Atmospheric seismology of the Sun with the HELLRIDE Instrument at the Vacuum Tower Telescope (VTT)

This doctoral dissertation is primarily focused on the study of the solar atmosphere, the outermost part of our Star. The main goal of this work is to improve our understanding of the solar atmospheric structure using helioseismic methods. The research conducted for this thesis is based primarily on sound waves generated inside the Sun. The presented results shed new light on the problem of determining limiting frequency of acoustic waves and uncovers some new view on the current understanding of solar flares.

This thesis is focused on helioseismology and methods of data analysis. As part of this scientific study, it was necessary to obtain relevant data, namely spectrometric measurements of the solar surface that allow to determine the solar eigen-oscillations. This was accomplished using high spatially resolved spectroscopic observations, taken with the specifically developed multi-wavelength interferometer HelLRIDe at the Vacuum Tower Telescope (VTT), where the local wave field above the solar surface is studied. The schematic set-up of the HelLRIDe spectrometer and the basic principles of obtaining measurements are discussed, as well as examples of measured data are presented.

Observations were made for different levels of the solar atmosphere. In practice, such measurements are achieved for different ranges in the solar spectrum, using narrow-band optical filters for this purpose. This means that each filter allows observation of slightly different spectral lines excited in different heights above the Sun’s surface. It is therefore highly important to accurately determine the height of the formation of the spectral line in which the measurement is carried out. The methods and results for individual wavelengths are discussed in this thesis.

Currently, the technology of simultaneous measurement of many wavelengths has become very popular. The measurement uniqueness is based on a large number of spectral lines (from 1 to 21, and in most measurements 9) combined with high spatial resolution and short cadence (5 to 60 seconds). Additionally, at least three different magnetic regions of the Sun were subjected to probing. It is worth mentioning that the measurements cover not only the solar photosphere from a height of about 40 km, but also reach the high chromosphere layers up to about 2000 km above the solar surface. This allowed accurate investigation of the propagation of acoustic waves between these layers.

The research results shed new light on the physical models of the solar atmosphere by examining existing theoretical models against observational data. Based on the compilation of five theoretical models of solar atmospheric and observational results, none of the previous models presented were fully consistent with the curve obtained experimentally. With the measurement data collected, we also manage to analyze the magnetic active areas such as a sunspot and a small pore.

Finally we investigate a solar flare and the properties of acoustic waves and their dominant frequencies. Measurements for seven levels in atmosphere were taken, which allow the production of a three-dimensional analysis of the solar flare and the early phase of chromospheric loop formation. One of the key results was an accurate spatial distribution map of the oscillations ranging from 3 to 14 mHz, within seven atmospheric heights in the flare region and in the surrounding plasma. The results obtained, built a new context of the occurrence of the flare phenomenon and its mechanisms.