High-resolution observations of the solar photosphere and chromosphere

Details High-resolution observations of the solar photosphere and chromosphere High-resolution observations of the solar photosphere and chromosphere

Mar 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007phdt.........6w&link_type=abstract

PhDT, 1

Physics

Optics

2

Solar Physics, Atmospheric Turbulence, Adaptive Optics

Scientific paper

Observations of the sun are almost always impaired by the turbulent motion of air in Earth's atmosphere. The turbulence would limit the theoretical resolution of modern large telescopes to that of amateur telescopes without additional tools. Today however, high-resolution data of the Sun are necessary to invesitgate its small-scale structure. This structure is likely to be connected to the radially outward increasing temparature distribution of the solar atmosphere.
An introduction into further details of this topic that has also been the motivation for this work is presented in Chapt. 1. A theory of atmospheric turbulence that builds the basis for several results of this work is described in Chapt. 2. Here, two modern tools to enhance the resolution of groundbased observations are reviewed, on the one hand adaptive optics (AO) systems and on the other hand speckle interferometry. Until recently, these two techniques were only used separately. In Chapt. 3 the necessary modifications for analytical models of transfer functions are developed that include the changes made by an AO system to the incoming wave front, thus making a combination of AO systems and speckle interferometry possible. The models were compared to measured data using different techniques, and a good agreement was found. In order to apply speckle interferometry to the observational data acquired for this work, a computer program package was developed that can reduce vast amount of data within a reasonable time in a parallel way (App. 1).
Speckle interferometry needs very shortly exposed data in order to compute a reconstruction. However, a part of the data observed for this work had to be exposed rather long because of technical problems, making the use of this reconstruction technique impossible. This motivated the development of an algorithm to estimate instantaneous point spread functions from speckle reconstructions. The point spread functions permit the deconvolution of the long exposed data making use of well known techniques. The algorithm is developed in Chapt. 4, along with a presentation of an examination of usability.
In Chapt. 5 the observational data that were reduced using the algorithms developed in the course of this work were analyzed. It was found that bright points within the chromospheric network are correlated both spatially and temporally to those in the photospheric network. The phenomena appear to overlay almost vertically. The ratio of their sizes is = 3.0 with a standard deviation of 0.7. The analysis of life times of structures within the chromosphere revealed that network and inter-network regions can be separated more accurately using a life time rather than the commonly used intensity criterium. The combination of high spectral and spatial resolution within this dataset revealed the existance of an up to now undetected pattern of granular size in the chromspheric inter-network that evolves too rapidly (with time scales of approx. 53s) to be reversed granulation. This finding supports recent models of the non-magnetic solar chromosphere that could explain this pattern as signature of propagating and interacting shock waves that are excited in the photosphere as an acoustic phenomenon. This is supported by the detailed investigation of the solar oscillations in the chromospheric network and inter-network that shows that the main contributions to the 3min oscillations in the chromosphere can be attributed to the inter-network. The chromospheric network mainly contributes to 5min oscillations, which are typical for the photosphere.

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