Multi-component structure of the solar transition region

Observations with the SUMER spectrograph onboard SOHO allowed to deconvolve the emission from small coronal loops in the network and coronal funnels that form the base of the large scale corona and the solar wind. This was done by fitting the observed line profiles by two Gaussians. As it turned out, one of the Gaussians is much broader than the other one and the relation of line shift and width to line intensity is very different for the two components. The latter result indicates that the two source regions are heated by different mechanisms. The variation of the line width of the emission from the coronal funnels is increasing with line formation (or electron) temperature. This might be interpreted either as a strong non-linear wave passing the transition region or it might indicate that the ions are much hotter that the electrons, even deep in the transition region. The latter interpretation would be along the lines of the UVCS observations above the limb giving several hundred million K for the ions. Further support for high ion temperatures deep in the transition region is provided by recent kinetic models for the transition region and solar wind. This new spectral technique to deconvolve emission from different atmospheric regions holds promise to be valuable for future EUV spectroscopic observations, like onboard the Solar Orbiter.