arXiv subjects

Ludovic Petitdemange

Publications and source records attributed to Ludovic Petitdemange.

At least 19 recordsLinked to original sources

arXiv2026

Probing magnetic fields in stars: A 2D oscillation framework including rotation and magnetism

Understanding the role of internal magnetic fields in stars remains a major challenge for the description of angular momentum transport and stellar evolution. It is therefore essential to probe these magnetic fields within the star, and asteroseismology provides a powerful means to do so. In this work, we present a new implementation of the 2D oscillation code ( ), which incorporates the effects of both stellar rotation and magnetic fields. Adaptive Code of Oscillations towards Realistic modeling The code has been rendered modular, making it possible to specify the set of equations to solve and the assumptions through a symbolic calculus approach. The full set of adiabatic, non-radial pulsation equations is solved using a spectral approach for the angular part of the modes and high-order finite differences for the radial part. As a first step, we focus on a magnetic field that is purely toroidal and axisymmetric about the star's rotation axis. The numerical results are compared against first-order perturbative predictions in the weak-field regime and with the traditional approximation of rotation and magnetism (TARM) in the case of stronger magnetic fields. We validate this new implementation of against perturbative and TARM approaches, showing good agreement and demonstrating the robustness of the code. The breakdown of these methods provides general validity limits. We show that internal magnetic fields leave signatures in the period spacing of g modes. These features provide a promising seismic diagnostic to probe deep stellar magnetism in γ Dor stars. Future work will aim to extend this framework to more realistic magnetic field topologies and a broader range of pulsating stars, including red giants.

astro-ph.SR↗

arXiv2026

Nonlinear interaction between dynamo-generated magnetic fields, mean flows and internal gravity waves in stellar stably-stratified layers: From 3D to 1D

Magnetic fields have been constrained at the surface of several massive and intermediate-mass stars, but their origin and properties in deep stellar radiative interiors are still debated, despite recent detections in the core of some red giant stars. Therefore, the modelling of AM transport in stellar radiative layers only relies on theoretical and numerical estimates of magnetic fields. Recent 3D numerical simulations show that a dynamo could occur in deep radiative regions. A realistic setup for understanding AM transport in such layers thus requires to take into account the mutual interactions of IGW and dynamo-generated magnetic field. We model the dynamics induced by IGW and dynamo in rotating radiative stellar layers using a simple description applicable to various evolutionary stages. As dynamo action and the propagation of IGW are 3D processes that have characteristic timescales short compared to periods associated with structural evolution of stars, we propose a mean-field 1D model by taking advantage of the dynamo coefficients computed from 3D spherical simulations. In this model, the necessary mean shear flow to trigger the dynamo results from the dissipation of monochromatic IGW generated in existing adjacent convective layers, which are expected to drive the formation of an oscillating rotational shear layer, the so-called Shear Layer Oscillation (SLO). In turn, magnetic effects can act on the mean flow through the Lorentz force. We show that the inclusion of magnetic fields adds up to the already very complex nonlinear problem and gives rise to the emergence of new dynamical regimes. Particularly, the fast SLO generated very close to the place where IGW are generated is perturbed by magnetic fields. This dynamical change can filter the wave energy spectrum transmitted towards further layers, with potential influence on the long-term evolution of the inner rotation.

astro-ph.SR↗

arXiv2025

A few ideas to promote inclusion

Promoting diversity, equity, inclusion and accessibility (DEIA) is both a legal and professional responsibility in French research institutions. This paper presents practical strategies to foster inclusive work environments within French research units. We summarize the regulatory context, key findings from the INSU-AA prospective on discrimination, and fundamental principles for promoting equity. We discuss approaches to mitigate implicit biases across all career stages, from early education to retirement, and outline strategies for equitable recruitment and career advancement. Concrete initiatives in one of our units (LESIA/LIRA) are described, including internal communications, exhibitions, and accessible pedagogical activities. The creation of a dedicated commission within the unit council ensures coordinated DEIA efforts, legitimized by institutional support and methodical planning. By sharing these experiences, we provide actionable guidance for research units seeking to advance DEIA in science.

