Off-limb EUV line profiles and the search for wave activity in the low corona

Abstract

Two extreme-ultraviolet (EUV) observations are used to examine the off-limb characteristics of emission line widths of the million K Mg X 625 Å line. To obtain sufficient statistical accuracy, the observations were made over long periods of time and a pixel summing technique is used. The observations are made above the western limb, for quiet corona. The most significant result is the discovery of emission line narrowing as a function of altitude and intensity at altitudes above 50 000 km. The results are compared to past observations, which suggest that emission line broadening, at lower altitudes, is due to the outward propagation of undamped Alfvén waves, in open field regions with decreasing density with altitude. The narrowing at higher altitudes, determined by the current study is interpreted as further evidence for coronal wave activity, but in closed field regions, and most likely the first evidence of the dissipation of Alfvén waves in the corona. An additional result of this work is the identification of a significant line broadening across a narrow region on the limb, which may be due to flows in low-lying loop systems.