Dipolar dynamos in stratified systems

Observations of low-mass stars reveal a variety of magnetic field topologies ranging from large-scale, axial dipoles to more complex magnetic fields. At the same time, three-dimensional spherical simulations of convectively driven dynamos reproduce a similar diversity, which is commonly obtained either with Boussinesq models or with more realistic models based on the anelastic approximation, which take into account the variation of the density with depth throughout the convection zone. Nevertheless, a conclusion from different anelastic studies is that dipolar solutions seem more difficult to obtain as soon as substantial stratifications are considered. In this paper, we aim at clarifying this point by investigating in more detail the influence of the density stratification on dipolar dynamos. To that end, we rely on a systematic parameter study that allows us to clearly follow the evolution of the stability domain of the dipolar branch as the density stratification is increased. The impact of the density stratification both on the dynamo onset and the dipole collapse is discussed and compared to previous Boussinesq results. Furthermore, our study indicates that the loss of the dipolar branch does not ensue from a specific modification of the dynamo mechanisms related to the background stratification, but could instead result from a bias as our observations naturally favour a certain domain in the parameter space characterized by moderate values of the Ekman number, owing to current computational limitations. Moreover, we also show that the critical magnetic Reynolds number of the dipolar branch is scarcely modified by the increase of the density stratification, which provides an important insight into the global understanding of the impact of the density stratification on the stability domain of the dipolar dynamo branch

Mechanisms of Planetary and Stellar Dynamos

We review some of the recent progress on modeling planetary and stellar dynamos. Particular attention is given to the dynamo mechanisms and the resulting properties of the field. We present direct numerical simulations using a simple Boussinesq model. These simulations are interpreted using the classical mean-field formalism. We investigate the transition from steady dipolar to multipolar dynamo waves solutions varying different control parameters, and discuss the relevance to stellar magnetic fields. We show that owing to the role of the strong zonal flow, this transition is hysteretic. In the presence of stress-free boundary conditions, the bistability extends over a wide range of parameters.Comment: Proceedings of IAUS 294 "Solar and Astrophysical Dynamos and Magnetic Activity" Editors A.G. Kosovichev, E.M. de Gouveia Dal Pino, & Y.Yan, Cambridge University Press, to appear (2013

Dipole Collapse and Dynamo Waves in Global Direct Numerical Simulations

Magnetic fields of low-mass stars and planets are thought to originate from self-excited dynamo action in their convective interiors. Observations reveal a variety of field topologies ranging from large-scale, axial dipole to more structured magnetic fields. In this article, we investigate more than 70 three-dimensional, self-consistent dynamo models obtained by direct numerical simulations. The control parameters, the aspect ratio and the mechanical boundary conditions have been varied to build up this sample of models. Both, strongly dipolar and multipolar models have been obtained. We show that these dynamo regimes can in general be distinguished by the ratio of a typical convective length scale to the Rossby radius. Models with a predominantly dipolar magnetic field were obtained, if the convective length scale is at least an order of magnitude larger than the Rossby radius. Moreover, we highlight the role of the strong shear associated with the geostrophic zonal flow for models with stress-free boundary conditions. In this case, the above transition disappears and is replaced by a region of bistability for which dipolar and multipolar dynamos co-exist. We interpret our results in terms of dynamo eigenmodes using the so-called test-field method. We can thus show that models in the dipolar regime are characterized by an isolated 'single mode'. Competing overtones become significant as the boundary to multipolar dynamos is approached. We discuss how these findings relate to previous models and to observations.Comment: 35 pages, 16 figure

Topology and field strength in spherical, anelastic dynamo simulations

Numerical modelling of convection driven dynamos in the Boussinesq approximation revealed fundamental characteristics of the dynamo-generated magnetic fields and the fluid flow. Because these results were obtained for an incompressible fluid, their validity for gas planets and stars remains to be assessed. A common approach is to take some density stratification into account with the so-called anelastic approximation. The validity of previous results obtained in the Boussinesq approximation is tested for anelastic models. We point out and explain specific differences between both types of models, in particular with respect to the field geometry and the field strength, but we also compare scaling laws for the velocity amplitude, the magnetic dissipation time, and the convective heat flux. Our investigation is based on a systematic parameter study of spherical dynamo models in the anelastic approximation. We make use of a recently developed numerical solver and provide results for the test cases of the anelastic dynamo benchmark. The dichotomy of dipolar and multipolar dynamos identified in Boussinesq simulations is also present in our sample of anelastic models. Dipolar models require that the typical length scale of convection is an order of magnitude larger than the Rossby radius. However, the distinction between both classes of models is somewhat less explicit than in previous studies. This is mainly due to two reasons: we found a number of models with a considerable equatorial dipole contribution and an intermediate overall dipole field strength. Furthermore, a large density stratification may hamper the generation of dipole dominated magnetic fields. Previously proposed scaling laws, such as those for the field strength, are similarly applicable to anelastic models. It is not clear, however, if this consistency necessarily implies similar dynamo processes in both settings.Comment: 14 pages, 11 figure

