The middle corona – resonant photoexcitation in EUV lines

ABSTRACT Simple equations are derived with various approximations and are used to calculate the local emissivity of solar coronal lines with the inclusion of resonant photoexcitation (RPE). A simple expression for the scattering factor is obtained. Several incorrect formulations found in previous literature are clarified. Codes to be used within the CHIANTI framework are provided, benchmarked against previous literature, and applied to predict extreme ultraviolet (EUV) emission in the quiet Sun and coronal holes. The RPE is a fundamental process that enhances the radiances of all the strong EUV lines even relatively close to the Sun, and offers a wide range of unexplored diagnostics to probe the middle corona. Some examples are presented to show that information on the electron density and radial outflow can be obtained. EUV observations obtained from the literature [Skylab; Solar and Heliospheric Observatory (SOHO)] or analysed here (Hinode) confirm the predictions from the models.