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Following the survey Well-being in astrophysics that was sent out in March 2021, to establish how astrophysics researchers, primarily in France, experience their career, some of the results were published in Webb et al. (2021). Here we further analyse the data to determine if gender can cause different experiences in astrophysics. We also study the impact on the well-being of temporary staff (primarily PhD students and postdocs), compared to permanent staff. Whilst more temporary staff stated that they felt permanently overwhelmed than permanent staff, the experiences in astrophysics for the different genders were in general very similar, except in one area. More than three times more females than males experienced harassment or discrimination, rising sharply for gender discrimination and sexual harassment, where all of those having experienced sexual harassment and who had provided their gender in the survey, were female. Further, as previously reported (Webb et al. 2021), 20% of the respondents had suffered mental health issues before starting their career in astrophysics. We found that whilst this group was split approximately equally with regards to males and females, the number rose sharply to almost 45% of astronomers experiencing mental health issues since starting in astrophysics. Of this population, there were 50% more females than males. This excess of females was almost entirely made up of the population of women that had been harassed or discriminated against., Comment: 5 pages, 1 figure, published in the SF2A-2022: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics. Eds.: J. Richard, A. Siebert, E. Lagadec, N. Lagarde, O. Venot, J. Malzac, J.-B. Marquette, M. N'Diaye, D. Briot, pp.171-175

We consider a conventional $\alpha-\Omega$-dynamo model with meridional circulation that exhibits typical features of the solar dynamo, including a Hale cycle period of around 20 years and a reasonable shape of the butterfly diagram. With regard to recent ideas of a tidal synchronization of the solar cycle, we complement this model by an additional time-periodic $\alpha$-term that is localized in the tachocline region. It is shown that amplitudes of some dm/s are sufficient for this $\alpha$-term to become capable of entraining the underlying dynamo. We argue that such amplitudes of $\alpha$ may indeed be realistic, since velocities in the range of m/s are reachable, e.g., for tidally excited magneto-Rossby waves., Comment: 14 pages, 6 figures

It has become clear that early career astrophysics researchers (doctoral researchers, post-docs, etc) have a very diverse appreciation of their career, with some declaring it the best job that you can have and others suffering from overwork, harrassment and stress from the precarity of their job, and associated difficulties. In order to establish how astrophysics researchers, primarily in France, experience their career, we sent out a survey to understand the impact that their job has on their well-being. 276 people responded to the survey. Whilst around half of the respondents expressed pleasure derived from their career, it is clear that many (early career) researchers are suffering due to overwork, with more than a quarter saying that they work in excess of 50 hours per week and 2\% in excess of 90 h per week. Almost 30\% professed to having suffered harrassment or discrimination in the course of their work. Further, whilst only 20\% had suffered mental health issues before starting their career in astrophysics, $\sim$45\% said that they suffered with mental health problems since starting in astrophysics. Here we provide results from the survey as well as possible avenues to explore and a list of recommendations to improve (early) careers in astrophysics., Comment: 6 page conference proceedings on well-being in French astrophysics

Some contracting or expanding stars are thought to host a large-scale magnetic field in their radiative interior. By interacting with the contraction-induced flows, such fields may significantly alter the rotational history of the star. They thus constitute a promising way to address the problem of angular momentum transport during the rapid phases of stellar evolution. In this work, we aim at studying the interplay between flows and magnetic fields in a contracting radiative zone. We propose a scenario that may account for the post-main sequence evolution of solar-like stars, in which a quasi-solid rotation can be maintained by a large-scale magnetic field during a contraction timescale. Then, an axisymmetric instability would destroy this large-scale structure and enables the differential rotation to set in. Such a contraction driven instability could also be at the origin of the observed dichotomy between strongly and weakly magnetic intermediate-mass stars., Comment: 31 pages, 29 figures, 3 tables, accepted for publication in Astronomy & Astrophysics

Observations of surface magnetic fields of cool stars reveal a large diversity of configurations. Although there is now a consensus that these fields are generated through dynamo processes occurring within the convective zone, the physical mechanism driving such a variety of field topologies is still debated. This paper discusses the possible origins of dipole and multipole-dominated morphologies using three-dimensional numerical simulations of stratified systems where the magnetic feedback on the fluid motion is significant. Our main result is that dipolar solutions are found at Rossby numbers up to 0.4 in strongly stratified simulations, where previous works suggested that only multipolar fields should exist. We argue that these simulations are reminiscent of the outlier stars observed at Rossby numbers larger than 0.1, whose large-scale magnetic field is dominated by their axisymmetric poloidal component. As suggested in previous Boussinesq calculations, the relative importance of inertial over Lorentz forces is again controlling the dipolar to multipolar transition. Alternatively, we find that the ratio of kinetic to magnetic energies can equally well capture the transition in the field morphology. We test the ability of this new proxy to predict the magnetic morphology of a few M-dwarf stars whose internal structure matches that of our simulations and for which homogeneous magnetic field characterization is available. Finally, the magnitude of the differential rotation obtained in our simulations is compared to actual measurements reported in the literature for M-dwarfs. In our simulations, we find a clear relationship between anti-solar differential rotation and the emergence of dipolar fields., Comment: 15 pages, 15 figures, accepted for publication in MNRAS

