Publications HAL

2020

Journal articles

titre
Magnetar formation through a convective dynamo in protoneutron stars
auteur
Raphaël Raynaud, Jérôme Guilet, Hans-Thomas Janka, Thomas Gastine
article
Science Advances , American Association for the Advancement of Science (AAAS), 2020, 6 (eaay2732), ⟨10.1126/sciadv.aay2732⟩
resume
The release of spin-down energy by a magnetar is a promising scenario to power several classes of extreme explosive transients. However, it lacks a firm basis because magnetar formation still represents a theoretical challenge. Using the first three-dimensional simulations of a convective dynamo based on a protoneutron star interior model, we demonstrate that the required dipolar magnetic field can be consistently generated for sufficiently fast rotation rates. The dynamo instability saturates in the magnetostrophic regime with the magnetic energy exceeding the kinetic energy by a factor of up to 10. Our results are compatible with the observational constraints on galactic magnetar field strength and provide strong theoretical support for millisecond protomagnetar models of gamma-ray burst and superluminous supernova central engines.
DOI
DOI : 10.1126/sciadv.aay2732
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https://hal.archives-ouvertes.fr/hal-02428428/file/raynaud_2020.pdf BibTex

2019

Conference papers

titre
Magnetar formation through convective dynamos in protoneutron stars
auteur
Raphaël Raynaud, Jérôme Guilet, Hans-Thomas Janka, Thomas Gastine
article
Multi-Messenger Astrophysics in the Gravitational Wave Era, Yukawa Institute for Theoretical Physics, Sep 2019, Kyoto, Japan
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titre
Convective dynamos in protoneutron stars
auteur
Raphaël Raynaud, Jérôme Guilet, Hans-Thomas Janka, Thomas Gastine
article
Dynamo action from the lab to the stars, PLAS@PAR Thematic Days, Sep 2019, Paris, France
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titre
Protoneutron star dynamos
auteur
Raphaël Raynaud, Jérôme Guilet
article
Gamma-ray bursts and supernovae: from the central engines to the observer, PSI2 thematic program, Jun 2019, Orsay, France
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titre
Convection and dynamo modeling in protoneutron stars
auteur
Raphaël Raynaud
article
Bursting the Bubble: Connecting Thermonuclear Burst Research to a Wider Community, Lorentz Center, Jun 2019, Leiden, Netherlands
resume
If the Boussinesq approximation performs well in modeling convection in laboratory experiments, it is however unsatisfactory for large natural systems, the lower part of which is compressed by the overlying material. The anelastic approximation enables us to overcome this difficulty by taking into account the mean stratification of the system, while filtering out sound waves for a faster numerical integration. It has been successfully used to study planetary or stellar convection and I will show how it can be further adapted to develop a realistic interior model of a newborn neutron star.
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titre
Magnetar formation through convective dynamos in protoneutron stars
auteur
Raphaël Raynaud, Jérôme Guilet, Hans-Thomas Janka, Thomas Gastine
article
Journées de la SF2A 2019, Société Francaise d’Astronomie et d’Astrophysique, May 2019, Nice, France
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titre
A global model of the magnetorotational instability in proto-neutron stars
auteur
Alexis Reboul-Salze, Jérôme Guilet, Raphaël Raynaud, Matteo Bugli
article
Journées de la SF2A, Société Francaise d’Astronomie et d’Astrophysique, May 2019, Nice, France
resume
The magnetorotational instability (MRI) is a promising mechanism to amplify the magnetic field in fast-rotating proto-neutron stars. Many local studies have shown that the magnetic field could be amplified on small scales. However, the efficiency of the MRI at generating a large-scale field similar to the dipolar magnetic field of magnetars (10 14 − 10 15 G) is still unknown. We used a three dimensional pseudo-spectral code to develop an idealized global model of the MRI in a proto-neutron star. We show that a dipole field strength consistent with the values of magnetar field intensity can be generated by the MRI, even though it is lower than the small-scale magnetic field. Overall, our results support the ability of the MRI to form magnetar-like dipolar magnetic fields.
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https://hal.archives-ouvertes.fr/hal-02428439/file/reboul-salze_2019_sf2a.pdf BibTex
titre
Magnetar formation through convective dynamos in protoneutron stars
auteur
Raphaël Raynaud, Jérôme Guilet, Hans-Thomas Janka, Thomas Gastine
article
XIXth Workshop on Nuclear Astrophysics, Mar 2019, Tegernsee, Germany
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Directions of work or proceedings

