Morphology of the quiet solar upper atmosphere in the 4 x 104 < Te < 1.4 x 106 K temperature regime

Studies on the morphology of the solar upper atmosphere began over three decades ago. Early models assumed that the temperature structure of the solar upper atmosphere was continuous with a thin transition region connecting the chromosphere with the corona. Over the years it became apparent that the original depiction of the solar upper atmosphere was too simplistic. In this paper we present a morphological study of the solar upper atmosphere over a wide range of temperatures (4 x 10(4) less than or equal to T-e less than or equal to 1.4 x 10(6) K) using high-resolution images (1 "-2 ") taken by TRACE, the SUMER spectrometer on SOHO, and the NRL spectroheliograph on Skylab. The images dearly show that the 4 x 10(4) less than or equal to T-e less than or equal to 1.4 x 10(6) K temperature domain of the solar upper atmosphere consists of a hierarchy of isothermal loop structures. While at the T-e < 8 x 10(5) K temperature regime the looplike structures are more abundant along the chromospheric network, at higher temperatures (T-e > 9 x 10(5) K) no association between them and the chromospheric network is apparent. The hottest (T-e approximate to 1.4 x 10(6) K), which are also the longest among the quiet-Sun loop structures, form a canopy over the lower temperature loop structures. We discuss in the paper possible relationships between the morphology of the solar upper atmosphere, its elemental abundance, and ideas regarding the origin of the slow-speed solar wind.