It has become clear that early career astrophysics researchers (doctoral researchers, post-docs, etc) have a very diverse appreciation of their career, with some declaring it the best job that you can have and others suffering from overwork, harrassment and stress from the precarity of their job, and associated difficulties. In order to establish how astrophysics researchers, primarily in France, experience their career, we sent out a survey to understand the impact that their job has on their well-being. 276 people responded to the survey. Whilst around half of the respondents expressed pleasure derived from their career, it is clear that many (early career) researchers are suffering due to overwork, with more than a quarter saying that they work in excess of 50 hours per week and 2\% in excess of 90 h per week. Almost 30\% professed to having suffered harrassment or discrimination in the course of their work. Further, whilst only 20\% had suffered mental health issues before starting their career in astrophysics, $\sim$45\% said that they suffered with mental health problems since starting in astrophysics. Here we provide results from the survey as well as possible avenues to explore and a list of recommendations to improve (early) careers in astrophysics., Comment: 6 page conference proceedings on well-being in French astrophysics

Some contracting or expanding stars are thought to host a large-scale magnetic field in their radiative interior. By interacting with the contraction-induced flows, such fields may significantly alter the rotational history of the star. They thus constitute a promising way to address the problem of angular momentum transport during the rapid phases of stellar evolution. In this work, we aim at studying the interplay between flows and magnetic fields in a contracting radiative zone. We propose a scenario that may account for the post-main sequence evolution of solar-like stars, in which a quasi-solid rotation can be maintained by a large-scale magnetic field during a contraction timescale. Then, an axisymmetric instability would destroy this large-scale structure and enables the differential rotation to set in. Such a contraction driven instability could also be at the origin of the observed dichotomy between strongly and weakly magnetic intermediate-mass stars., Comment: 31 pages, 29 figures, 3 tables, accepted for publication in Astronomy & Astrophysics

The present study aims at studying the flow and field produced by a stellar radiative zone which is initially made to rotate differentially in the presence of a large-scale poloidal magnetic field threading the whole domain. We focus both on the axisymmetric configurations produced by the initial winding-up of the magnetic field lines and on the possible instabilities of those configurations. We aim in particular at assessing the role of the stratification at stabilising the system. We perform 2D and 3D global Boussinesq numerical simulations started from an initial radial or cylindrical differential rotation and a large-scale poloidal magnetic field. Under the conditions of a large rotation frequency compared to the Alfv\'en frequency, a magnetic configuration strongly dominated by its toroidal component is built. The parameters of the simulations are chosen to respect the ordering of time scales of a typical stellar radiative zone. In this framework, the axisymmetric evolution is studied by varying the relative effects of the thermal diffusion, the Br\"unt-V\"ais\"al\"a frequency, the rotation and the initial poloidal field strength. We find that the axisymmetric state only depends on $t_{es}/t_{Ap}$, the ratio between the Eddington-Sweet circulation time scale and the Alfv\'en time scale. A scale analysis of the Boussinesq equations allows us to recover this result. In the cylindrical case, a magneto-rotational instability develops when the thermal diffusivity is sufficiently high to enable the favored wavenumbers to be insensitive to the effects of the stable stratification. In the radial case, the magneto-rotational instability is driven by the latitudinal shear created by the back-reaction of the Lorentz force on the flow. Increasing the level of stratification then leaves the growth rate of the instability mainly unaffected while its horizontal length scale grows., Comment: 20 pages, 13 figures, accepted for publication in A&A

Context. Stars experience rapid contraction or expansion at different phases of their evolution. Modelling the angular momentum and chemical elements transport occurring during these phases remains an unsolved problem. Aims. We study a stellar radiative zone undergoing radial contraction and investigate the induced differential rotation and meridional circulation. Methods. We consider a rotating spherical layer crossed by an imposed radial velocity field that mimics the contraction and solve numerically the axisymmetric hydrodynamical equations in both the Boussinesq and anelastic approximations. An extensive parametric study is conducted to cover regimes of contraction, rotation, stable stratification and density stratification that are relevant for stars. Results. The differential rotation and the meridional circulation result from a competition between the contraction-driven inward transport of angular momentum and an outward transport dominated by either viscosity or an Eddington-Sweet type circulation, depending on the value of the $P_r \left( N_0 / \Omega_0 \right)^2 $ parameter, where $P_r$ is the Prandtl number, $N_0$ the Brunt-V\"ais\"ail\"a frequency and $\Omega_0$ the rotation rate. Taking the density stratification into account is important to study more realistic radial contraction fields but also because the resulting flow is less affected by unwanted effects of the boundary conditions. In these different regimes and for a weak differential rotation, we derive scaling laws that relate the amplitude of the differential rotation to the contraction timescale., Comment: 24 pages, 16 figures, 8 tables

The present study aims at studying the flow and field produced by a stellar radiative zone which is initially made to rotate differentially in the presence of a large-scale poloidal magnetic field threading the whole domain. We focus both on the axisymmetric configurations produced by the initial winding-up of the magnetic field lines and on the possible instabilities of those configurations. We aim in particular at assessing the role of the stratification at stabilising the system. We perform 2D and 3D global Boussinesq numerical simulations started from an initial radial or cylindrical differential rotation and a large-scale poloidal magnetic field. Under the conditions of a large rotation frequency compared to the Alfv\'en frequency, a magnetic configuration strongly dominated by its toroidal component is built. The parameters of the simulations are chosen to respect the ordering of time scales of a typical stellar radiative zone. In this framework, the axisymmetric evolution is studied by varying the relative effects of the thermal diffusion, the Br\"unt-V\"ais\"al\"a frequency, the rotation and the initial poloidal field strength. We find that the axisymmetric state only depends on $t_{es}/t_{Ap}$, the ratio between the Eddington-Sweet circulation time scale and the Alfv\'en time scale. A scale analysis of the Boussinesq equations allows us to recover this result. In the cylindrical case, a magneto-rotational instability develops when the thermal diffusivity is sufficiently high to enable the favored wavenumbers to be insensitive to the effects of the stable stratification. In the radial case, the magneto-rotational instability is driven by the latitudinal shear created by the back-reaction of the Lorentz force on the flow. Increasing the level of stratification then leaves the growth rate of the instability mainly unaffected while its horizontal length scale grows., Comment: 20 pages, 13 figures, accepted for publication in A&A