titre
Dipole collapse in rotating stratified dynamos
auteur
Ludovic Petitdemange, Raphaël Raynaud
article
Astro Fluid: An International Conference in Memory of Professor Jean-Paul Zahn's Great Scientific Achievements, Jun 2016, Paris, 2019, ⟨10.1051/eas/1982031⟩
resume
Numerical modelling of convection driven dynamos in the Boussinesq approximation revealed fundamental characteristics of the dynamo-generated magnetic fields, but the relevance of these results remains to be assessed for highly stratified systems, like gas planets and stars. The common approach is then to rely on the anelastic approximation to model the background density stratification. A conclusion from different anelastic studies is that dipolar solutions seem more difficult to obtain in presence of a substantial density contrast. We review some important results obtained by Raynaud et al. (2015), who investigated the influence of the density stratification on the stability of dipolar dynamos. This 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. In strongly stratified systems, the force balance may vary with depth, and a local increase of inertia close to the outer surface can explain the loss of the dipolar branch, while volume-averaged measures may underestimate the role of inertia on the field topology.
DOI
DOI : 10.1051/eas/1982031
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https://hal.archives-ouvertes.fr/hal-02426720/file/Petitdemange_Raynaud_2019_Proc_AstroFluid.pdf BibTex

2018

Journal articles

titre
Gravity darkening in late-type stars. I. The Coriolis effect
auteur
Raphaël Raynaud, Michel Rieutord, Ludovic Petitdemange, Thomas Gastine, Bertrand Putigny
article
Astronomy and Astrophysics - A&A, EDP Sciences, 2018, 609 (A124), ⟨10.1051/0004-6361/201731729⟩
resume
Context. 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. Aims. 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. Methods. We carry out a systematic parameter study of three-dimensional, direct numerical simulations of anelastic convection in rotating spherical shells. As a first step, we neglect the centrifugal acceleration and retain only the Coriolis force. The fluid instability is driven by a fixed entropy drop between the inner and outer boundaries where stress-free boundary conditions are applied for the velocity field. 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. Results. 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.
DOI
DOI : 10.1051/0004-6361/201731729
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https://hal.archives-ouvertes.fr/hal-01706693/file/raynaud2017.pdf BibTex

Conference papers

titre
Convective dynamos in protoneutron stars
auteur
Raphaël Raynaud, Jérôme Guilet, Hans-Thomas Janka, Thomas Gastine
article
Magnetic field formation and evolution in neutron stars, PHAROS WG4+WG5 meeting and CoCoNuT Meeting, Nov 2018, Saclay, France
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titre
The pseudo-spectral code MagIC
auteur
Raphaël Raynaud
article
Multi-Dimensional processes in stellar physics, École Évry Schatzman du PNPS, Oct 2018, Roscoff, France
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titre
Gravity darkening in late-type stars
auteur
Raphaël Raynaud, Michel Rieutord, Ludovic Petitdemange, Thomas Gastine, Bertrand Putigny
article
7th BCool meeting, Apr 2018, Dublin, Ireland
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Directions of work or proceedings

