Physics
Scientific paper
Aug 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt.......417s&link_type=abstract
Doctoral Thesis, ETH Zurich, 2008
Physics
Magnetic Fields Of The Sun, Polarization, Solar Atmosphere, Radiation Transport
Scientific paper
Coherent scattering in the solar atmosphere leads to the formation of the linearly polarized solar spectrum, just like Rayleigh scattering leads to the polarization of the blue sky. One of the most prominent features of the linearly polarized solar spectrum is the CN violet system as it is also in the unpolarized spectrum. This thesis is devoted to the modeling and interpretation of this system in both spectra and developing it into a very sensitive tool for studying the magnetic fields and the temperature structure of the solar atmosphere.
The understanding of the solar magnetic field structure is very important as it is connected with and even controls most of the solar activity phenomena. Zeeman effect diagnostics allows to measure strong directed magnetic fields which only cover about 1% of the solar atmosphere. The remaining part is occupied by weak entangled magnetic fields with mixed polarity, which might significantly contribute to the overall solar magnetic energy. These fields are invisible to the Zeeman effect due to signal cancellation. Therefore the discovery of the linearly polarized solar spectrum opened a new epoch in solar physics. The polarization due to the scattering processes is modified by weak entangled magnetic fields via the Hanle effect and thus, provides us with a unique possibility to access and study such "hidden" magnetic fields.
Molecular lines are very useful for probing magnetic fields as, due to their strong temperature sensitivity, different molecules sample different, narrow layers of the solar atmosphere. Therefore the extension of the atomic Hanle effect to molecular lines can provide the 3D structure of the solar turbulent magnetic field. Moreover, due to the broad range of magnetic sensitivities within narrow spectral regions molecular lines can be used for employing the differential Hanle effect technique, which allows dramatically reduced model dependence of the obtained magnetic field. This thesis consists of two main parts which reflect the two consecutive steps in the modeling of the polarized solar spectrum. First, the physical properties of the molecular scattering process have to be understood. For the CN violet system, it implies taking into account the Paschen-Back effect on the fine structure (which results in a change of intensities and line positions) and interference effects between the fine structure components itself. Both effects influence the Hanle effect and lead to the consequences which are analyzed in detail in the first part of this thesis.
Then, to connect the coherent scattering and physical properties of the solar plasma with the emerged solar radiation, which is measured by our detectors, radiative transfer theory has to be applied. The presence of the scattering processes implies the non-equilibrium nature of the problem. Its self-consistent solution is especially complicated for molecular bands as they usually consist of a huge number of transitions which couple the numerous vibrational-rotational molecular levels. This, for example, makes the two-level approximation, often used in atomic calculations unsuitable. Moreover, there is a strong lack of information about the molecular collision rates so they basically enter the calculations as additional free parameter. In the second part of the thesis we present two radiative transfer models with different degrees of complexity and applicability. These models allow us to successfully fit the observations of the CN violet system in both polarized and unpolarized spectra and provide us with a magnetic field estimation. We discuss in detail the model-dependence of our results and general problems of the 1D solar spectrum modeling.
The enormous potential of the linearly polarized solar spectrum makes it one of the main tools for studying solar magnetic fields, which becomes very urgent nowadays, as they can affect the Earth's climate and, hence, our life.
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