Magnetohydrodynamic interactions between plasma flows and magnetic fields is fundamental to the origin and sustenance of the 11-year sunspot cycle. These processes are intrinsically three-dimensional (3D) in nature. Our goal is to construct a 3D solar dynamo model that on the one hand captures the buoyant emergence of tilted bipolar sunspot pairs, and on the other hand produces cyclic large-scale field reversals mediated via surface flux-transport processes -- that is, the Babcock-Leighton mechanism. We perform kinematic dynamo simulations where the prescribed velocity field is a combination of solar-like differential rotation and meridional circulation, along with a parametrized turbulent diffusivity. We use a novel methodology for modeling magnetic buoyancy through field-strength-dependent 3D helical up-flows that results in the formation of tilted bipolar sunspots. The bipolar spots produced in our simulations participate in the process of poloidal-field generation through the Babcock-Leighton mechanism, resulting in self-sustained and periodic large-scale magnetic field reversal. Our parameter space study varying the amplitude of the meridional flow, the convection zone diffusivity, and parameters governing the efficiency of the magnetic buoyancy mechanism reveal their relative roles in determining properties of the sunspot cycle such as amplitude, period, and dynamical memory relevant to solar cycle prediction. We also derive a new dynamo number for the Babcock-Leighton solar dynamo mechanism which reasonably captures our model dynamics. This study elucidates the relative roles of different flux-transport processes in the Sun's convection zone in determining the properties and physics of the sunspot cycle and could potentially lead to realistic, data-driven 3D dynamo models for solar-activity predictions and exploration of stellar magnetism and starspot formation in other stars., Comment: Accepted for publication in A&A

Magnetohydrodynamic interactions between plasma flows and magnetic fields is fundamental to the origin and sustenance of the 11-year sunspot cycle. These processes are intrinsically three-dimensional (3D) in nature. Our goal is to construct a 3D solar dynamo model that on the one hand captures the buoyant emergence of tilted bipolar sunspot pairs, and on the other hand produces cyclic large-scale field reversals mediated via surface flux-transport processes -- that is, the Babcock-Leighton mechanism. We perform kinematic dynamo simulations where the prescribed velocity field is a combination of solar-like differential rotation and meridional circulation, along with a parametrized turbulent diffusivity. We use a novel methodology for modeling magnetic buoyancy through field-strength-dependent 3D helical up-flows that results in the formation of tilted bipolar sunspots. The bipolar spots produced in our simulations participate in the process of poloidal-field generation through the Babcock-Leighton mechanism, resulting in self-sustained and periodic large-scale magnetic field reversal. Our parameter space study varying the amplitude of the meridional flow, the convection zone diffusivity, and parameters governing the efficiency of the magnetic buoyancy mechanism reveal their relative roles in determining properties of the sunspot cycle such as amplitude, period, and dynamical memory relevant to solar cycle prediction. We also derive a new dynamo number for the Babcock-Leighton solar dynamo mechanism which reasonably captures our model dynamics. This study elucidates the relative roles of different flux-transport processes in the Sun's convection zone in determining the properties and physics of the sunspot cycle and could potentially lead to realistic, data-driven 3D dynamo models for solar-activity predictions and exploration of stellar magnetism and starspot formation in other stars., Comment: Accepted for publication in A&A

This study aims at investigating the ability of strong interactions between magnetic field concentrations during their rise through the convection zone to produce complex active regions at the solar surface. To do so, we perform numerical simulations of buoyant magnetic structures evolving and interacting in a model solar convection zone. We first produce a 3D model of rotating convection and then introduce idealized magnetic structures close to the bottom of the computational domain. These structures possess a certain degree of field line twist and they are made buoyant on a particular extension in longitude. The resulting twisted $\Omega$-loops will thus evolve inside a spherical convective shell possessing large-scale mean flows. We present results on the interaction between two such loops with various initial parameters (mainly buoyancy and twist) and on the complexity of the emerging magnetic field. In agreement with analytical predictions, we find that if the loops are introduced with opposite handedness and same axial field direction or same handedness but opposite axial field, they bounce against each other. The emerging region is then constituted of two separated bipolar structures. On the contrary, if the loops are introduced with the same direction of axial and peripheral magnetic fields and if sufficiently close, they merge while rising. This more interesting case produces complex magnetic structures, with a high degree of non-neutralized currents, especially when the convective motions act significantly on the magnetic field. This indicates that those interactions could be good candidates to produce eruptive events like flares or CMEs., Comment: 39 pages, 20 figures, accepted for publication in ApJ

This study aims at investigating the ability of strong interactions between magnetic field concentrations during their rise through the convection zone to produce complex active regions at the solar surface. To do so, we perform numerical simulations of buoyant magnetic structures evolving and interacting in a model solar convection zone. We first produce a 3D model of rotating convection and then introduce idealized magnetic structures close to the bottom of the computational domain. These structures possess a certain degree of field line twist and they are made buoyant on a particular extension in longitude. The resulting twisted $\Omega$-loops will thus evolve inside a spherical convective shell possessing large-scale mean flows. We present results on the interaction between two such loops with various initial parameters (mainly buoyancy and twist) and on the complexity of the emerging magnetic field. In agreement with analytical predictions, we find that if the loops are introduced with opposite handedness and same axial field direction or same handedness but opposite axial field, they bounce against each other. The emerging region is then constituted of two separated bipolar structures. On the contrary, if the loops are introduced with the same direction of axial and peripheral magnetic fields and if sufficiently close, they merge while rising. This more interesting case produces complex magnetic structures, with a high degree of non-neutralized currents, especially when the convective motions act significantly on the magnetic field. This indicates that those interactions could be good candidates to produce eruptive events like flares or CMEs., Comment: 39 pages, 20 figures, accepted for publication in ApJ