Impacts of the Oyster Creek Nuclear Generating Station on the Zooplankton Community Structure of Barnegat Bay, New Jersey

Estuaries are biologically diverse and productive marine ecosystems, but many have been degraded as a result of anthropogenic activity, which can also negatively impact sensitive aquatic organisms like zooplankton. Zooplankton represent the crucial link between phytoplankton and higher trophic-level organisms. They are sensitive to environmental variation and increasing water temperature can cause dramatic shifts in zooplankton community structure. Climate change and coastal development favor species that are more tolerant of poor water quality. Barnegat Bay, New Jersey is an eutrophied estuarine lagoon in the Mid-Atlantic Ocean region. A major stress on Barnegat Bay was the Oyster Creek Nuclear Generating Station (OCNGS), which relied on water from the bay for cooling. Power plants entrain organisms in the cooling process and discharge waste heat as thermal pollution, which can negatively impact planktonic community structure. OCNGS operation began in 1969 and closed in September 2018. The objective of this research was to assess the zooplankton community structure of Barnegat Bay in the year prior to and the year following the closure of OCNGS to determine its impacts on coastal zooplankton communities. The results show site-specific increases in the abundance of the scyphozoan C. chesapeakei, the ctenophore M. leidyi, and several zooplankton taxa including calanoid copepods, Brachyura larvae, and Caridea larvae. There was also a significant increase in the abundance of fish eggs along with larval Atlantic Silverside and Bay Anchovy, two important estuarine fish species. Overall, the closure of OCNGS appears to have reduced a significant stress on numerous zooplankton species within the Barnegat Bay estuary, but longerterm studies are necessary to determine whether populations will recover or if permanent community shifts have occurred

Subcritical transition to turbulence triggered by a magnetic dynamo

It has recently been shown that a significant slowdown of many stars can be attributed to the emergence of a strong magnetic field within the radiative region, where heat is transferred through radiation in a stably stratified layer. Here, we describe how this transition can be understood as a subcritical bifurcation to small-scale turbulence in linearly stable flows. The turbulence is sustained by a nonlinear mean-field dynamo and can be observed down to relatively small differential rotation, arbitrarily far from the linear onset of any hydrodynamic instability. In this regime, turbulent fluctuations provide diffusivity-free transfer of angular momentum that increases the transport generated by the magnetic field triggering the turbulence. Finally, we present a simple nonlinear model that captures this scenario and can be used as a general description of the transition to turbulence in astrophysical flows, as long as it involves a competition between a large-scale dynamo, and a small-scale magnetic instability.Comment: 17 pages, 9 figures, accepted in Phys. Rev. Fluid

Tayler-Spruit dynamos in simulated radiative stellar layers

The Tayler-Spruit dynamo mechanism has been proposed two decades ago as a plausible mechanism to transport angular momentum in radiative stellar layers. Direct numerical simulations are still needed to understand its trigger conditions and the saturation mechanisms. The present study follows up on (Petitdemange et al. 2023), where we reported the first numerical simulations of a Tayler-Spruit dynamo cycle. Here we extend the explored parameter space to assess in particular the influence of stratification on the dynamo solutions. We also present numerical verification of theoretical assumptions made in (Spruit 2002), which are instrumental in deriving the classical prescription for angular momentum transport implemented in stellar evolution codes. A simplified radiative layer is modeled numerically by considering the dynamics of a stably-stratified, differentially rotating, magnetized fluid in a spherical shell. Our simulations display a diversity of magnetic field topologies and amplitudes depending on the flow parameters, including hemispherical solutions. The Tayler-Spruit dynamos reported here are found to satisfy magnetostrophic equilibrium and achieve efficient turbulent transport of angular momentum, following Spruit's heuristic prediction

