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 R _⊙

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 ⊙, 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 ⊙ 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 to 1.4 MK in coronal holes and up to 1.65 MK in the core of streamers. The line brightnesses and inferred T e values are then quantitatively compared to the Predictive Science Inc. 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%–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.

Coronal Densities, Temperatures, and Abundances during the 2019 Total Solar Eclipse: The Role of Multiwavelength Observations in Coronal Plasma Characterization

“…The narrowband images are credited to S. Habbal and M. Druckmüller and were provided to J. Samra in a private communication from S. Habbal. It is clear from the Fe XIII and Fe XIV overlays that AIR-Spec sampled hotter material on the east limb than on the west, which agrees with the electron temperature inferred in Boe et al (2022). Section 3.3 provides quantitative support for this conclusion.…”

“…Semiempirical models of the electron temperature in quiescent streamers also indicated nearly constant values (see, e.g., Withbroe 1991). Finally, narrowband images in iron visible lines during eclipses also indicate a nearly constant ionization temperature (see, e.g., Boe et al 2022).…”

“…In Figure 3, we show the AIR-Spec slits overlaid on the narrowband images described in Boe et al (2022). The narrowband images are credited to S. Habbal and M. Druckmüller and were provided to J. Samra in a private communication from S. Habbal.…”

“…We note that other studies of the 2019 eclipse, based on observations in the visible, have been published. Boe et al (2022) presented narrowband images, and Hanaoka et al (2021) used pB continuum images.…”

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

“…Recent developments have proven that EUV lines can be detected out to 2 R e with Proba-2/SWAP (Goryaev et al 2014) and 2.5 R e with GOES/SUVI (Seaton et al 2021), but the EUV lines are difficult to observe beyond 1.5 R e , and the relative contribution of resonant scattering is not as well characterized since its source brightness (i.e., emission from the low corona) has spatial and temporal variation. The visible and infrared coronal lines, on the other hand, can be radiatively excited by photospheric emission and, in turn, can often be observed at helioprojective distances of up to 3.4 R e or more (see Boe et al 2022).…”