An extremely weak circularly polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A. A weak surface magnetic field was proposed to account for the observed polarized signal, but the shape of the phase-averaged signature, dominated by a prominent positive lobe, is not expected in the standard theory of the Zeeman effect. We aim at verifying the presence of weak circularly polarized signatures in two other bright Am stars, beta UMa and theta Leo, and investigating the physical origin of Sirius-like polarized signals further. We present here a set of deep spectropolarimetric observations of beta UMa and theta Leo, observed with the NARVAL spectropolarimeter. We analyzed all spectra with the Least Squares Deconvolution multiline procedure. To improve the signal-to-noise ratio and detect extremely weak signatures in Stokes V profiles, we co-added all available spectra of each star (around 150 observations each time). Finally, we ran several tests to evaluate whether the detected signatures are consistent with the behavior expected from the Zeeman effect. The line profiles of the two stars display circularly polarized signatures similar in shape and amplitude to the observations previously gathered for Sirius A. Our series of tests brings further evidence of a magnetic origin of the recorded signal. These new detections suggest that very weak magnetic fields may well be present in the photospheres of a significant fraction of intermediate-mass stars. The strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant single-sign lobe) are not expected in the standard theory of the Zeeman effect and may be linked to sharp vertical gradients in photospheric velocities and magnetic field strengths.

An extremely weak circularly polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A. A weak surface magnetic field was proposed to account for the observed polarized signal, but the shape of the phase-averaged signature, dominated by a prominent positive lobe, is not expected in the standard theory of the Zeeman effect. We aim at verifying the presence of weak circularly polarized signatures in two other bright Am stars, beta UMa and theta Leo, and investigating the physical origin of Sirius-like polarized signals further. We present here a set of deep spectropolarimetric observations of beta UMa and theta Leo, observed with the NARVAL spectropolarimeter. We analyzed all spectra with the Least Squares Deconvolution multiline procedure. To improve the signal-to-noise ratio and detect extremely weak signatures in Stokes V profiles, we co-added all available spectra of each star (around 150 observations each time). Finally, we ran several tests to evaluate whether the detected signatures are consistent with the behavior expected from the Zeeman effect. The line profiles of the two stars display circularly polarized signatures similar in shape and amplitude to the observations previously gathered for Sirius A. Our series of tests brings further evidence of a magnetic origin of the recorded signal. These new detections suggest that very weak magnetic fields may well be present in the photospheres of a significant fraction of intermediate-mass stars. The strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant single-sign lobe) are not expected in the standard theory of the Zeeman effect and may be linked to sharp vertical gradients in photospheric velocities and magnetic field strengths.

We study the magnetic flux carried by pores located outside active regions with sunspots and investigate their possible contribution to the reversal of the global magnetic field of the Sun. We find that they contain a total flux of comparable amplitude to the total magnetic flux contained in polar caps. The pores located at distances of 40--100~Mm from the closest active region have systematically the correct sign to contribute to the polar cap reversal. These pores can predominantly be found in bipolar magnetic regions. We propose that during grand minima of solar activity, such a systematic polarity trend, akin to a weak magnetic (Babcock-Leighton-like) source term could still be operating but was missed by the contemporary observers due to the limited resolving power of their telescopes., Comment: 11 pages, 9 figures, accepted for publication in Astronomy&Astrophysics

We study the magnetic flux carried by pores located outside active regions with sunspots and investigate their possible contribution to the reversal of the global magnetic field of the Sun. We find that they contain a total flux of comparable amplitude to the total magnetic flux contained in polar caps. The pores located at distances of 40--100~Mm from the closest active region have systematically the correct sign to contribute to the polar cap reversal. These pores can predominantly be found in bipolar magnetic regions. We propose that during grand minima of solar activity, such a systematic polarity trend, akin to a weak magnetic (Babcock-Leighton-like) source term could still be operating but was missed by the contemporary observers due to the limited resolving power of their telescopes., Comment: 11 pages, 9 figures, accepted for publication in Astronomy&Astrophysics

We present three-dimensional numerical simulations of a magnetic loop evolving in either a convectively stable or unstable rotating shell. The magnetic loop is introduced in the shell in such a way that it is buoyant only in a certain portion in longitude, thus creating an \Omega-loop. Due to the action of magnetic buoyancy, the loop rises and develops asymmetries between its leading and following legs, creating emerging bipolar regions whose characteristics are similar to the ones of observed spots at the solar surface. In particular, we self-consistently reproduce the creation of tongues around the spot polarities, which can be strongly affected by convection. We moreover emphasize the presence of ring-shaped magnetic structures around our simulated emerging regions, which we call "magnetic necklace" and which were seen in a number of observations without being reported as of today. We show that those necklaces are markers of vorticity generation at the periphery and below the rising magnetic loop. We also find that the asymmetry between the two legs of the loop is crucially dependent on the initial magnetic field strength. The tilt angle of the emerging regions is also studied in the stable and unstable cases and seems to be affected both by the convective motions and the presence of a differential rotation in the convective cases., Comment: 23 pages (ApJ 2-column format), 19 figures, accepted for publication in ApJ

