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The predictions of mean-field electrodynamics can now be probed using direct numerical simulations of random flows and magnetic fields. When modelling astrophysical magnetohydrodynamics, it is important to verify that such simulations are in agreement with observations. One of the main challenges in this area is to identify robust quantitative measures to compare structures found in simulations with those inferred from astrophysical observations. A similar challenge is to compare quantitatively results from different simulations. Topological data analysis offers a range of techniques, including the Betti numbers and persistence diagrams, that can be used to facilitate such a comparison. After describing these tools, we first apply them to synthetic random fields and demonstrate that, when the data are standardized in a straightforward manner, some topological measures are insensitive to either large-scale trends or the resolution of the data. Focusing upon one particular astrophysical example, we apply topological data analysis to H i observations of the turbulent interstellar medium (ISM) in the Milky Way and to recent magnetohydrodynamic simulations of the random, strongly compressible ISM. We stress that these topological techniques are generic and could be applied to any complex, multi-dimensional random field.
This issue commemorates an outstanding scientist of the twentieth century, Yakov Borisovich Zeldovich, in connection with the centenary of his birth (8 March 1914), with a collection of reviews and research articles broadly related to large-scale random phenomena in astrophysical plasmas.
This article deals with a quadrilingual lexicon from a unique manuscript in the Bibliothéque Nationale in Paris, which was compiled in 1439 and contains abundant material on Medieval Greek vocabulary and phraseology. The article analyses Oriental loan-words in the Greek part of the lexicon as evidence of Oriental influences on the Greek language during the late Byzantine period.
The mechanisms by which agriculture spread across Europe in the Neolithic, and the speed at which it happened, have long been debated. Attempts to quantify the process by constructing spatio-temporal models have given a diversity of results. In this paper, a new approach to the problem of modelling is advanced. Data from over 300 Neolithic sites from Asia Minor and Europe are used to produce a global picture of the emergence of farming across Europe which also allows for variable local conditions. Particular attention is paid to coastal enhancement: the more rapid advance of the Neolithic along coasts and rivers, as compared with inland or terrestrial domains. The key outcome of this model is hence to confirm the importance of waterways and coastal mobilities in the spread of farming in the early Neolithic, and to establish the extent to which this importance varied regionally.
The theory of mean-field galactic dynamos is generalized by allowing for a finite response time of the mean electromotive force (emf) to variations in the mean magnetic field and small-scale turbulence. A non-axisymmetric forcing of the dynamo by a spiral pattern (either stationary or transient) is invoked. The resulting magnetic spiral arms are phase-shifted from the spiral arms of the pattern by an angle 15°–40°, opposite to the sense of galactic rotation. Our findings may help to explain the phase shift between material and magnetic arms observed in NGC 6946 and other galaxies.
We present a study of the Milky Way halo magnetic field, determined from observations of Faraday rotation measure (RM) of extragalactic radio sources (EGS) in Galactic longitude range 100°–117° within 30° of the Galactic plane. We find negative median RMs in both the northern and southern Galactic hemispheres for |b|>15°, outside the latitude range where the disk field dominates. This suggest that the halo magnetic field towards the outer Galaxy does not reverse direction across the mid-plane. An azimuthal magnetic field at heights 0.8−2 kpc above/below the Galactic plane between the local and the Perseus spiral arm can reproduce the observed trend of RM against Galactic latitude. We propose that the Milky Way could have a halo magnetic field similar to that observed in M51.
This work demonstrates the patterning of polymethylmethacrylate (PMMA) by ablation with Ar8+ ion laser (λ = 46.9 nm) pumped by pulse, high-current, capillary-discharge. For focusing a long-focal spherical mirror (R = 2100 mm) covered by 14 double-layer Sc-Si coating was used. The ablated focal spots demonstrate not only that the energy of our laser is sufficient for such experiments, but also that the design of focusing optics must be more sophisticated: severe aberrations were revealed — an irregular spot shape and strong astigmatism with astigmatic difference as large as 16 mm. In some cases, on the bottom of ablated spots a laser-induced periodic surface structure appeared. Finally, an illumination of the sample through quadratic hole 7.5 × 7.5 µm, standing in contact with PMMA substrate ablated from the surface a strongly developed two-dimensional diffraction pattern (period in the center about 125 nm).
Newly available radiocarbon dates show the early signs of pottery-making in the North Caspian area, the Middle-Lower Volga, and the Lower Don at 8–7 kyr cal BC. Stable settlements, as indicated by “coeval subsamples,” are recognized in the Middle-Lower Volga (Yelshanian) at 6.8 kyr cal BC and the Caspian Lowland at about 6 kyr cal BC. The ages of the Strumel-Gostyatin, Surskian, and Bug-Dniesterian sites are in the range of 6.6–4.5 kyr BC, overlapping with early farming entities (Starčevo-Körös-Criş and Linear Pottery), whose influence is perceptible in archaeological materials. Likewise, the 14C-dated pollen data show that the spread of early pottery-making coincided with increased precipitation throughout the forest-steppe area.