physics.soc-ph↗

arXiv2025

Enabling Blind and Visually Impaired Individuals to Pursue Careers in Science

Blind and Visually Impaired (BVI) Individuals face significant challenges in science due to the discipline's reliance on visual elements such as graphs, diagrams, and laboratory work. Traditional learning materials, such as Braille and large-print textbooks, are often scarce or delayed, while practical experiments are rarely adapted for accessibility. Additionally, mainstream educators lack the training to effectively support BVI students, and Teachers for the Visually Impaired (TVIs) often lack scientific expertise. As a result, BVI individuals remain underrepresented in scientific jobs, reinforcing a cycle of exclusion. However, technological advancements and inclusive initiatives are opening new opportunities. Outreach programs aim to make science engaging and accessible for BVI individuals through multi-sensory learning experiences. Hands-on involvement in these activities fosters confidence and interest in scientific careers. Beyond sparking interest, equipping BVI students with the right tools and skills is crucial for their academic success. Early exposure to assistive technologies enables BVI students to navigate scientific studies independently. Artificial Intelligence (AI) tools further enhance accessibility by converting visual data into descriptive text and providing interactive assistance. Several learning sessions demonstrated the effectiveness of these interventions, with participants successfully integrating into university-level science programs. Educating BVI and their teachers on these tools and good pratices is the aim of our project AccesSciencesDV. Research careers offer promising opportunities for BVI, especially in computational fields. By leveraging coding, data analysis, and AI-driven tools, BVI researchers can conduct high-level scientific work without relying on direct visual observations. The presence of BVI scientists enriches research environments.

physics.ed-ph↗

arXiv2025

New constraint on Europa's ice shell: magnetic signature from the ocean

Jupiter's icy moons are believed to host subsurface liquid oceans, and among them, Europa stands out as one of the most promising candidates for extraterrestrial life. Yet, the processes driving oceanic flows beneath its ice shell, as well as the factors controlling the thickness of this ice, remain incompletely understood. One especially distinctive feature of Europa is that its salty ocean is electrically conducting and thus influenced by Jupiter's time-varying magnetic field, which is believed to drive a large-scale zonal flow. Here, we examine hos this magnetically-induced jet affects both the heat flux and the dynamics of the convective flow within Europa's ocean. We first show that the magnetically-driven jet efficiently transports heat in stably stratified regions near the top of the ocean, and may alter the expected convective scaling laws in deeper layers. Second, by analysing the latitudinal distribution of heat flux and relating it to ice-thickness variations, we make predictions that can be compared with current observations. In anticipation of the upcoming JUICE and Europa Clipper missions, we discuss how improved measurement precision could help further constrain the ocean's properties and refine our model-based forecasts.

astro-ph.EP↗

arXiv2025

The Future of Solar modelling: requirements for a new generation of solar models

Helioseismology and solar modelling have enjoyed a golden era thanks to decades-long surveys from ground-based networks such as for example GONG, BiSON, IRIS and the SOHO and SDO space missions which have provided high-quality helioseismic observations that supplemented photometric, gravitational, size and shape, limb-darkening and spectroscopic constraints as well as measurements of neutrino fluxes. However, the success of solar models is also deeply rooted in progress in fundamental physics (equation of state of the solar plasma, high-quality atomic physics computations and opacities, description of convection and the role of macroscopic transport processes of angular momentum and chemicals, such as for example meridional circulation, internal gravity waves, shear-induced turbulence or even convection. In this paper, we briefly outline some key areas of research that deserve particular attention in solar modelling. We discuss the current uncertainties that need to be addressed, how these limit our predictions from solar models and their impact on stellar evolution in general. We outline potential strategies to mitigate them and how multidisciplinary approaches will be needed in the future to tackle them.

astro-ph.SR↗

arXiv2025

Run-away transition to turbulent strong-field dynamo

Planets and stars are able to generate coherent large-scale magnetic fields by helical convective motions in their interiors. This process, known as hydromagnetic dynamo, involves nonlinear interaction between the flow and magnetic field. Nonlinearity facilitates existence of bi-stable dynamo branches: a weak field branch where the magnetic field is not strong enough to enter into the leading order force balance in the momentum equation at large flow scales, and a strong field branch where the field enters into this balance. The transition between the two with enhancement of convection can be either subcritical or supercritical, depending on the strength of magnetic induction. In both cases, it is accompanied by topological changes in velocity field across the system; however, it is yet unclear how these changes are produced. In this work, we analyse transitions between the weak and strong dynamo regimes using a data-driven approach, separating different physical effects induced by dynamically active flow scales. Using Dynamic Mode Decomposition, we decompose the dynamo data from direct numerical simulations into different components (modes), identify the ones relevant for transition, and estimate relative magnitudes of their contributions Lorentz force and induction term. Our results suggest that subcritical transition to a strong dynamo is facilitated by a subharmonic instability, allowing for a more efficient mode of convection, and provide a modal basis for reduced-order models of this transition.

physics.flu-dyn↗

arXiv2025

Magnetic field morphologies in convective zones influenced by a turbulent surface layer

