Magnetic flux generation and transport in cool stars
Başlık çevirisi mevcut değil.
- Tez No: 400216
- Danışmanlar: PROF. DR. FRANZ KNEER, PROF. DR. MANFRED SCHÜSSLER
- Tez Türü: Doktora
- Konular: Bilim ve Teknoloji, Fizik ve Fizik Mühendisliği, Science and Technology, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2007
- Dil: İngilizce
- Üniversite: Georg-August-Universität Göttingen
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Fizik Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 107
Özet
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Özet (Çeviri)
Magnetic activity in stars with outer convection zones, such as the Sun, results from theinteraction of turbulent convection, large-scale flows, and magnetic fields. In this dissertation,I consider problems concerning the generation, storage, emergence, and surfacetransport of solar and stellar magnetic flux.? Surface flux transport simulations have been carried out to investigate the effectsof differential rotation and meridional flow on the lifetimes of bipolar magnetic regionson dwarf and subgiant stars, in order to study the structure and evolution ofstarspots. It is found that the lifetime does not change considerably with emergencelatitude, rotational shear, and meridional flow, but depends strongly on the tilt anglesof the emerging bipolar magnetic regions, i.e., on the angle between the lineconnecting the two regions of opposite polarity and the local latitudinal circle. Asustained emergence of bipolar magnetic regions at mid-latitudes can lead to theformation of long-lived polar spots on stars.? Magnetic flux storage at the bottom of the solar convection zone has been studiedby analytical calculations and numerical simulations of toroidal magnetic flux tubesunder the influence of perpendicular flows. I have considered the problem whetherflux tubes can be stored for times of the order of years under the effects of perpendicularand longitudinal flows in the convective overshoot region. First, relationsbetween the flow velocity and the resulting displacement and deformation of a fluxtube have been obtained as a function of flow and tube parameters. Second, thedependence of the nonlinear Parker instability and the friction-induced instabilityon the perturbation amplitude have been investigated by numerical simulations. Acomparison of the simulations with the analytical results indicates that a flux tubewith a radius of 1000 km and field strength 7 · 104 G can be stored in the bottom ofthe convection zone for times on the order of a few years, if the convective velocitiesare of the order of 10 m s?1.? A coupled model of magnetic flux generation and transport in cool stars has beendeveloped in order to test solar and stellar dynamo models by comparison withobservations, and to gain a better understanding of the relationships between thephysical processes involved. The model consists of three components: 1) field generationby hydromagnetic dynamo action at the bottom of the convection zone, 2)instability and emergence of magnetic flux tubes from the dynamo layer, 3) surfacetransport of the emerged flux under the influence of differential rotation, meridionalflow, and turbulent diffusion. The combined model has been applied first to theSun, using the large-scale flow pattern in the solar interior and at the surface. It isfound that the dynamo waves at the bottom of the convection zone and the surfaceemergence pattern match, because the deflection of rising flux tubes is small in arelatively slow rotator such as the Sun. This result supports an implicit assumptionthat is often made when interpreting solar dynamo models, namely that the surfaceactivity pattern reflects the deep-seated dynamo wave pattern.? The combined model has been applied to stars of solar structure which rotate 3 and13 times faster than the Sun. For such rapidly rotating stars, it is found that thedifference between the magnetic field patterns in the deep convection zone and onthe surface can become quite significant. This stresses the importance of consideringemergence and surface transport processes jointly with stellar dynamo models.It turns out that, for rapidly rotating stars, a cyclic dynamo at the bottom of theconvection zone may lead to a non-cyclic surface activity. This is caused by acombination of strong overlapping between consecutive cycles, large tilt angles ofemerging flux tubes, and surface flux transport. The combined model developedhere can be easily extended to study magnetic activity in stars in a wide range ofrotation, mass, and evolutionary state.
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