Solar Transition Region Features Observed with Hinode/EIS

Abstract

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 $3\times 10^9$ cm$^{-3}$ at the base to 1.5$\times$10$^9$ 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 $4\times 10^{10}$ cm$^{-3}$, and emission lines of Mg VI and Mg VII show line profiles broadened by 50 km s$^{-1}$ and wings extending beyond $\pm$200 km s$^{-1}$. Continuum emission in the short wavelength band is also found to be enhanced, and is suggested to be free-bound emission from recombination onto He$^+$.