Properties of the quiet Sun EUV network

Abstract

Observations of the quiet Sun network in a small region at Sun centre taken with the Coronal Diagnostic Spectrometer (CDS) on board SOHO are reported for EUV lines with T_e between ~ 10(4) and 10(6) K. The changing structure of the network in the upper chromosphere, transition region, up to the corona was examined using intensity distributions which were decomposed into two normal components using a mixture-modelling technique. This enabled areas of high and low intensities to be separated, and hence averaged network properties including area, intensity, contrast, and fractal dimension to be derived as a function of ion temperature. The network area and emission were found to be more concentrated in the transition region than in the chromosphere and in the corona, although the results for the chromospheric He lines appear to be affected by resonance scattering. At ~ 10(6) K, the area and emission of bright structures dramatically increase, partly due to the appearance of small coronal loops. There is also a discrete change in the fractal dimension at coronal temperatures, signifying a change from network to simpler coronal structures. Furthermore, the contrast of bright to dark regions is at a maximum for T_e ~ 2.5x 10(5) K and falls to its lowest values for coronal temperatures. The properties of several individual network structures were found to follow the same general behaviour as in the statistical analysis. Our results including physical dimensions are broadly consistent with the transition region model of Gabriel, although we cannot exclude the existence of low-lying loops as in the model of Dowdy et al.