Observations show that magnetic fields in the interstellar medium (ISM) often do not respond to increases in gas density as would be naively expected for a frozen-in field. This may suggest that the magnetic field in the diffuse gas becomes detached from dense clouds as they form. We have investigated this possibility using theoretical estimates, a simple magneto-hydrodynamic model of a flow without mass conservation and numerical simulations of a thermally unstable flow. Our results show that significant magnetic flux can be shed from dense clouds as they form in the diffuse ISM, leaving behind a magnetically dominated diffuse gas.
We calculate the relative magnitudes of the fluctuations in total synchrotron intensity in the interstellar medium, both from observations and from theory under various assumptions about the correlation or anticorrelation between cosmic rays and interstellar magnetic fields. The results are inconsistent with local energy equipartition between cosmic rays and magnetic fields. The distribution of cosmic rays must be rather uniform at scales of order 1 kpc, whereas interstellar magnetic fields vary at much smaller scales.
Recent radio polarization observations have revealed a plethora of
unexpected features in the polarized Galactic radio background that
arise from propagation effects in the random (turbulent) interstellar
medium. The canals are especially striking among them, a random network
of very dark, narrow regions clearly visible in many directions against
a bright polarized Galactic synchrotron background. There are no obvious
physical structures in the ISM that may have caused the canals, and so
they have been called Faraday ghosts. They evidently carry information
about interstellar turbulence but only now is it becoming clear how this
information can be extracted. Two theories for the origin of the canals
have been proposed; both attribute the canals to Faraday rotation, but
one invokes strong gradients in Faraday rotation in the sky plane
(specifically, in a foreground Faraday screen) and the other only relies
on line-of-sight effects (differential Faraday rotation). In this review
we discuss the physical nature of the canals and how they can be used to
explore statistical properties of interstellar turbulence. This opens
studies of magnetized interstellar turbulence to new methods of
analysis, such as contour statistics and related techniques of
computational geometry and topology. In particular, we can hope to
measure such elusive quantities as the Taylor microscale and the
effective magnetic Reynolds number of interstellar MHD turbulence.
Newly obtained radiocarbon measurements are used to suggest that the initial settlement of the northeastern Baltic area was largely controlled by the Ladoga-Baltic waterway in the north of the Karelian Isthmus, which emerged ∼11,500 cal BP and remained in action for ∼7000 yr. The transgression of Ladoga Lake started ∼5000 cal BP and reached its maximum at ∼3000 cal BP (∼1100–1000 cal BC). The formation of a new outlet via the Neva River led to a rapid regression of the lake that stimulated the spread of farming populations.
We argue that interstellar gas in elliptical galaxies can be turbulent, with turbulent scale and velocity of 400 pc and 20 km s−1 respectively. An upper limit on turbulent velocity, ≃ 50 km s−1, follows from the requirement that the turbulence dissipation rate does not exceed the X-ray gas luminosity. The turbulence can generate random magnetic fields of 0.3 μG strength at the above scale via fluctuation dynamo action. The resulting Faraday rotation is random, with a typical value of 5-30 rad m−2, consistent with observational evidence available.
Maarit J. Korpi, Astronomy Division, University of Oulu, P.O. Box 333, 90571 Oulu, Finland,
Axel Brandenburg, Department of Mathematics, University of Newcastle upon Tyne, NE1 7RU, UK,
Anvar Shukurov, Department of Mathematics, University of Newcastle upon Tyne, NE1 7RU, UK,
Ilkka Tuominen, Astronomy Division, University of Oulu, P.O. Box 333, 90571 Oulu, Finland
Jose Franco, Universidad Nacional Autónoma de México,Alberto Carraminana, Instituto Nacional de Astrofisica, Optica y Electronica, Tonantzintla, Mexico
We investigate the generation of vorticity in supernova driven interstellar turbulence using a local three-dimensional MHD model. Our model includes the effects of density stratification, compressibility, magnetic fields, large-scale shear due to galactic differential rotation, heating via supernova explosions and parameterized radiative cooling of the interstellar medium; we also include viscosity and resistivity. We allow for multiple supernovae, which are distributed randomly in the galactic disc and exponentially in the vertical direction. When supernovae are infrequent, so that there is no interactions between supernova remnants, the dynamics of the system is dominated by strong shocks driven by the young remnants. Supernova interactions, where shock fronts from younger remnants encounter the dense shells of the older remnants, were found to produce vorticity via the baroclinic effect. Vorticity generated by the baroclinic effect was observed to be amplified by the stretching of vortex lines, these two vorticity production mechanisms being of equal importance after 1.5 × 108 years. Motions driven by the supernova explosions also amplify the magnetic field via stretching and compression. This generates a random component from a uniform azimuthal magnetic field prescribed as an initial condition and maintains it against Ohmic losses.
