The Inadequacy of Temperature Measurements in the Solar Corona through Narrowband Filter and Line Ratios

Abstract

We analyze the determination of coronal line-of-sight temperatures with the technique of narrowband filter ratios that is currently employed for data obtained with the Transition Region and Coronal Explorer and the EUV Imaging Telescope on board the Solar and Heliospheric Observatory. We demonstrate that the simple fact that the observed differential emission measure curves in coronal loops have a broad plateau everywhere along the length of the loop leads to the finding of isothermal loops with different temperatures for each pair of filters. We show that none of the temperatures thus obtained correctly describe the state of the loop plasma, which instead must be characterized by the full differential emission measure per pixel. We conclude that the recent discovery of a new class of isothermal loops is probably a mere artifact of the narrowband filter ratio method and show that the shift in the location of the plateau in the differential emission measure along the loop indicates significant heating near the loop tops.