The Solar Minimum Eclipse of 2019 July 2: II. The First Absolute Brightness Measurements and MHD Model Predictions of Fe X, XI and XIV out to 3.4 Rs

TL;DR

This study provides the first absolute brightness measurements of Fe X, XI, and XIV lines during the 2019 solar eclipse, comparing observations with MHD model predictions to improve understanding of coronal properties and solar wind origins.

Contribution

It presents the first absolute brightness measurements of specific coronal emission lines during a solar eclipse and compares these with MHD model predictions to refine coronal and solar wind models.

Findings

Fe XI is the brightest emission line observed.

Line brightness varies between streamers and coronal holes, with Fe XIV showing the largest variation.

The MHD model generally predicts the observed line brightnesses well, with some discrepancies in polar coronal holes.

Abstract

We present the spatially resolved absolute brightness of the Fe X, Fe XI and Fe XIV visible coronal emission lines from 1.08 to 3.4 $R_\odot$, observed during the 2019 July 2 total solar eclipse (TSE). The morphology of the corona was typical of solar minimum, with a dipole field dominance showcased by large polar coronal holes and a broad equatorial streamer belt. The Fe XI line is found to be the brightest, followed by Fe X and Fe XIV (in disk $B_\odot$ units). All lines had brightness variations between streamers and coronal holes, where Fe XIV exhibited the largest variation. However, Fe X remained surprisingly uniform with latitude. The Fe line brightnesses are used to infer the relative ionic abundances and line of sight averaged electron temperature ($T_e$) throughout the corona, yielding values from 1.25 - 1.4 MK in coronal holes up to 1.65 MK in the core of streamers. The line brightnesses and inferred $T_e$ values are then quantitatively compared to the PSI Magnetohydrodynamic model prediction for this TSE. The MHD model predicted the Fe lines rather well in general, while the forward modeled line ratios slightly underestimated the observationally inferred $T_e$ within 5 to 10 % averaged over the entire corona. Larger discrepancies in the polar coronal holes may point to insufficient heating and/or other limitations in the approach. These comparisons highlight the importance of TSE observations for constraining models of the corona and solar wind formation.