Le transport non urgent des patients entre les hôpitaux de Montréal : un prototype pour l’automatisation du choix du transport et de l’accompagnement

RÉSUMÉ : Le transport externe des patients dans un établissement de santé est un centre de coût majeur dans la logistique des établissements de santé. En 2015, la loi 10 engendre la création des CIUSSS et des CISSS ( Centre Intégré (Universitaire) de Soins et Santé sociale) au Québec. De ce fait, les organisations de santé ont décidé d’utiliser ce contexte pour uniformiser leurs pratiques afin d’améliorer le niveau de service et de diminuer les coûts du transport externe des patients. Le transport externe des patients concerne les transferts interétablissements, les retours au domicile ainsi que les transports pour les examens des patients. Comme ce sont des transports entre établissements de santé, ils sont aux frais du système de santé. Aujourd’hui, chaque établissement a sa propre gestion du transport. Ceci engendre une diversité des pratiques ainsi qu’une absence d’uniformité dans le réseau de la santé. Par exemple, lorsqu’un patient a besoin d’être transporté (vers un autre établissement ou à son domicile), il faut choisir le mode de transport, déterminer s’il a besoin d’un accompagnement (et si oui, de quel type) puis effectuer la réservation. Il se trouve qu’il existe plusieurs options avec des coûts différents. Ainsi un transport non approprié peut engendrer un surcoût. Afin d’uniformiser les pratiques, nous avons cartographié l’état actuel du fonctionnement du transport de patient à travers 29 établissements, nous permettant ainsi de tracer plus de 70 processus de choix du transport et de sa réservation. Ces entrevues nous ont aussi permise de recenser les critères de choix du transport et de l’accompagnement. Avec cette connaissance du domaine, nous avons développé un prototype d’outil d’aide à la décision pour le choix du transport et de l’accompagnement des patients. Le choix du transport est automatisé grâce aux critères qui ont été récupérés, une base de données a été crée à l’aide de MySql, sa gestion et les interfaces qui permettent d’interagir avec les données sont générées avec Excel 2007. Des tests utilisateurs ont été menés sur cette interface pour s’assurer qu’elle répondait bien aux besoins des utilisateurs. L’outil se base sur un arbre de décision construit suite à plusieurs entrevues sur le terrain avec les professionnels de la santé. Une fois la demande de transport effectuée, un répartiteur décidera du transport à utiliser. Cet outil d’aide à la décision permet donc de choisir le transport, mais aussi d’éviter un surclassement dispendieux et non adapté au patient. Il permet aussi un meilleur accès aux données sur le transport dans un format standardisé. Suite aux tests effectués, nous avons constaté qu’une amélioration du choix du transport était possible pour 27% des véhicules choisis, ainsi il est possible de sauver des coûts en automatisant le choix du transport. Le transport externe des patients dans un établissement de santé est un centre de coût majeur dans la logistique des établissements de santé. En 2015, la loi 10 engendre la création des CIUSSS et des CISSS ( Centre Intégré (Universitaire) de Soins et Santé sociale) au Québec. De ce fait, les organisations de santé ont décidé d’utiliser ce contexte pour uniformiser leurs pratiques afin d’améliorer le niveau de service et de diminuer les coûts du transport externe des patients. Le transport externe des patients concerne les transferts interétablissements, les retours au domicile ainsi que les transports pour les examens des patients. Comme ce sont des transports entre établissements de santé, ils sont aux frais du système de santé. Aujourd’hui, chaque établissement a sa propre gestion du transport. Ceci engendre une diversité des pratiques ainsi qu’une absence d’uniformité dans le réseau de la santé. Par exemple, lorsqu’un patient a besoin d’être transporté (vers un autre établissement ou à son domicile), il faut choisir le mode de transport, déterminer s’il a besoin d’un accompagnement (et si oui, de quel type) puis effectuer la réservation. Il se trouve qu’il existe plusieurs options avec des coûts différents. Ainsi un transport non approprié peut engendrer un surcoût. Afin d’uniformiser les pratiques, nous avons cartographié l’état actuel du fonctionnement du transport de patient à travers 29 établissements, nous permettant ainsi de tracer plus de 70 processus de choix du transport et de sa réservation. Ces entrevues nous ont aussi permise de recenser les critères de choix du transport et de l’accompagnement. Avec cette connaissance du domaine, nous avons développé un prototype d’outil d’aide à la décision pour le choix du transport et de l’accompagnement des patients. Le choix du transport est automatisé grâce aux critères qui ont été récupérés, une base de données a été crée à l’aide de MySql, sa gestion et les interfaces qui permettent d’interagir avec les données sont générées avec Excel 2007. Des tests utilisateurs ont été menés sur cette interface pour s’assurer qu’elle répondait bien aux besoins des utilisateurs. L’outil se base sur un arbre de décision construit suite à plusieurs entrevues sur le terrain avec les professionnels de la santé. Une fois la demande de transport effectuée, un répartiteur décidera du transport à utiliser. Cet outil d’aide à la décision permet donc de choisir le transport, mais aussi d’éviter un surclassement dispendieux et non adapté au patient. Il permet aussi un meilleur accès aux données sur le transport dans un format standardisé. Suite aux tests effectués, nous avons constaté qu’une amélioration du choix du transport était possible pour 27% des véhicules choisis, ainsi il est possible de sauver des coûts en automatisant le choix du transport.