titre
Supermodulation of the solar cycle
auteur
Raphaël Raynaud, Steven Tobias
article
Journées de la SF2A, Jul 2018, Bordeaux, France. 2018
resume
There is evidence from record of cosmogenic radionuclides that the Sun's activity switches between periods of strong modulation with clusters of deep grand minima and periods of weaker modulation. By considering dynamo action driven by three-dimensional rotating anelastic convection in a spherical shell, we demonstrate how interactions between convection, differential rotation and magnetic fields may lead to modulation of the basic cycle of an oscillatory dynamo. For some parameters, Type 1 modulation occurs by the transfer of energy between modes of different symmetries with little change in the overall amplitude; for other parameters, the modulation is of Type 2 where the amplitude is significantly affected (leading to grand minima in activity) without significant changes in symmetry. Most importantly we identify the presence of supermodulation in the solutions where the activity switches chaotically between Type 1 and Type 2 modulation; this is believed to be characteristic of the long term modulation of the solar activity.
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https://hal.archives-ouvertes.fr/hal-02426733/file/raynaud_tobias_2018.pdf BibTex
titre
Gravity darkening in late-type stars
auteur
Raphaël Raynaud, Michel Rieutord, Ludovic Petitdemange, T. Gastine, Bertrand Putigny
article
Journées de la SF2A, Jul 2018, Bordeaux, France. 2018
resume
Recent interferometric data have been able to constrain the brightness distribution at the surface of nearby stars, in particular the gravity darkening that makes fast rotating stars brighter at their poles than at their equator. However, good models of gravity darkening are missing when the stars own a convective envelope. In order to better understand how rotation affects the heat transfer in stellar convective envelopes, we studied the heat flux distribution in latitude at the outer surface of numerical models of anelastic convection in rotating sphericall shells. We found 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. However, in presence of a strong background density stratification, as expected in real stars, the increase of the local Rossby number in the outer layers leads to the uniformisation of the surface heat flux distribution.
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https://hal.archives-ouvertes.fr/hal-02426744/file/raynaud_et_al_2018.pdf BibTex

2017

Conference papers

titre
Latitudinal flux variations of thermal anelastic convection in a rotating spherical shell
auteur
Raphaël Raynaud, Michel Rieutord, Ludovic Petitdemange, Thomas Gastine, Bertrand Putigny
article
Compressible Convection Conference, Sep 2017, Lyon, France
resume
Following interferometric observations of fast rotating stars, which now give new constraints on the latitudinal heat flux distribution, we investigate the influence of rotation on this quantity in solar-type stars, which own an outer convective envelope. To that end, we carried out a systematic parameter study of three-dimensional direct numerical simulations of anelastic convection in rotating spherical shells. As a first step, we neglect the centrifugal acceleration and retain only the Coriolis force. The fluid instability is driven by a fixed entropy drop between the inner and outer boundaries where stress-free boundary conditions are applied for the velocity field. Restricting our investigations to hydrodynamical models with a thermal Prandtl number fixed to unity, we consider both thick and thin shells with different degrees of stratification. 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 luminosity contrast is mainly controlled by the surface value of the local Rossby number: when the Coriolis force dominates the force balance, 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. But in presence of a strong background stratification, the radial dependence of the conductive entropy profile favours the sharp increase of the local Rossby number in the outer layers, which ultimately leads to the uniformisation of the surface heat flux distribution.
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titre
Dynamo theory
auteur
Raphaël Raynaud
article
10th National Meeting on Astronomy and Astrophysics, Iranian Astronomical Society, Jan 2017, Zahedan, Iran
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Poster communications

titre
Magnetars
auteur
Raphaël Raynaud, Jérôme Guilet
article
GDRI Dynamo Meeting, Nov 2017, Paris, France
resume
What is the origin of neutron star magnetic fields ? Although the fossil field hypothesis (i.e. the amplification of the neutron star progenitor field by flux conservation) would be the simplest mechanism, it seems unlikely to explain the existence of magnetars, which are a peculiar type of neutron stars endowed with unusually strong dipolar magnetic fields. Their intensity is currently deduced from stellar spin-down rate measurements and estimated to be in the order of $10^{15}$ Gauss, which makes them the strongest magnetic fields ever observed in the Universe. Understanding the formation process of magnetars is of particular importance, since they are believed to power a variety of outstanding explosive events observed with ongoing and future transient surveys (LSST, SVOM).
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https://hal.archives-ouvertes.fr/hal-01856043/file/poster.pdf BibTex

