Photometric Properties Of Complex Magnetic Elements In The Solar Photosphere
Creators: name: Criscuoli, Serena
Publisher: name: Zenodo
Year: 2007
Subjects: Solar Photosphere; Solar magnetism; Magnetic flux tube numerical model; Irradiance; Fractal dimension

description: In this thesis I investigate the photometric and geometric properties of bright<br>
magnetic features in the lower solar atmosphere. The contribution of these features<br>
to Total Solar Irradiance (TSI here after) variations observed at different<br>
temporal scales has been broadly showed during the last years. Nevertheless,<br>
measurements and theoretical investigations of their properties, on which reconstructions<br>
of TSI variations are based, have produced discrepant results.<br>
In order to interpret discrepancies presented in the literature and to improve<br>
our understanding of physical properties of magnetic elements, both experimental<br>
and theoretical aspects have been investigated.<br>
In the first part of the thesis I show results obtained by the analysis of full<br>
disk PSPT broad band images from Rome and Hawaii. Geometric properties<br>
and the possible connection with photometric properties have been investigated<br>
through the measurement of fractal dimension of features observed in chromosphere.<br>
Results I obtain agree very well with the ones presented in the literature<br>
carried out on similar data and with the same fractal dimension estimator. The<br>
fractal dimension increases in fact with features area and reaches a plateau at<br>
areas larger than about 1000-2000 Mm2. Nevertheless, by the analyses of images<br>
of fractals whose dimension is known by the theory, I show that fractal<br>
dimension estimation is critically effected by pixelization, technique employed<br>
to select magnetic structures on images and resolution. In particular the increase<br>
of fractal dimension with object size is an effect of pixelization and thus<br>
some conclusions previously drawn in the literature should be revisited.<br>
Photometric properties are investigated by the analyses of contrast of identified<br>
features in two photospheric bands and in the chromosphere. In particular<br>
the variation of the contrast with position on the solar disk and with object<br>
size is investigated. I show that the contrast in the chromosphere is not dependent<br>
on disk position and that in the photosphere monotonically increases from<br>
the center toward the limb. A comparison with previously published results<br>
shows a better agreement with authors that employed an identification methods<br>
similar to the ones I employed to select magnetic features on images. The<br>
contrast, especially at the limb, is also critically affected by seeing. Comparison<br>
of the scaling of average and maximum contrast with object size suggests that<br>
the smaller magnetic elements, whose clustering forms the features analyzed,<br>
are characterized by different photometric properties. The increase of average<br>
contrast with object size, very similar to the increase observed for the fractal<br>
dimension, is instead an effect of filling factor.<br>
In order to investigate the physical origin of the results and validate some<br>
of the conclusions drawn, 2D numerical codes based on the magnetic flux tube<br>
model have been developed. Plane parallel gray atmosphere in LTE is supposed<br>
and radiative and convective energy transport mechanisms have been taken into<br>
account. In particular two classes of models are investigated. In the first one<br>
convection is modelled by the Mixing length theory and radiation by the radiative<br>
diffusion approximation. In the second one only radiation is taken into<br>
account, but radiative diffusion approximation is dropped and radiative equilibrium<br>
is imposed by an iterative scheme. The presence of the magnetic field<br>
is mimicked by imposing a lower pressure and density in the magnetic region.<br>
In order to evaluate the radiation field a numerical code, based on the short<br>
characteristic technique, was developed. A detailed description of the code,<br>
as well as results obtained by tests aimed to investigate and compare different<br>
numerical techniques and spurious effects, are presented. The radiative flux is<br>
finally evaluated by a quadrature scheme. At this aim two schemes have been<br>
developed and compared. The software developed has allowed to investigate<br>
radiation field through the flux tube models studied. I show that the presence<br>
of a magnetic structure generates areas of different shapes and contrast around<br>
it. These features vary with the position of the structures on the solar disk (the<br>
sight angle) and have spatial scales smaller than the typical scale of a flux tube<br>
(about 100 km), so resolution better than 0.1 arcsec is required to observe them.<br>
The contrast of magnetic features varies also in function of the optical depth, so<br>
that for the same model different center to limb variations of the contrast can<br>
be observed. This indicates that when observing magnetic structures at different<br>
wavelengths the contrast can be very different, thus partially explaining the<br>
discrepant results obtained in the literature. Investigation of the results also<br>
shows that the center to limb variation of the contrast reflects the temperature<br>
stratification inside and outside the tube. Measurements carried out at different<br>
wavelengths are thus fundamental for the determination of temperature of<br>
magnetic structures and for the investigation of their physical properties.<br>
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