The interstellar medium (ISM) is in a state of a compressible, inhomogeneous and anisotropic turbulent flow. There are several energy sources for the interstellar turbulence. Stellar winds, supernova (SN) explosions and superbubbles heat, accelerate and compress the ISM driving shock waves (e.g. Ostriker & McKee 1988).
The emperor Manuel I Komnenos (1143-80) asked a certain fortune teller, how long the dynasty of Alexios I Komnenos (1081-1118) would reign. The prophetic response was the word (blood), the first three letters designating the first initials of the Komnenian emperors in the order of succession, namely, Alexios I, John II (1118-43), Manuel I, the last alpha for the name of Manuel’s future successor.
Using the asymptotic forms of the eigenfunctions, we solve, for Rm ≫ 1 and t → ∈ (with Rm the magnetic Reynolds number), the Cauchy problem for the kinematic screw dynamo. It is demonstrated that for a spatially localized seed magnetic field the field grows at different rates within the region of localization and outside it.
The screw dynamo is one of the simplest examples of a conducting fluid flow in which magnetic field can be self-excited provided the magnetic Reynolds number is sufficiently large (see, e.g., Roberts 1993). Such a flow can be encountered in some astrophysical objects and also in such technological devices as breeder reactors. For example, jet outflows in active galaxies and near young stars can be swirling. A flow of this type is used for modelling the dynamo effects in laboratory conditions (Gailitis 1993). The generation of magnetic fields by a laminar flow with helical streamlines was discussed by Lortz (1968), Ponomarenko (1973), Gailitis & Freiberg (1976), Gilbert (1988), Ruzmaikin et al. (1988) and other authors as an eigenvalue problem. Below we use the results of the asymptotic analysis of this problem for large Rm by Ruzmaikin et al.
We introduce an axisymmetric velocity field whose cylindrical polar components are (0, rω(r), v2(r)), with (r, φ, z) the cylindrical coordinates. We
consider smooth functions v2(r) and ω(r) vanishing as r → ∞. Both v2 (0) and ω(0) are assumed to be of order unity.
For Rm ≫ 1, an eigenmode of the screw dynamo represents a dynamo wave concentrated in a cylindrical shell of thickness ≃Rm−1/4 a certain radius r0.
The origin of a regular magnetic field in astrophysical jets is discussed. It is shown that jet plasma flow can generate a magnetic field provided the streamlines are helical. The dynamo of this type, known as the screw dynamo, generates magnetic fields with the dominant azimuthal wave number m = 1 whose field lines also have a helical shape. The field concentrates into a relatively thin cylindrical shell and its configuration is favorable for the collimation and confinement of the jet plasma.
Trebizond, on the southern coast of the Black Sea is separated by the distance of nearly one thousand kilometres from Ardabīl, the native town of Safawids, situated not far from the Caspian Sea. Despite that, in the midst of the 15th century fate amazingly brought together two royal families: the Grand Komnenoi of Trebizond and the Safawids of Iran. Shāh Ismācīl (1501–1524), the founder of the Safawid state was born in 1487/892 of Halima-begum, the daughter of the Trepezuntine despoina Theodora and the Aq Quyunlu ruler Uzun Hasan (1457–1478). Shaykh Djunayd Ṣafawī (d. 1460), the grandfather of Shāh Ismācīl, had nearly seized the city of Trebizond, about thirty years before the birth of his famous grandson. The Trapezuntine nobles left their sovereigns, the emperor John IV (1429–1459) and, likely, his sister Theodora, the future wife of Uzun Ḥasan and grandmother of Ismācīl. Trebizond remained unprotected and, if shaykh Djunayd were more persistent in his attempts to capture the city, who knows what would be the future of both celebrated dynasties.
A nonlinear thin-disk galactic dynamo model based on α-quenching is proposed. Assuming that the mean helicity depends on the magnetic field strength averaged across the disk, we derive a universal form of nonlinearity in the radial dynamo equation. We discuss the evolution of the regular magnetic field in the Milky Way and the Andromeda Nebula. It is argued that the reversals of the regular magnetic field in the Galaxy are a relic inherited from the structure of the seed field. We also briefly discuss the role of the turbulent diamagnetism and the effects of galactic evolution on the dynamo.