2016

Journal articles

titre
Multi-stage high order semi-Lagrangian schemes for incompressible flows in Cartesian geometries
auteur
Alexandre Cameron, Raphaël Raynaud, Emmanuel Dormy
article
International Journal for Numerical Methods in Fluids, Wiley, 2016, ⟨10.1002/fld.4245⟩
resume
Efficient transport algorithms are essential to the numerical resolution of incompressible fluid flow problems. Semi-Lagrangian methods are widely used in grid based methods to achieve this aim. The accuracy of the interpolation strategy then determines the properties of the scheme. We introduce a simple multi-stage procedure which can easily be used to increase the order of accuracy of a code based on multi-linear interpolations. This approach is an extension of a corrective algorithm introduced by Dupont & Liu (2003, 2007). This multi-stage procedure can be easily implemented in existing parallel codes using a domain decomposition strategy, as the communications pattern is identical to that of the multi-linear scheme. We show how a combination of a forward and backward error correction can provide a third-order accurate scheme, thus significantly reducing diffusive effects while retaining a non-dispersive leading error term.
DOI
DOI : 10.1002/fld.4245
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https://hal.archives-ouvertes.fr/hal-01335662/file/1606.06117v1.pdf BibTex
titre
Convective dynamo action in a spherical shell: symmetries and modulation
auteur
Raphaël Raynaud, Steven Tobias
article
Journal of Fluid Mechanics, Cambridge University Press (CUP), 2016, 799 (R6), ⟨10.1017/jfm.2016.407⟩
resume
We consider dynamo action driven by three-dimensional rotating anelastic convection in a spherical shell. Motivated by the behaviour of the solar dynamo, we examine the interaction of hydromagnetic modes with different symmetries and demonstrate how complicated interactions between convection, differential rotation and magnetic fields may lead to modulation of the basic cycle. For some parameters, Type 1 modulation occurs by the transfer of energy between modes of different symmetries with little change in the overall amplitude; for other parameters, the modulation is of Type 2, where the amplitude is significantly affected (leading to grand minima in activity) without significant changes in symmetry. Most importantly, we identify the presence of 'supermodulation' in the solutions, where the activity switches chaotically between Type 1 and Type 2 modulation; this is believed to be an important process in solar activity.
DOI
DOI : 10.1017/jfm.2016.407
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https://hal.archives-ouvertes.fr/hal-01342498/file/1607.01066v1.pdf BibTex

Conference papers

titre
Convective dynamos: symmetries and modulation
auteur
Raphaël Raynaud, Steven Tobias
article
Advances in Geophysical and Astrophysical Turbulence, Jul 2016, Cargèse, France
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titre
Convective dynamos: symmetries and modulation
auteur
Raphaël Raynaud, Steven Tobias
article
European GdR Dynamo, Jun 2016, Barcelone, Spain
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titre
New insights into the modulation of the Sun activity
auteur
Raphaël Raynaud, Steven Tobias
article
19th Meeting on Research in Astronomy, Institute for Advanced Studies in Basic Sciences (IASBS), May 2016, Zanjan, Iran
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2015

Journal articles

titre
Dipolar dynamos in stratified systems
auteur
Raphaël Raynaud, Ludovic Petitdemange, Emmanuel Dormy
article
Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2015, 448, pp.2055-2065. ⟨10.1093/mnras/stv122⟩
resume
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.
DOI
DOI : 10.1093/mnras/stv122
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https://hal.archives-ouvertes.fr/hal-01122919/file/raynaud2015.pdf BibTex

Conference papers

titre
Dipolar dynamos in stratified systems
auteur
Raphaël Raynaud, Ludovic Petitdemange, Emmanuel Dormy
article
Stellar and planetary dynamos, May 2015, Goettingen, Germany
resume
Observations of low-mass stars reveal a variety of magnetic topologies ranging from large-scale, axial dipoles to more complex magnetic fields. Spherical simulations of convectivedynamos reproduce a similar diversity, either with Boussinesq or anelastic models taking intoaccount the variation of the density with depth throughout the convection zone. A conclusionfrom anelastic studies is that dipolar solutions are difficult to obtain as soon as substantialstratifications are considered. We rely on a systematic parameter study to investigate inmore detail the impact of the density stratification both on the dynamo onset and thedipole collapse. Our study indicates that the loss of the dipolar branch does not ensue froma modification of the dynamo mechanisms related to the background stratification, butcould instead result from a bias as our observations naturally favour a certain domain in theparameter space characterized by moderate values of the Ekman number, owing to currentcomputational limitations. We also show that the critical magnetic Reynolds number ofthe dipolar branch is scarcely modified, which provides an important insight into the globalunderstanding of the impact of the density stratification on the stability domain of thedipolar dynamo branch.
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Directions of work or proceedings

titre
Dynamos stellaires : symétries et modulations
auteur
Raphaël Raynaud
article
Rencontre du Non Linéaire, Mar 2015, Paris, France. 2015
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Theses

