Diagnostics of Suprathermal Electrons in Active-Region Plasmas Using He-like UV Lines

Abstract

In the present paper we use UV lines emitted by He-like ions as a tool to test the presence of nonthermal high-energy electrons and to quantify their number and energy. The He-like lines we consider are the He-like 1s2s³S-1s2p³P lines observed in the UV, and their use capitalizes on the high excitation energies for the 1s2p³P levels and on the near-unity ion abundances of the He-like ions for large temperature ranges. We investigate the presence of nonthermal high-energy electrons in solar active regions, placing upper limits on the presence of electrons capable of exciting the upper levels of Ne IX, Mg XI, and Si XIII.