----------ABSTRACT : Patient interhospital transportation is a major portion of the budget of logistics in health care facilities in Quebec. Since April 1st 2015, the facilities of Montreal are merged into CIUSSSs (centre intégré universitaire de santé et services sociaux) which include several health care institutions (hospital centers, clinics, day centers, nursing homes. . . ). Therefore, these organizations have decided to use this context to uniformize their decision process regarding the booking and the choice of patient interhospital transportation. Currently, these differ from place to place. For example, when a patient needs to be transported (to another institution or to his/her home), it is necessary to choose the mode of transport, to determine whether it needs an accompaniment (and if so of what type), and then make the reservation. It happens that there are several options with different costs. Thus, inappropriate transport can lead to an additional cost. Optimizing the transportation system itself will lead to a better balance between the quality of the transfers and their cost. To evaluate the potential gain of a standardized process for the transportation of non-urgent patients, we used in this research the transportation problem of non-urgent patient at Montreal, Quebec, Canada. On the first phase of the project, we mapped the current state. The analysis was based on observations and interviews conducted with health professionals and clerks from 29 facilities which belong to five CIUSSSs (Centre intégré universitaire de santé et de services sociaux) of the Island of Montreal. We were able to retrieve more than 70 processes. None the less, the transportation process of non-urgent patients can be divided on two stages (1) Type of companionship (2) type of vehicle. On the second phase of our project, we conducted an in-depth analysis in order to identify the key criteria in choosing the best type of companionship and the most appropriate type of vehicle for the patient. On the third phase of the project, we developed a prototype of a support decision tool for the choice of transport and patient’s accompaniment based a decision tree created from the 17 criteria determined during phase two. The tool was developed using a database coded on textit MySql. Its management and the interfaces that allow to interact with the data are generated with textit Excel 2007. This choice is justified among other things because of the ease of the MySql language and because the excel tool is favored in medical establishment for its ease of use and low cost. On the final phase of the project, tests were conducted on the interface to ensure that it meets the needs of the final users and the final beneficiary, the patient. As a result of the tests carried out, we were able to determine the practicality of a standardized process. In fact, in 27% of the cases we were able to demonstrate an improvement in the choice of transport which translate in a direct costs saving. Moreover, this decision-making tool not only guides the choice the transport but avoids an expensive upgrade not adapted for the patient, and provides better access to transportation data in a standardized format which can be used in future researches

Caroline et l’apprentissage de l’allemand L2 dans une école maternelle bilingue

We present a longitudinal observation of a 4 and-a-half-year-old pupil. She is in the middle section of a kindergarten with a partial immersion curriculum. We will discover the strategies Caroline sets up in order to communicate, express her desires and intentions. To achieve the speech acts, different types of code switching are used in alternation. Her very subtle use of her L1 (French) and L2 (German) shows a particularly high level of linguistic awareness on her part.Presentiamo qui l’osservazione longitudinale di un’alunna di 4 anni e mezzo. È al secondo anno di scuola materna in una didattica curricolare ad immersione parziale. Vedremo quali strategie di comunicazione vengono messe in atto dal bambino al fine di comunicare, di esprimere i propri desideri e le proprie intenzioni rispetto all’azione. Per effettuare i suoi atti di parola la bambina mette in opera diversi tipi di code switching e impiega in modo molto sottile sia la sua prima lingua (il francese), sia la L2 (tedesco), dando così prova di un livello di coscienza linguistica particolarmente alta