We here report calculations of magnetic buoyancy instabilities of a sheared magnetic layer where two separate zones are unstable. The idea is to study the possible generation of large-scale helical structures which could then rise through a stellar convection zone and emerge at the surface to create active regions. The calculations shown here are a follow-up of the work of Favier et al. (2012) where the instability developed in a weakly magnetized atmosphere, consisting of a uniform field oriented in a different direction from the unstable layer below. Here, the top layer representing the atmosphere is itself unstable to buoyancy instabilities and thus quickly creates a more complex magnetic configuration with which the layer below will interact. We also find in this case that the accumulation of magnetic tension between the two unstable layers favors the creation of large-scale helical structures., Comment: French Astrophysics Society Conference Proceedings, 4 pages, 4 figures

We consider the three-dimensional instability of a layer of horizontal magnetic field in a polytropic atmosphere where, contrary to previous studies, the field lines in the initial state are not unidirectional. We show that if the twist is initially concentrated inside the unstable layer, the modifications of the instability reported by several authors (see e.g. Cattaneo et al. (1990)) are only observed when the calculation is restricted to two dimensions. In three dimensions, the usual interchange instability occurs, in the direction fixed by the field lines at the interface between the layer and the field-free region. We therefore introduce a new configuration: the instability now develops in a weakly magnetised atmosphere where the direction of the field can vary with respect to the direction of the strong unstable field below, the twist being now concentrated at the upper interface. Both linear stability analysis and non-linear direct numerical simulations are used to study this configuration. We show that from the small-scale interchange instability, large-scale twisted coherent magnetic structures are spontaneously formed, with possible implications to the formation of active regions from a deep-seated solar magnetic field., Comment: 11 pages, 10 figures

We here report calculations of magnetic buoyancy instabilities of a sheared magnetic layer where two separate zones are unstable. The idea is to study the possible generation of large-scale helical structures which could then rise through a stellar convection zone and emerge at the surface to create active regions. The calculations shown here are a follow-up of the work of Favier et al. (2012) where the instability developed in a weakly magnetized atmosphere, consisting of a uniform field oriented in a different direction from the unstable layer below. Here, the top layer representing the atmosphere is itself unstable to buoyancy instabilities and thus quickly creates a more complex magnetic configuration with which the layer below will interact. We also find in this case that the accumulation of magnetic tension between the two unstable layers favors the creation of large-scale helical structures., Comment: French Astrophysics Society Conference Proceedings, 4 pages, 4 figures

We consider the three-dimensional instability of a layer of horizontal magnetic field in a polytropic atmosphere where, contrary to previous studies, the field lines in the initial state are not unidirectional. We show that if the twist is initially concentrated inside the unstable layer, the modifications of the instability reported by several authors (see e.g. Cattaneo et al. (1990)) are only observed when the calculation is restricted to two dimensions. In three dimensions, the usual interchange instability occurs, in the direction fixed by the field lines at the interface between the layer and the field-free region. We therefore introduce a new configuration: the instability now develops in a weakly magnetised atmosphere where the direction of the field can vary with respect to the direction of the strong unstable field below, the twist being now concentrated at the upper interface. Both linear stability analysis and non-linear direct numerical simulations are used to study this configuration. We show that from the small-scale interchange instability, large-scale twisted coherent magnetic structures are spontaneously formed, with possible implications to the formation of active regions from a deep-seated solar magnetic field., Comment: 11 pages, 10 figures

We present three-dimensional numerical simulations of a magnetic loop evolving in either a convectively stable or unstable rotating shell. The magnetic loop is introduced in the shell in such a way that it is buoyant only in a certain portion in longitude, thus creating an \Omega-loop. Due to the action of magnetic buoyancy, the loop rises and develops asymmetries between its leading and following legs, creating emerging bipolar regions whose characteristics are similar to the ones of observed spots at the solar surface. In particular, we self-consistently reproduce the creation of tongues around the spot polarities, which can be strongly affected by convection. We moreover emphasize the presence of ring-shaped magnetic structures around our simulated emerging regions, which we call "magnetic necklace" and which were seen in a number of observations without being reported as of today. We show that those necklaces are markers of vorticity generation at the periphery and below the rising magnetic loop. We also find that the asymmetry between the two legs of the loop is crucially dependent on the initial magnetic field strength. The tilt angle of the emerging regions is also studied in the stable and unstable cases and seems to be affected both by the convective motions and the presence of a differential rotation in the convective cases., Comment: 23 pages (ApJ 2-column format), 19 figures, accepted for publication in ApJ

We aim at a highly sensitive search for weak magnetic fields in main sequence stars of intermediate mass, by scanning classes of stars with no previously reported magnetic members. After detecting a weak magnetic field on the normal, rapidly rotating A-type star Vega, we concentrate here on the bright star Sirius A, taken as a prototypical, chemically peculiar, moderately rotating Am star. We employed the NARVAL and ESPaDOnS high-resolution spectropolarimeters to collect 442 circularly polarized spectra, complemented by 60 linearly polarized spectra. Using a list of about 1,100 photospheric spectral lines, we computed a cross correlation line profile from every spectrum, leading to a signal-to-noise ratio of up to 30,000 in the polarized profile. We report the repeated detection of circularly polarized, highly asymmetric signatures in the line profiles, interpreted as Zeeman signatures of a large-scale photospheric magnetic field, with a line-of-sight component equal to $0.2 \pm 0.1$ G. This is the first polarimetric detection of a surface magnetic field on an Am star. Using rough estimates of the physical properties of the upper layers of Sirius A, we suggest that a dynamo operating in the shallow convective envelope cannot account for the field strength reported here. Together with the magnetic field of Vega, this result confirms that a new class of magnetic objects exists among non Ap/Bp intermediate-mass stars, and it may indicate that a significant fraction of tepid stars are magnetic., Comment: Accepted by Astronomy and Astrophysics