titre
Modélisation numérique du magnétisme solaire et stellaire
auteur
Raphaël Raynaud
article
Physique [physics]. Observatoire de Paris, 2015. Français
resume
Dynamo action, i.e. the self-amplification of a magnetic field by the flow of an electrically conducting fluid, is considered to be the main mechanism for the generation of the magnetic fields of stars and planets. Intensive and systematic param- eter studies by direct numerical simulations using the Boussinesq approximation revealed fundamental properties of these models. However, this approximation considers an incompressible conducting fluid, and is therefore not adequate to describe convection in highly stratified systems like stars or gas giants. A common approach to overcome this difficulty is then to use the anelastic approximation, that allows for a reference density profile while filtering out sound waves for a faster numerical integration. Through a systematic parameter study of spherical anelastic dynamo models, we investigate the influence of the stratification on the dynamo mechanisms, and compare them with previous results obtained in the Boussinesq approximation. We discuss the influence of the density stratification on the field geometry and the field strength. Our results are in overall agreeement with the previous geodynamo studies, and thus the main established results seem to be still relevant to the study of stellar dynamos.
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https://hal.archives-ouvertes.fr/tel-01196516/file/these_raynaud.pdf BibTex

2014

Journal articles

titre
Topology and field strength in spherical, anelastic dynamo simulations
auteur
Martin Schrinner, Ludovic Petitdemange, Raphaël Raynaud, Emmanuel Dormy
article
Astronomy and Astrophysics - A&A, EDP Sciences, 2014, 564 (A78), ⟨10.1051/0004-6361/201322801⟩
resume
Context: 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 of constant density, 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. Aims: 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. Methods: 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. Results: 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.
DOI
DOI : 10.1051/0004-6361/201322801
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https://hal.archives-ouvertes.fr/hal-01122251/file/schrinner2014.pdf BibTex
titre
Influence of the mass distribution on the magnetic field topology
auteur
Raphaël Raynaud, Ludovic Petitdemange, Emmanuel Dormy
article
Astronomy and Astrophysics - A&A, EDP Sciences, 2014, 567 (A107), ⟨10.1051/0004-6361/201423902⟩
resume
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 a different mass distribution. Indeed, it is likely to have as large consequences as taking a stratified reference state into account would, 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.
DOI
DOI : 10.1051/0004-6361/201423902
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https://hal.archives-ouvertes.fr/hal-01122419/file/raynaud2014.pdf BibTex

Conference papers

titre
Influence of the mass distribution on the magnetic field topology
auteur
Raphaël Raynaud, Ludovic Petitdemange, Emmanuel Dormy
article
European GdR Dynamo, Sep 2014, Cambridge, United Kingdom
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titre
Influence of the mass distribution on the magnetic field topology
auteur
Raphaël Raynaud, Ludovic Petitdemange, Emmanuel Dormy
article
UK MHD Meeting, May 2014, Exeter, United Kingdom
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Directions of work or proceedings

titre
Influence of the mass distribution on the magnetic field topology in spherical, anelastic dynamo simulations
auteur
Raphaël Raynaud, Ludovic Petitdemange, Emmanuel Dormy
article
Journées de la SF2A, Jun 2014, Paris, France. 2014
resume
Numerical modelling of convection driven dynamos in the Boussinesq approximation revealed fundamental characteristics of the dynamo-generated magnetic fields. However, Boussinesq models are not adequate for describing convection in stratified systems, and thus the validity of these previous results remains to be assessed for gas giants and stars. To that end, we carried out a systematic parameter study of spherical dynamo models in the so-called anelastic approximation, which allows for a reference density profile while filtering out sound waves for faster numerical integration. We show that the dichotomy of dipolar and multipolar dynamos identified in Boussinesq simulations is still present in anelastic models, and dipolar dynamos require that the typical length scale of convection is an order of magnitude larger than the Rossby radius. However, the established distinction between dipolar and multipolar dynamos tends to be less clear than it was in Boussinesq studies, since we found a large number of models with a considerable equatorial dipole contribution together with an intermediate overall dipole field strength. This tendency can be found in very weakly stratified models, but it was not reported for previous Boussinesq models assuming a homogeneous mass distribution. In contrast, anelastic models usually assume a central mass distribution, which leads to a gravity profile proportional to 1/r**2. Actually, we show that this choice can result in changes in the magnetic field topology that are mainly due to the concentration of convective cells close to the inner sphere.
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https://hal.archives-ouvertes.fr/hal-01126612/file/raynaud_sf2a2014.pdf BibTex

