High resolution spectroscopy of photospheric bright points

Photospheric bright points are proxies for small-scale magnetic flux concentrations. <br>These magnetic elements are thought to be the building blocks of the solar magnetic field <br>and are important to understand many physical processes on the Sun, e.g. the variability of the solar irradiance. <br>Because of their small size (100-200 km in diameter) the observations of photospheric bright points put special requirements on the seeing conditions, instrumental setup and post-processing techniques. <br> <br> The motivation of this work comes from theoretical predictions concerning flows associated with magnetic elements. Solar magneto-convection simulations indicate that the formationof these structures is accompanied with large downflows that can reach velocities of several km/s. In some cases, the downflowing gas may rebound in the deeper layers which can cause <br>an upward-propagating shock. <br>MHD simulations have usually much better resolution that observational data, so advanced observational techniques need to be applied to compare them. <br> <br> Speckle reconstruction techniques are widely used to improve the quality of ground-based solar observations. To date, there have been no comprehensive analysis of photometric errors of the speckle reconstruction process and of the influence of the speckle deconvolution on spectral line profiles. In this thesis, the results of such an analysis are presented, followed by a conclusion that speckle-reconstructed spectral line profiles can be used for quantitative spectroscopy. <br> <br> The data set presented in this thesis consists of speckle-reconstructed G-band images and quasi-simultaneous two-dimensional spectra in the neutral iron line at 557.6 nm. The choice of the G-band - a molecular bandhead at around 430 nm - for observations of photospheric bright points is motivated by their increased contrast with respect to the surroundings at this wavelength range. <br> <br> In this analysis, spectral profiles were used to determine line-of-sight velocities at photospheric bright points locations. I found that the majority of bright points are associated with downflow regions, which is consistent with previous findings. Velocities derived from the line core (formed in higher photospheric layers)are reduced in comparison with velocities of the line wing which is formed lower in the atmosphere, in agreement with some theoretical predictions. Individual cases of photospheric bright points were studied in order to understand their interaction with surrounding granular flow. The maximum downflow usually do not coincides with the maximum G-band intensity. No downflows with velocities of few km/s as predicted by MHD simulations <br>have been found, but it may be due to the lack of sufficient spatial resolution. <br>There were only very few photospheric bright points in the analyzed data which showed upflows with respect to their immediate surroundings.

Author dc:creator

• Mikurda, Katarzyna

Contributors dc:contributor

• Lühe, Oskar von der

• Photosphäre, Photosphärische Bright Points
• Sun, Photosphere, Spectroscopy, Photospheric Bright Points, Speckle Imaging

Repository record source_url

https://freidok.uni-freiburg.de/data/2459

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:2459