3D MHD simulations of subsurface convection in OB stars

During their main sequence evolution, massive stars can develop convective regions very close to their surface. These regions are caused by an opacity peak associated with iron ionization. Cantiello et al. (2009) found a possible connection between the presence of sub-photospheric convective motions and small scale stochastic velocities in the photosphere of early-type stars. This supports a physical mechanism where microturbulence is caused by waves that are triggered by subsurface convection zones. They further suggest that clumping in the inner parts of the winds of OB stars could be related to subsurface convection, and that the convective layers may also be responsible for stochastic excitation of non-radial pulsations. Furthermore, magnetic fields produced in the iron convection zone could appear at the surface of such massive stars. Therefore subsurface convection could be responsible for the occurrence of observable phenomena such as line profile variability and discrete absorption components. These phenomena have been observed for decades, but still evade a clear theoretical explanation. Here we present preliminary results from 3D MHD simulations of such subsurface convection.

Rekonnektio Maan magnetosfäärissä

Tässä väitöskirjassa perehdytään magneettisen rekonnektion ilmenemismuotoihin ja vaikutuksiin Maan magnetosfäärissä. Keskeisenä tutkimusvälineenä käytetään magnetohydrodynaamista (MHD) Gumics-magnetosfäärisimulaatiota. Työssä kehitetään myös uusia menetelmiä simulaatiossa ilmenevän rekonnektion tunnistamiseksi ja mittaamiseksi. MHD-simulaatio sopii suuren mittakaavan ilmiöiden tarkasteluun, joten kuvaa rekonnektiosta täydennetään pienen mittakaavan piirteiden osalta Cluster-satelliittien avulla. Tärkein tutkimuksen tuoma edistysaskel menetelmien saralla on rekonnektioviivan paikallistaminen topologisesti erityyppisten magneettikenttäviivojen alueiden liitoskohdassa olevana erottajaviivana neljän kentän tienoon menetelmää käyttäen. Tämä topologinen lähestymistapa on hyödyllinen erityisesti magnetopausilla, jonka monimutkainen geometria tekee magneettikentän paikalliseen käyttäytymiseen perustuvien rekonnektioviivan etsintätapojen soveltamisen hankalaksi. Topologisesti määritelty rekonnektioviiva on myös helppo tunnistaa magnetosfäärin globaalin konvektion solmukohdaksi. Magnetopausin rekonnektioviivan käyttäytyminen Gumicsissa noudattaa komponenttirekonnektio-olettamaan pohjautuvia teoreettisia ennusteita. Rekonnektion kvantitatiivinen tarkastelu Gumics-simulaatiossa perustuu energian muuntumiseen, joka lasketaan Poyntingin vektorin divergenssinä tai Poyntingin vuona valitun umpinaisen pinnan läpi. Rekonnektioon liittyvän energian muuntumisen jakautumista magnetopausilla tarkastellaan energian muuntumisen pintatiheyden avulla ja rekonnektion kokonaismäärää rekonnektiotehon avulla. Magnetopausin ja pyrstön rekonnektiotehot ovat simulaatiossa samaa suuruusluokkaa. Tärkeimmät magnetopausin rekonnektiotehoa säätelevät parametrit ovat aurinkotuulen nopeus ja aurinkotuulen magneettikentän suunta. Magnetopausin rekonnektio puolestaan säätelee energian ja aineen pääsyä magnetosfääriin, joskaan magnetopausin läpäisevät vuot eivät ole aivan suoraan verrannollisia rekonnektiotehoon. Pyrstön rekonnektioteho sen sijaan on suoraan verrannollinen magnetopausilta tulevaan energiavuohon; pyrstörekonnektio Gumicsissa on siis ulkoista pakotetta seuraava passiivinen energian käsittelijä. Simulaation tuottama rekonnektio on realistinen magnetosfäärin globaalissa mittakaavassa tarkasteltuna, mutta satelliittihavainnot paljastavat rekonnektiosta simulaation erottelukykyä pienimittakaavaisempia piirteitä. Havaintopuolella tämän väitöstutkimuksen tärkein löytö on protonien diffuusioalueen rakenteeseen kuuluvien Hallin kenttien kääntyminen pyrstön virtalevyn aaltoilun mukana.

Local numerical modelling of magnetoconvection and turbulence - implications for mean-field theories

During the last decades mean-field models, in which large-scale magnetic fields and differential rotation arise due to the interaction of rotation and small-scale turbulence, have been enormously successful in reproducing many of the observed features of the Sun. In the meantime, new observational techniques, most prominently helioseismology, have yielded invaluable information about the interior of the Sun. This new information, however, imposes strict conditions on mean-field models. Moreover, most of the present mean-field models depend on knowledge of the small-scale turbulent effects that give rise to the large-scale phenomena. In many mean-field models these effects are prescribed in ad hoc fashion due to the lack of this knowledge. With large enough computers it would be possible to solve the MHD equations numerically under stellar conditions. However, the problem is too large by several orders of magnitude for the present day and any foreseeable computers. In our view, a combination of mean-field modelling and local 3D calculations is a more fruitful approach. The large-scale structures are well described by global mean-field models, provided that the small-scale turbulent effects are adequately parameterized. The latter can be achieved by performing local calculations which allow a much higher spatial resolution than what can be achieved in direct global calculations. In the present dissertation three aspects of mean-field theories and models of stars are studied. Firstly, the basic assumptions of different mean-field theories are tested with calculations of isotropic turbulence and hydrodynamic, as well as magnetohydrodynamic, convection. Secondly, even if the mean-field theory is unable to give the required transport coefficients from first principles, it is in some cases possible to compute these coefficients from 3D numerical models in a parameter range that can be considered to describe the main physical effects in an adequately realistic manner. In the present study, the Reynolds stresses and turbulent heat transport, responsible for the generation of differential rotation, were determined along the mixing length relations describing convection in stellar structure models. Furthermore, the alpha-effect and magnetic pumping due to turbulent convection in the rapid rotation regime were studied. The third area of the present study is to apply the local results in mean-field models, which task we start to undertake by applying the results concerning the alpha-effect and turbulent pumping in mean-field models describing the solar dynamo.

On global solar dynamo simulations

Global dynamo simulations solving the equations of magnetohydrodynamics (MHD) have been a tool of astrophysicists who try to understand the magnetism of the Sun for several decades now. During recent years many fundamental issues in dynamo theory have been studied in detail by means of local numerical simulations that simplify the problem and allow the study of physical effects in isolation. Global simulations, however, continue to suffer from the age-old problem of too low spatial resolution, leading to much lower Reynolds numbers and scale separation than in the Sun. Reproducing the internal rotation of the Sun, which plays a crucual role in the dynamo process, has also turned out to be a very difficult problem. In the present paper the current status of global dynamo simulations of the Sun is reviewed. Emphasis is put on efforts to understand how the large-scale magnetic fields, i.e. whose length scale is greater than the scale of turbulence, are generated in the Sun. Some lessons from mean-field theory and local simulations are reviewed and their possible implications to the global models are discussed. Possible remedies to some of the current issues of the solar simulations are put forward.