We aim at a highly sensitive search for weak magnetic fields in main sequence stars of intermediate mass, by scanning classes of stars with no previously reported magnetic members. After detecting a weak magnetic field on the normal, rapidly rotating A-type star Vega, we concentrate here on the bright star Sirius A, taken as a prototypical, chemically peculiar, moderately rotating Am star. We employed the NARVAL and ESPaDOnS high-resolution spectropolarimeters to collect 442 circularly polarized spectra, complemented by 60 linearly polarized spectra. Using a list of about 1,100 photospheric spectral lines, we computed a cross correlation line profile from every spectrum, leading to a signal-to-noise ratio of up to 30,000 in the polarized profile. We report the repeated detection of circularly polarized, highly asymmetric signatures in the line profiles, interpreted as Zeeman signatures of a large-scale photospheric magnetic field, with a line-of-sight component equal to $0.2 \pm 0.1$ G. This is the first polarimetric detection of a surface magnetic field on an Am star. Using rough estimates of the physical properties of the upper layers of Sirius A, we suggest that a dynamo operating in the shallow convective envelope cannot account for the field strength reported here. Together with the magnetic field of Vega, this result confirms that a new class of magnetic objects exists among non Ap/Bp intermediate-mass stars, and it may indicate that a significant fraction of tepid stars are magnetic., Comment: Accepted by Astronomy and Astrophysics

The Sun is a magnetic star whose cyclic activity is thought to be linked to internal dynamo mechanisms. A combination of numerical modelling with various levels of complexity is an efficient and accurate tool to investigate such intricate dynamical processes. We investigate the role of the magnetic buoyancy process in 2D Babcock-Leighton dynamo models, by modelling more accurately the surface source term for poloidal field. Methods. To do so, we reintroduce in mean-field models the results of full 3D MHD calculations of the non-linear evolution of a rising flux tube in a convective shell. More specifically, the Babcock-Leighton source term is modified to take into account the delay introduced by the rise time of the toroidal structures from the base of the convection zone to the solar surface. We find that the time delays introduced in the equations produce large temporal modulation of the cycle amplitude even when strong and thus rapidly rising flux tubes are considered. Aperiodic modulations of the solar cycle appear after a sequence of period doubling bifurcations typical of non-linear systems. The strong effects introduced even by small delays is found to be due to the dependence of the delays on the magnetic field strength at the base of the convection zone, the modulation being much less when time delays remain constant. We do not find any significant influence on the cycle period except when the delays are made artificially strong. A possible new origin of the solar cycle variability is here revealed. This modulated activity and the resulting butterfly diagram are then more compatible with observations than what the standard Babcock-Leighton model produces., Comment: 13 pages, 10 figures, accepted for publication in A&A

The Sun is a magnetic star whose cyclic activity is thought to be linked to internal dynamo mechanisms. A combination of numerical modelling with various levels of complexity is an efficient and accurate tool to investigate such intricate dynamical processes. We investigate the role of the magnetic buoyancy process in 2D Babcock-Leighton dynamo models, by modelling more accurately the surface source term for poloidal field. Methods. To do so, we reintroduce in mean-field models the results of full 3D MHD calculations of the non-linear evolution of a rising flux tube in a convective shell. More specifically, the Babcock-Leighton source term is modified to take into account the delay introduced by the rise time of the toroidal structures from the base of the convection zone to the solar surface. We find that the time delays introduced in the equations produce large temporal modulation of the cycle amplitude even when strong and thus rapidly rising flux tubes are considered. Aperiodic modulations of the solar cycle appear after a sequence of period doubling bifurcations typical of non-linear systems. The strong effects introduced even by small delays is found to be due to the dependence of the delays on the magnetic field strength at the base of the convection zone, the modulation being much less when time delays remain constant. We do not find any significant influence on the cycle period except when the delays are made artificially strong. A possible new origin of the solar cycle variability is here revealed. This modulated activity and the resulting butterfly diagram are then more compatible with observations than what the standard Babcock-Leighton model produces., Comment: 13 pages, 10 figures, accepted for publication in A&A

Aims: To understand stellar magnetism and to test the validity of the Babcock-Leighton flux transport mean field dynamo models with stellar activity observations Methods: 2-D mean field dynamo models at various rotation rates are computed with the STELEM code to study the sensitivity of the activity cycle period and butterfly diagram to parameter changes and are compared to observational data. The novelty is that these 2-D mean field dynamo models incorporate scaling laws deduced from 3-D hydrodynamical simulations for the influence of rotation rate on the amplitude and profile of the meridional circulation. These models make also use of observational scaling laws for the variation of differential rotation with rotation rate. Results: We find that Babcock-Leighton flux transport dynamo models are able to reproduce the change in topology of the magnetic field (i.e. toward being more toroidal with increasing rotation rate) but seem to have difficulty reproducing the cycle period vs activity period correlation observed in solar-like stars if a monolithic single cell meridional flow is assumed. It may however be possible to recover the $P_{cyc}$ vs $P_{rot}$ relation with more complex meridional flows, if the profile changes in a particular assumed manner with rotation rate. Conclusions: The Babcock-Leighton flux transport dynamo model based on single cell meridional circulation does not reproduce the $P_{cyc}$ vs $P_{rot}$ relation unless the amplitude of the meridional circulation is assumed to increase with rotation rate which seems to be in contradiction with recent results obtained with 3-D global simulations., Comment: 12 pages, 8 figures, accepted for publication by A&A 1: AIM, CEA/DSM-CNRS-Univ. Paris 7, IRFU/SAp, France, 2: D.A.M.T.P., Centre for Mathematical Sciences, Univ. of Cambridge, UK, 3: JILA and Department of Astrophysical and Planetary Sciences, Univ. of Colorado, USA

