Solar Transition Region Features Observed with Hinode/EIS

Abstract

Two types of solar active region feature prominent at transition region temperatures are identified in Hinode/EIS data of AR 10938 taken on 2007 January 20. The footpoints of 1 MK TRACE loops are shown to emit strongly in emission lines formed at log T = 5.4-5.8, allowing the temperature increase along the footpoints to be clearly seen. A density diagnostic of Mg VII yields the density in the footpoints, with one loop showing a decrease from 3 × 10⁹ cm^-3 at the base to 1.5 × 10⁹ cm^-3 at a projected height of 20 Mm. The second feature is a compact active region transition region brightening which is particularly intense in O V emission (log T = 5.4) but also has a signature at temperatures up to log T = 6.3. The Mg VII diagnostic gives a density of 4 × 10¹⁰ cm^-3, and emission lines of Mg VI and Mg VII show line profiles broadened by 50kms^-1 and wings extending beyond ± 200kms^-1. Continuum emission in the short wavelength band is also found to be enhanced, and is suggested to be free-bound emission from recombination onto He⁺.