Spectropolarimetric observations show that many low-mass stars possess large-scale poloidal magnetic fields with considerable dipole component, which in some cases exhibit temporal dynamics - cycles or reversals. Although it is widely accepted that their magnetic fields are generated by the dynamo process, it is hard to reproduce coherent oscillations of large-scale magnetic fields with a dipolar symmetry as observed for the Sun when turbulent and compressible regimes are explored. Most previous 3D numerical studies partially avoided this problem by considering a numerical domain with low density stratification, which may correspond to neglecting surface effects where density drops considerably. To address this question, we perform direct numerical simulations of convective dynamos in extreme parameter regimes of both strong turbulence and strong density stratification, using software MagIC. Our simulations exhibit rotationally-influenced large-scale convective motions surrounded by a turbulent compressible surface layer. We find complex time variations of the magnetic field in flow regimes of predominantly dipolar configuration with respect to the few large-scale harmonics. In such regimes, turbulent surface layer induces global magnetic pumping mechanism, transporting magnetic energy into the deep interiors of our dynamo model. Dipole magnetic fields are found in regimes of transition between solar- and anti-solar differential rotation, and interact dynamically with it. The spatial distribution and temporal behavior of the large-scale fields is consistent with observations of low-mass stars, which suggest magnetic pumping could promote time-dependent magnetic fields with a dipolar symmetry as observed for the Sun and other solar-like stars. Our results suggest a parameter path in which dynamo models with a complex multiscale dynamics should be explored.

astro-ph.SR↗

arXiv2024

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.

astro-ph.SR↗

arXiv2023

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.

physics.flu-dyn↗

arXiv2023

Spin-down by dynamo action in simulated radiative stellar layers

The evolution of a star is influenced by its internal rotation dynamics through transport and mixing mechanisms, which are poorly understood. Magnetic fields can play a role in transporting angular momentum and chemical elements, but the origin of magnetism in radiative stellar layers is unclear. Using global numerical simulations, we identify a subcritical transition to turbulence due to the generation of a magnetic dynamo. Our results have many of the properties of the theoretically-proposed Tayler-Spruit dynamo mechanism, which strongly enhances transport of angular momentum in radiative zones. It generates deep toroidal fields that are screened by the stellar outer layers. This mechanism could produce strong magnetic fields inside radiative stars, without an observable field on their surface.

astro-ph.SR↗

arXiv2020

Magnetic effects on fields morphologies and reversals in geodynamo simulations

The dynamo effect is the most popular candidate to explain the non-primordial magnetic fields of astrophysical objects. Although many systematic studies of parameters have already been made to determine the different dynamical regimes explored by direct numerical geodynamo simulations, it is only recently that the regime corresponding to the outer core of the Earth characterized by a balance of forces between the Coriolis and Lorentz forces is accessible numerically. In most previous studies, the Lorentz force played a relatively minor role. For example, they have shown that a purely hydrodynamic parameter (the local Rossby number $Ro_\ell$ determines the stability domain of dynamos dominated by the axial dipole (dipolar dynamos). In this study, we show that this result cannot hold when the Lorentz force becomes dominant. We model turbulent geodynamo simulations with a strong Lorentz force by varying the important parameters over several orders of magnitude. This method enables us to question previous results and to argue on the applications of numerical dynamos in order to better understand the geodynamo problem. Strong dipolar fields considerably affect the kinetic energy distribution of convective motions which enables the maintenance of this field configuration. The relative importance of each force depends on the spatial length scale, whereas $Ro_\ell$ is a global output parameter which ignores the spatial dependency. We show that inertia does not induce a dipole collapse as long as the Lorentz and the Coriolis forces remain dominant at large length scales.

physics.flu-dyn↗

arXiv2019

Inverse cascade of hybrid helicity in $B Ω$-MHD turbulence

We investigate the impact of a solid-body rotation $Ω_0$ on the large-scale dynamics of an incompressible magnetohydrodynamic turbulent flow in presence of a background magnetic field $\bf B_0$ and at low Rossby number. Three-dimensional direct numerical simulations are performed in a periodic box, at unit magnetic Prandtl number and with a forcing at intermediate wavenumber $k_f=20$. When $Ω_0$ is aligned with $\bf B_0$ (i.e. $θ\equiv \widehat{\left(Ω_{0}, \bf B_0 \right)} = 0$), inverse transfer is found for the magnetic spectrum at $k 0$ and becomes weak when $θ\ge 35^o$. These properties are understood as the consequence of an inverse cascade of hybrid helicity which is an inviscid/ideal invariant of this system when $θ=0$. Hybrid helicity emerges, therefore, as a key element for understanding rotating dynamos. Implication of these findings on the origin of the alignment of the magnetic dipole with the rotation axis in planets and stars is discussed.

physics.flu-dyn↗

arXiv2019

A magnetically driven equatorial jet in Europa's ocean

During recent decades, data from space missions have provided strong evidence of deep liquid oceans underneath a thin outer icy crust on several moons of Jupiter, particularly Europa. But these observations have also raised many unanswered questions regarding the oceanic motions generated under the ice, or the mechanisms leading to the geological features observed on Europa. By means of direct numerical simulations of Europa's interior, we show here that Jupiter's magnetic field generates a retrograde oceanic jet at the equator, which may influence the global dynamics of Europa's ocean and contribute to the formation of some of its surface features by applying a unidirectional torque on Europa's ice shell.

