On the nature of the transition region from the chromosphere to the coronaof the Sun

One of the keys to understand coronal heating is to understand the (magneti c) structure of the atmosphere below the respective coronal features. The E UV emission lines formed in the thin transition region from the chromospher e to the corona present a sensitive tool to study the structure and dynamic s of the lowermost corona and its connection to the chromosphere. Data from the SUMER spectrograph on SOHO show for the first time that broad componen ts are a common feature of emission line profiles formed from some 40 000 K to 10(6) K. The contribution of that tail component to the total intensity of the line exhibits a trend with line formation temperature that peaks in the middle transition region with smaller contribution at high and low tem peratures. The line width of the tail component shows a monotonic increase with temperature that is consistent with a passing Alfven wave, which is in contrast to the trend in width of the line core. Together with previous ob servations this presents evidence that the line core and the tail component are formed in radically different physically regimes. It is proposed that these are small closed loops and coronal funnels, respectively, with the la tter being the footpoints of large coronal loops. The new results on the st ructuring of the transition region will improve understanding on heating th e corona and accelerating the solar wind.