Aims: To understand stellar magnetism and to test the validity of the Babcock-Leighton flux transport mean field dynamo models with stellar activity observations Methods: 2-D mean field dynamo models at various rotation rates are computed with the STELEM code to study the sensitivity of the activity cycle period and butterfly diagram to parameter changes and are compared to observational data. The novelty is that these 2-D mean field dynamo models incorporate scaling laws deduced from 3-D hydrodynamical simulations for the influence of rotation rate on the amplitude and profile of the meridional circulation. These models make also use of observational scaling laws for the variation of differential rotation with rotation rate. Results: We find that Babcock-Leighton flux transport dynamo models are able to reproduce the change in topology of the magnetic field (i.e. toward being more toroidal with increasing rotation rate) but seem to have difficulty reproducing the cycle period vs activity period correlation observed in solar-like stars if a monolithic single cell meridional flow is assumed. It may however be possible to recover the $P_{cyc}$ vs $P_{rot}$ relation with more complex meridional flows, if the profile changes in a particular assumed manner with rotation rate. Conclusions: The Babcock-Leighton flux transport dynamo model based on single cell meridional circulation does not reproduce the $P_{cyc}$ vs $P_{rot}$ relation unless the amplitude of the meridional circulation is assumed to increase with rotation rate which seems to be in contradiction with recent results obtained with 3-D global simulations., Comment: 12 pages, 8 figures, accepted for publication by A&A 1: AIM, CEA/DSM-CNRS-Univ. Paris 7, IRFU/SAp, France, 2: D.A.M.T.P., Centre for Mathematical Sciences, Univ. of Cambridge, UK, 3: JILA and Department of Astrophysical and Planetary Sciences, Univ. of Colorado, USA

The Sun is a magnetic star whose magnetism and cyclic activity is linked to the existence of an internal dynamo. We aim to understand the establishment of the solar magnetic 22-yr cycle, its associated butterfly diagram and field parity selection through numerical simulations of the solar global dynamo. Inspired by recent observations and 3D simulations that both exhibit multicellular flows in the solar convection zone, we seek to characterise the influence of various profiles of circulation on the behaviour of solar mean-field dynamo models. We are using 2-D mean field flux transport Babcock-Leighton numerical models in which we test several types of meridional flows: 1 large single cell, 2 cells in radius and 4 cells per hemisphere. We confirm that adding cells in latitude tends to speed up the dynamo cycle whereas adding cells in radius more than triples the period. We find that the cycle period in the four cells model is less sensitive to the flow speed than in the other simpler meridional circulation profiles studied. Moreover, our studies show that adding cells in radius or in latitude seems to favour the parity switching to a quadrupolar solution. According to our numerical models, the observed 22-yr cycle and dipolar parity is easily reproduced by models including multicellular meridional flows. On the contrary, the resulting butterfly diagram and phase relationship between the toroidal and poloidal fields are affected to a point where it is unlikely that such multicellular meridional flows persist for a long period of time inside the Sun, without having to reconsider the model itself., Comment: 12 pages, 10 ps figures

We present recent 3-D MHD numerical simulations of the non-linear dynamical evolution of magnetic flux tubes in an adiabatically stratified convection zone in spherical geometry, using the anelastic spherical harmonic (ASH) code. We seek to understand the mechanism of emergence of strong toroidal fields from the base of the solar convection zone to the solar surface as active regions. We confirm the results obtained in cartesian geometry that flux tubes that are not twisted split into two counter vortices before reaching the top of the convection zone. Moreover, we find that twisted tubes undergo the poleward-slip instability due to an unbalanced magnetic curvature force which gives the tube a poleward motion both in the non-rotating and in the rotating case. This poleward drift is found to be more pronounced on tubes originally located at high latitudes. Finally, rotation is found to decrease the rise velocity of the flux tubes through the convection zone, especially when the tube is introduced at low latitudes., Comment: 4 pages, 6 ps figures

The Sun is a magnetic star whose magnetism and cyclic activity is linked to the existence of an internal dynamo. We aim to understand the establishment of the solar magnetic 22-yr cycle, its associated butterfly diagram and field parity selection through numerical simulations of the solar global dynamo. Inspired by recent observations and 3D simulations that both exhibit multicellular flows in the solar convection zone, we seek to characterise the influence of various profiles of circulation on the behaviour of solar mean-field dynamo models. We are using 2-D mean field flux transport Babcock-Leighton numerical models in which we test several types of meridional flows: 1 large single cell, 2 cells in radius and 4 cells per hemisphere. We confirm that adding cells in latitude tends to speed up the dynamo cycle whereas adding cells in radius more than triples the period. We find that the cycle period in the four cells model is less sensitive to the flow speed than in the other simpler meridional circulation profiles studied. Moreover, our studies show that adding cells in radius or in latitude seems to favour the parity switching to a quadrupolar solution. According to our numerical models, the observed 22-yr cycle and dipolar parity is easily reproduced by models including multicellular meridional flows. On the contrary, the resulting butterfly diagram and phase relationship between the toroidal and poloidal fields are affected to a point where it is unlikely that such multicellular meridional flows persist for a long period of time inside the Sun, without having to reconsider the model itself., Comment: 12 pages, 10 ps figures