astro-ph.EP↗

arXiv2018

Systematic parameter study of dynamo bifurcations in geodynamo simulations

We investigate the nature of the dynamo bifurcation in a configuration applicable to the Earth's liquid outer core. Numerical studies on the stability domain of dipolar magnetic fields found a dichotomy between non-reversing dipole-dominated dynamos and the reversing non-dipole-dominated multipolar solutions. We show that, by considering weak initial fields, the above transition disappears and is replaced by a region of bistability. Such a result was also observed in models with free-slip boundaries in which the geostrophic zonal flow can develop and participate to the dynamo mechanism for non-dipolar fields. We show that a similar process develops in no-slip models when viscous effects are reduced sufficiently. The following three regimes are distinguished: (i) Close to the onset of convection ($Ra_c$) with only the most critical convective mode (wave number) being present, dynamos set in supercritically and are dipole-dominated. (ii) in the range $3 10)$, the relative importance of zonal flows increases with $Ra$ in non-magnetic models. The field topology depends on the magnitude of the initial magnetic field. The dipolar branch has a subcritical behavior whereas the multipolar branch has a supercritical behavior. By approaching more realistic parameters, the extension of this bistable regime increases. A hysteretic behavior questions the common interpretation for geomagnetic reversals. Far above the dynamo threshold (by increasing the magnetic Prandtl number), Lorentz forces contribute to the first order force balance, as predicted for planetary dynamos. When $Ra$ is sufficiently high, dipolar fields affect significantly the flow speed, the flow structure and heat transfer which is reduced by the Lorentz force regardless of the field strength.

physics.geo-ph↗

arXiv2017

Three Branches of Dynamo Action

In addition to the weak-dipolar state and to the fluctuating-multipolar state, widely discussed in the literature, a third regime has been identified in Dormy (2016). It corresponds to a strong-dipolar branch which appears to approach, in a numerically affordable regime, the magnetostrophic limit relevant to the dynamics of the Earth's core. We discuss the transitions between these states and point to the relevance of this strong-dipolar state to Geodynamo modelling.

physics.geo-ph↗

arXiv2017

Gravity darkening in late-type stars. The Coriolis effect

Recent interferometric data have been used to constrain the brightness distribution at the surface of nearby stars, in particular the so-called gravity darkening that makes fast rotating stars brighter at their poles than at their equator. However, good models of gravity darkening are missing for stars that posses a convective envelope. In order to better understand how rotation affects the heat transfer in stellar convective envelopes, we focus on the heat flux distribution in latitude at the outer surface of numerical models. We carry out a systematic parameter study of three-dimensional, direct numerical simulations of anelastic convection in rotating spherical shells. Restricting our investigations to hydrodynamical models with a thermal Prandtl number fixed to unity, we consider both thick and thin (solar-like) shells, and vary the stratification over three orders of magnitude. We measure the heat transfer efficiency in terms of the Nusselt number, defined as the output luminosity normalised by the conductive state luminosity. We report diverse Nusselt number profiles in latitude, ranging from brighter (usually at the onset of convection) to darker equator and uniform profiles. We find that the variations of the surface brightness are mainly controlled by the surface value of the local Rossby number: when the Coriolis force dominates the dynamics, the heat flux is weakened in the equatorial region by the zonal wind and enhanced at the poles by convective motions inside the tangent cylinder. In the presence of a strong background density stratification however, as expected in real stars, the increase of the local Rossby number in the outer layers leads to uniformisation of the surface heat flux distribution.

astro-ph.SR↗

arXiv2014

Influence of the mass distribution on the magnetic field topology

Context. Three-dimensional spherical dynamo simulations carried out within the framework of the anelastic approximation have revealed that the established distinction between dipolar and multipolar dynamos tends to be less clear than it was in Boussinesq studies. This result was first interpreted as a direct consequence of the existence of a larger number of models with a high equatorial dipole contribution, together with an intermediate dipole field strength. However, this finding has not been clearly related to specific changes that would have been introduced by the use of the anelastic approximation. Aims. In this paper, we primarily focus on the effects of choosing of a different mass distribution. Indeed, it is likely to have as large consequences as would taking a stratified reference state into account, especially when comparing our results to previous Boussinesq studies. Methods. Our investigation is based on a systematic parameter study of weakly stratified anelastic dynamo models. Results. We show that the tendencies highlighted in previous anelastic dynamo simulations are already present in the Boussinesq limit. Thus they cannot be systematically related to anelastic effects. Actually, a central mass distribution can result in changes in the magnetic field topology that are mainly due to the concentration of convective cells close to the inner sphere.

astro-ph.SR↗