1,720,990 research outputs found
Quasi-geostrophic approximation of anelastic convection
Full text linkThe onset of convection in a rotating cylindrical annulus with parallel ends filled with a compressible fluid is studied in the anelastic approximation. Thermal Rossby waves propagating in the azimuthal direction are found as solutions. The analogy to the case of Boussinesq convection in the presence of conical end surfaces of the annular region is emphasised. As in the latter case, the results can be applied as an approximation for the description of the onset of anelastic convection in rotating spherical fluid shells. Reasonable agreement with three-dimensional numerical results published by Jones, Kuzanyan & Mitchell (J. Fluid Mech., vol. 634, 2009, pp. 291–319) for the latter problem is found. As in those results, the location of the onset of convection shifts outwards from the tangent cylinder with increasing number Nρof density scale heights until it reaches the equatorial boundary. A new result is that at a much higher number Nρ the onset location returns to the interior of the fluid shell
Flows and dynamos in a model of stellar radiative zones
Full text linkStellar radiative zones are typically assumed to be motionless in standard
models of stellar structure but there is sound theoretical and
observational evidence that this cannot be the case. We investigate
by direct numerical simulations a three-dimensional and time-dependent
model of stellar radiation zones consisting of an electrically-conductive
and stably-stratified anelastic fluid confined to a rotating spherical
shell and driven by a baroclinic torque. As the baroclinic driving is
gradually increased a sequence of transitions from an axisymmetric and
equatorially-symmetric time-independent flow to flows with a strong
poloidal component and lesser symmetry are found. It is shown that
all flow regimes characterised with significant non-axisymmetric
components are capable of generating self-sustained magnetic field.
As the value of the Prandtl number is decreased and the value of the
Ekman number is decreased flows become strongly time-dependent with
progressively complex spatial structure and dynamos can be generated
at lower values of the magnetic Prandtl number
Uncovering the secrets of the Sun’s magnetic field
No full textHumans have admired the Sun for as long as they have existed because life simply would not be possible without it. Today, we know that the Sun is not just a blazing ball of light but an extremely complex space laboratory in our cosmic neighborhood. In this article, you will read about the mystery of the Sun—how it acts as a gigantic magnet through a process called the solar dynamo. We will also describe the methods scientists use to understand this mystery
Regimes of thermo-compositional convection and related dynamos in rotating spherical shells
Full text linkConvection and magnetic field generation in the Earth and planetary interiors are driven by both thermal and compositional gradients. In this work numerical simulations of finite-amplitude double-diffusive convection and dynamo action in rapidly rotating spherical shells full of incompressible two-component electrically-conducting fluid are reported. Four distinct regimes of rotating double-diffusive convection identified in a recent linear analysis (Silva, Mather and Simitev, Geophys. Astrophys. Fluid Dyn. 2019, 113, 377) are found to persist significantly beyond the onset of instability while their regime transitions remain abrupt. In the semi-convecting and the fingering regimes characteristic flow velocities are small compared to those in the thermally- and compositionally-dominated overturning regimes, while zonal flows remain weak in all regimes apart from the thermally-dominated one. Compositionally-dominated overturning convection exhibits significantly narrower azimuthal structures compared to all other regimes while differential rotation becomes the dominant flow component in the thermally-dominated case as driving is increased. Dynamo action occurs in all regimes apart from the regime of fingering convection. While dynamos persist in the semi-convective regime they are very much impaired by small flow intensities and very weak differential rotation in this regime which makes poloidal to toroidal field conversion problematic. The dynamos in the thermally-dominated regime include oscillating dipolar, quadrupolar and multipolar cases similar to the ones known from earlier parameter studies. Dynamos in the compositionally-dominated regime exhibit subdued temporal variation and remain predominantly dipolar due to weak zonal flow in this regime. These results significantly enhance our understanding of the primary drivers of planetary core flows and magnetic fields
Flute and kink instabilities in a dynamically twisted flux tube with anisotropic plasma viscosity
Full text linkMagnetic flux tubes such as those in the solar corona are subject to a number of instabilities. Important among them is the kink instability that plays a central part in the nanoflare theory of coronal heating, and for this reason in numerical simulations, it is usually induced by tightly controlled perturbations and studied in isolation. In contrast, we find that fluting modes of instability are readily excited when disturbances are introduced in our magnetohydrodynamic flux tube simulations by dynamic twisting of the flow at the boundaries. We also find that the flute instability, which has been theorized but rarely observed in the coronal context, is strongly enhanced when plasma viscosity is assumed anisotropic. We proceed to investigate the co-existence and competition between flute and kink instabilities for a range of values of the resistivity and of the parameters of the anisotropic and isotropic models of viscosity. We conclude that while the flute instability cannot prevent the kink from ultimately dominating, it can significantly delay its development especially at strong viscous anisotropy induced by intense magnetic fields
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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