PERRI, Barbara (2015) Spontaneous magnetic flux concentrations beneath a radiative stellar surface PRE - Research Project, ENSTA.
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Abstract
The Negative Effective Magnetic Pressure Instability (NEMPI) effect is an alternative theory to flux tubes rising to explain the formation of sunspots. Numerical simulations have demonstrated its efficiency for isothermal and polytropic atmospheres. In order to complete these studies, we use a more realistic model where a polytropic zone is interfaced with a radiative surface, using a Kramers-like opacity law to ensure a smooth transition between the diffusion approximation and the radiative transfer equation. We study whether NEMPI can operate in such conditions and if it is the case, how the generated magnetic field is affected compared to previous studies. We complete the 1D model implemented by Barekat and Brandenburg to have 2D results using the Pencil-Code and MFS. We set the Kramers parameters a and b to 1 and 0 to have a polytropic index of 3/2, which is the most realistic value for hydrogen. For the normalized Kramers opacity factor, we choose a value of 10^7 Mm^-1 cm^3 g^-1 because it gives us the most realistic value for the surface temperature. We observe NEMPI with an expected general behavior but three main differences came unforeseen: not one, but between two and three layers of magnetic structures are to be seen, organized in diagonals; structures seem to get bigger with time and not with depth; there is a threshold value for beta_* (between 0.72 and 0.79) to observe NEMPI. This model is useful to study the impact of radiative transfer on NEMPI, although a slightly unidentified stabilizing effect is suspected to slow and weaken NEMPI more than it should.
Item Type: | Thesis (PRE - Research Project) |
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Uncontrolled Keywords: | Radiative transfer, hydrodynamics , sun atmosphere |
Subjects: | Physics, Optics |
ID Code: | 6572 |
Deposited By: | Barbara Perri |
Deposited On: | 31 janv. 2017 15:56 |
Dernière modification: | 31 janv. 2017 15:56 |
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