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

Abstract

One of the keys to understand coronal heating is to understand the (magnetic) structure of the atmosphere below the respective coronal features. The EUV emission lines formed in the thin transition region from the chromosphere to the corona present a sensitive tool to study the structure and dynamics of the lowermost corona and its connection to the chromosphere. Data from the SUMER spectrograph on SOHO show for the first time that broad components are a common feature of emission line profiles formed from some 40 000 K to 10⁶ 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 temperatures. The line width of the tail component shows a monotonic increase with temperature that is consistent with a passing Alfvén wave, which is in contrast to the trend in width of the line core. Together with previous observations 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 latter being the footpoints of large coronal loops. The new results on the structuring of the transition region will improve understanding on heating the corona and accelerating the solar wind.