The Solar Minimum Eclipse of 2019 July 2. III. Inferring the Coronal $T_e$ with a Radiative Differential Emission Measure Inversion

Abstract

Differential Emission Measure (DEM) inversion methods use the brightness of a set of emission lines to infer the line-of-sight (LOS) distribution of the electron temperature ($T_e$) in the corona. DEM inversions have been traditionally performed with collisionally excited lines at wavelengths in the Extreme Ultraviolet (EUV) and X-ray. However, such emission is difficult to observe beyond the inner corona (1.5 R$_\odot$), particularly in coronal holes. Given the importance of the $T_e$ distribution in the corona for exploring the viability of different heating processes, we introduce an analog of the DEM specifically for radiatively excited coronal emission lines, such as those observed during total solar eclipses (TSEs) and with coronagraphs. This Radiative DEM (R-DEM) inversion utilizes visible and infrared emission lines which are excited by photospheric radiation out to at least 3 R$_\odot$. Specifically, we use the Fe X (637 nm), Fe XI (789 nm), and Fe XIV (530 nm) coronal emission lines observed during the 2019 July 2 TSE near solar minimum. We find that despite a large $T_e$ spread in the inner corona, the distribution converges to an almost isothermal yet bimodal distribution beyond 1.4 R$_\odot$, with $T_e$ ranging from 1.1 to 1.4 in coronal holes, and from 1.4 to 1.65 MK in quiescent streamers. Application of the R-DEM inversion to the Predictive Science Inc. magnetohydrodynamic (MHD) simulation for the 2019 eclipse validates the R-DEM method and yields a similar LOS Te distribution to the eclipse data.