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

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

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

The Near-Eastern Roots of the Neolithic in South Asia

<div><p>The Fertile Crescent in the Near East is one of the independent origins of the Neolithic, the source from which farming and pottery-making spread across Europe from 9,000 to 6,000 years ago at an average rate of about 1 km/yr. There is also strong evidence for causal connections between the Near-Eastern Neolithic and that further east, up to the Indus Valley. The Neolithic in South Asia has been far less explored than its European counterpart, especially in terms of absolute (¹⁴C) dating; hence, there were no previous attempts to assess quantitatively its spread in Asia. We combine the available ¹⁴C data with the archaeological evidence for early Neolithic sites in South Asia to analyze the spatio-temporal continuity of the Neolithic dispersal from the Near East through the Middle East and to the Indian subcontinent. We reveal an approximately linear dependence between the age and the geodesic distance from the Near East, suggesting a systematic (but not necessarily uniform) spread at an average speed of about 0.65 km/yr.</p></div