Poster communications

titre
Topology and field strength in spherical, anelastic dynamo simulations
auteur
Raphaël Raynaud, Martin Schrinner, Ludovic Petitdemange, Emmanuel Dormy
article
AGU Fall Meeting, Dec 2014, San Francisco, United States
resume
Dynamo action, i.e. the self-amplification of a magnetic field by the flow of an electrically conducting fluid, is considered to be the main mechanism for the generation of magnetic fields of stars and planets. Intensive and systematic parameter studies by direct numerical simulations using the Boussinesq approximation revealed fundamental properties of these models. However, this approximation considers an incompressible conducting fluid, and is therefore not adequate to describe convection in highly stratified systems like stars or gas giants. A common approach to overcome this difficulty is then to use the anelastic approximation, that allows for a reference density profile while filtering out sound waves for a faster numerical integration. We present the results of a systematic parameter study of spherical anelastic dynamo models, and compare them with previous results obtained in the Boussinesq approximation. We discuss the influence of the stratification on the field geometry and the field strength, and also compare the different scaling laws for the velocity amplitude, the magnetic dissipation time, and the convective heat flux.
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https://hal.archives-ouvertes.fr/hal-02428473/file/poster_agu.pdf BibTex
titre
Spherical, anelastic dynamo simulations
auteur
Raphaël Raynaud
article
Magnetic fields from the Sun to black holes - In memory of Jean Heyvaerts, Nov 2014, Paris, France
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2013

Journal articles

titre
Intermittency in spherical Couette dynamos
auteur
Raphaël Raynaud, Emmanuel Dormy
article
Physical Review Online Archive (PROLA), American Physical Society, 2013, 87 (033011), ⟨10.1103/PhysRevE.87.033011⟩
resume
We investigate dynamo action in three-dimensional numerical simulations of turbulent spherical Couette flows. Close to the onset of dynamo action, the magnetic field exhibits an intermittent behavior, characterized by a series of short bursts of the magnetic energy separated by low-energy phases. We show that this behavior corresponds to the so-called on-off intermittency. This behavior is here reported for dynamo action with realistic boundary conditions. We investigate the role of magnetic boundary conditions in this phenomenon.
DOI
DOI : 10.1103/PhysRevE.87.033011
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Conference papers

titre
The convective approximations
auteur
Raphaël Raynaud
article
Differential Rotation and Magnetism across the HR Diagram, Nordita Scientific Program, Apr 2013, Stockholm, Sweden
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Poster communications

titre
The convective approximations
auteur
Raphaël Raynaud, Ludovic Petitdemange, Emmanuel Dormy
article
European GdR Dynamo, Jul 2013, Ascona, Switzerland
resume
Convection can occur in a shallow layer of fluid with a small temperature contrast across the layer. Thus, it is natural to hope for a simpler description of buoyantly driven flows via an expansion of the fully compressible equations. The Boussinesq approximation performs well in modeling convection in laboratory experiments, but it is however unsatisfactory for large stratified systems, the lower part of which is compressed by the overlying material. More general approximations are then required to describe convection in natural systems like oceans, stars or planetary cores.
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2012

Poster communications

titre
On-off intermittency in spherical Couette flow
auteur
Raphaël Raynaud, Stephan Fauve, Emmanuel Dormy
article
European GDR Dynamo, Oct 2012, Nice, France
resume
We perform 3D numerical simulations of dynamos in a Couette flow generated by two contra-rotating spheres. We show that the magnetic field may display on-off inter- mittency at relatively low magnetic Prandtl number, close to the onset of dynamo ac- tion. The basic signature of this phe- nomenon lies in series of short bursts of the magnetic energy (“on” phases) separated by low energy phases (“off” phases). The length of the “off” phases increases as the system gets closer to the threshold. We successfully compare our observations to predictions based on a canonical model and find it provides an accurate estimate of the threshold.
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https://hal.archives-ouvertes.fr/hal-02428471/file/myposter.pdf BibTex