We present recent 3-D MHD numerical simulations of the non-linear dynamical evolution of magnetic flux tubes in an adiabatically stratified convection zone in spherical geometry, using the anelastic spherical harmonic (ASH) code. We seek to understand the mechanism of emergence of strong toroidal fields from the base of the solar convection zone to the solar surface as active regions. We confirm the results obtained in cartesian geometry that flux tubes that are not twisted split into two counter vortices before reaching the top of the convection zone. Moreover, we find that twisted tubes undergo the poleward-slip instability due to an unbalanced magnetic curvature force which gives the tube a poleward motion both in the non-rotating and in the rotating case. This poleward drift is found to be more pronounced on tubes originally located at high latitudes. Finally, rotation is found to decrease the rise velocity of the flux tubes through the convection zone, especially when the tube is introduced at low latitudes., Comment: 4 pages, 6 ps figures

We present recent 3-D MHD numerical simulations of the non-linear dynamical evolution of magnetic flux tubes in an adiabatically stratified convection zone in spherical geometry, using the anelastic spherical harmonic (ASH) code. We seek to understand the mechanism of emergence of strong toroidal fields from the base of the solar convection zone to the solar surface as active regions. We confirm the results obtained in cartesian geometry that flux tubes that are not twisted split into two counter vortices before reaching the top of the convection zone. Moreover, we find that twisted tubes undergo the poleward-slip instability due to an unbalanced magnetic curvature force which gives the tube a poleward motion both in the non-rotating and in the rotating case. This poleward drift is found to be more pronounced on tubes originally located at high latitudes. Finally, rotation is found to decrease the rise velocity of the flux tubes through the convection zone, especially when the tube is introduced at low latitudes., Comment: 4 pages, 6 ps figures

The Sun is a magnetic star whose magnetism and cyclic activity is linked to the existence of an internal dynamo. We aim to understand the establishment of the solar magnetic 22-yr cycle, its associated butterfly diagram and field parity selection through numerical simulations of the solar global dynamo. Inspired by recent observations and 3D simulations that both exhibit multicellular flows in the solar convection zone, we seek to characterise the influence of various profiles of circulation on the behaviour of solar mean-field dynamo models. We are using 2-D mean field flux transport Babcock-Leighton numerical models in which we test several types of meridional flows: 1 large single cell, 2 cells in radius and 4 cells per hemisphere. We confirm that adding cells in latitude tends to speed up the dynamo cycle whereas adding cells in radius more than triples the period. We find that the cycle period in the four cells model is less sensitive to the flow speed than in the other simpler meridional circulation profiles studied. Moreover, our studies show that adding cells in radius or in latitude seems to favour the parity switching to a quadrupolar solution. According to our numerical models, the observed 22-yr cycle and dipolar parity is easily reproduced by models including multicellular meridional flows. On the contrary, the resulting butterfly diagram and phase relationship between the toroidal and poloidal fields are affected to a point where it is unlikely that such multicellular meridional flows persist for a long period of time inside the Sun, without having to reconsider the model itself., Comment: 12 pages, 10 ps figures

Contient : Huber, R.. Lettre(s) ; Hucher, Eugène, directeur de musée. Lettre(s) ; Huet, Gédéon, bibliothécaire à la Bibliothèque nationale. Lettre(s) ; Huffer, Hermann, professeur à l'Université de Bonn. Lettre(s) ; Huguet, Edmond, professeur. Lettre(s) ; Huth, Alfred H.. Lettre(s) ; Ive, Antonio, professeur à l'Université de Graz. Lettre(s) ; Izoulet, Jean, professeur au Collège de France. Lettre(s) ; Jaberg, K.. Lettre(s) ; Jackson, T.W., professeur à Oxford. Lettre(s) ; Jacob, Alfred. Lettre(s) ; Jacobs, Joseph. Lettre(s) ; Jacottet, Léopold. Lettre(s) ; Jacquel, secrétaire de l'Institut. Lettre(s) ; Jadart, Henri, bibliothécaire à Reims. Lettre(s) ; Janet, Paul, professeur à la Sorbonne. Lettre(s) ; Jannet, P.. Lettre(s) ; Jarnik, H.. Lettre(s) ; Jarnik, Jean-Urbain, professeur à l'Université de Prague. Lettre(s) ; Jarry, Eugène, archiviste-paléographe. Lettre(s) ; Jastrow, Minis. Lettre(s) ; Jeanjaquet, J.. Lettre(s) ; Jeanroy, Alfred, professeur à la Sorbonne. Lettre(s) ; Jenkins, J.A.. Lettre(s) ; Jenkins, Thomas Atkinson. Lettre(s) ; Jensen, L.E.. Lettre(s) ; Jobin, Abbé J.B.. Lettre(s) ; Johansson, Alfred. Lettre(s) ; John, Ivar B.. Lettre(s) ; Johnston, Oliver M.. Lettre(s) ; Joly, Aristide, professeur à l'Université de Caen. Lettre(s) ; Joret, Charles, professeur à l'Université d'Aix. Lettre(s) ; Jorga, Nicolas, historien et homme politique. Lettre(s) ; Joulin, L., ingénieur. Lettre(s) ; Jourdanne, G.. Lettre(s) ; Jourde, Théophile. Lettre(s) ; Jouve, L.. Lettre(s), Numérisation effectuée à partir d'un document original : NAF 24443.