Studies on the solar chromosphere network a thesis submitted for the award of degree of doctor of philosophy in the faculty of science, Indian Institute of Scinece, Bangalore R. Srikanth [Ph.D Thesis]

Thesis Supervisor Prof. Jagdev Singh

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1998

This thesis is a study of morphological and dynamical properties of the chromospheric network cells. It investigates how the size, lifetime and dynamics of cells are related to each other and to activity level. The data for the observations consisted of an almost uninterrupted, time-series of Ca II K filtergrams obtained in 1989-1990 local summer in Antartica, and of spectroheliograms from Kodaikanal Solar Observatory.
Morphological properties of individual cells like size, shape and intensity contrast have been studied by visual inspection of individual cells. Shape of cells was quantified using the concept of area compactness of cells. It was found that smaller cells tend to have a regular polygonal or circular shape. We also found that the relative maximum contrast of intensity for larger cells tends to be greater. The mean intensity contrast in the cells is shown to be about 14%. The length-scale derived by direct inspection serves as a useful cross-check against estimates obtained by more complicated techniques like autocorrelation and tessellation.
Length-scales of network cells has been derived using the autocorrelation technique. The dependence of length-scales on latitude was studied using data from the Kodiakanal solar observatory spectroheliograms obtained during solar minima in the 1914-1974 period. The scales show a North-South symmetry. Under a. wide-bin latitude averaging, there also emerges a pole-ward fall in cell sizes. A simple physical model is proposed to incorporate both rotational and magnetic influence on cell sizes. Using peak width of the autocorrelation function as a measure of cell-wall thickness, we found that there was no correlation between thickness of walls and cell size.
Lifetime of cells was derived using cross-correlation technique. It is noted that the cross-correlation function does not fall monotonously, but is sometimes marked by small undulations, whose origins are as yet not clear. The influence of activity level on lifetimes was studied. We found that lifetimes are considerably longer near active regions. The cross-correlation and auto-correlation functions were used to estimate the diffusion rate of chromospheric fine-structures in quiet and enhanced network regions. The longer lifetimes near active regions is attributed to smaller diffusion rates in these regions.
The relationship between cell lifetimes and sizes has been studied using data obtained from 82 windows in the quiet Sun. The functional form of the relation has been determined by comparing the frequency functions of the two parameters. We found that lifetimes vary linearly with cell area, suggesting that diffusion process is responsible for the evolution of cells. The diffusion constant derived from this relation has been shown to be compatible with the diffusion rate for fluxtubes derived earlier.
The "steepest descent" algorithm has been used to tessellate the quiet chromosphere region. The algorithm minimizes the intensity I along the direction of the minimum gradient. The properties of the tiles derived in this manner have been studied. The geometric relation between these tiles and Ca K network is considered. It is shown that the tiles correspond not to cells but often to discrete mangetic elements.