archaeological evidence related to all major elements of the agricultural production cycle within the constraints provided by environmental and technological considerations. The starting point of our analysis is the palaeodiet structure suggested by archaeological data, stable isotope analyses of human remains, and palynology studies in the CTU area. We allow for the archeologically attested contributions of domesticated and wild animal products to the diet, develop plausible estimates of the yield of ancient cereal varieties cultivated with ancient techniques, and quantify the yield dependence on the time after initial planting and on rainfall (as a climate proxy). Our conclusions involve analysis of the labour costs of various seasonal parts of the agricultural cycle of both an individual and a family with a majority of members that do not engage in productive activities that require physical fitness, such as tillage. Finally, we put our results into the context of the exploitation territory and catchment analysis, to project various subsistence strategies into the exploitation territory of a farming settlement. The simplest economic complex based on cereals, domestic and wild animal products, with fallow cropping, appears to be capable of supporting an isolated, relatively small farming community of 50–300 people (2–10 ha in area) even without recourse to technological improvements such as the use of manure fertiliser. Our results strongly suggest that dairy products played a significant role in the dietary and labour balance. The smaller settlements are typical of the earliest Trypillia A but remain predominant at the later stages. A larger settlement of several hundred people could function in isolation, perhaps with a larger fraction of cereals in the diet, only with technological innovations, such as manure fertiliser and, most importantly, ard tillage. The ard relieves radically the extreme time pressure associated with soil preparation for sowing. It appears that very large settlements of a few hundred hectares in area, found in the CTU region, could function only if supported by satellite farming villages and stable exchange networks. In turn, this implies social division of labour and occupation, sufficiently complex social relations, stable exchange channels, etc.: altogether, a proto-urban character of such settlements. A model is proposed for the lifetime of a farming settlement assuming that it is limited by the soil fertility (the depleted resources model), that provides a lifetime estimate consistent with the archaeological evidence available (100–150 years). It is shown that the lifetime strongly depends on the fraction of the arable land area kept fallow. We also discuss, quantify and assess some strategies to mitigate the risks of arable agriculture associated with strong temporal fluctuations in the cereal yield, such as manure fertilisation, increased fraction of cereals in the diet combined with producing grain surplus for emergency storage

The effect of hyperdiffusivity on turbulent dynamos with helicity

In numerical studies of turbulence, hyperviscosity is often used as a tool to extend the inertial subrange and to reduce the dissipative subrange. By analogy, hyperdiffusivity (or hyperresistivity) is sometimes used in magnetohydrodynamics. The underlying assumption is that only the small scales are affected by this manipulation. In the present paper, possible side effects on the evolution of the large scale magnetic field are investigated. It is found that for turbulent flows with helicity, hyperdiffusivity causes the dynamo-generated magnetic field to saturate at a higher level than normal diffusivity. This result is successfully interpreted in terms of magnetic helicity conservation, which also predicts that full saturation is only reached after a time comparable to the large scale magnetic (hyper)diffusion time.Comment: 4 pages, 3 figure

A Pan-European model of the Neolithic

We present a mathematical model, based on a compilation of radiocarbon dates, of the transition to the Neolithic, from about 7000 to 4000 BC in Europe. With the arrival of the Neolithic, hunting and food gathering gave way to agriculture and stock breeding in many parts of Europe; pottery-making spread into even broader areas. We use a population dynamics model to suggest the presence of two waves of advance, one from the Near East, and another through Eastern Europe. Thus, we provide a quantitative framework in which a unified interpretation of the Western and Eastern Neolithic can be developed.Predstavljamo matematični model, ki temelji na kompilaciji radiokarbonskih datumov med 7000 in 4000 BC. Ti datumi so v Evropi povezani s prehodom v neolitik, ko sta poljedelstvo in živinoreja v mnogih regijah zamenjala lov in nabiralništvo; lončarstvo pa se je širilo še dlje. S pomočjo modela populacijske dinamike predstavljamo dva vala napredovanja, enega iz Bližnjega Vzhoda in drugega preko Vzhodne Evrope. Z njim zagotavljamo kvantitavni okvir, v katerem lahko razvijamo enovito interpretacijo 'zahodnega' in 'vzhodnega' neolitika