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 (
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Abstract T e ) in the corona. DEM inversions have been traditionally performed with collisionally excited lines at wavelengths in the extreme ultraviolet and X-ray. However, such emission is difficult to observe beyond the inner corona (1.5R ⊙), particularly in coronal holes. Given the importance of theT 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 that are excited by photospheric radiation out to at least 3R ⊙. Specifically, we use the Fex (637 nm), Fexi (789 nm), and Fexiv (530 nm) coronal emission lines observed during the 2019 July 2 TSE near solar minimum. We find that, despite a largeT e spread in the inner corona, the distribution converges to an almost isothermal yet bimodal distribution beyond 1.4R ⊙, withT 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 simulation for the 2019 eclipse validates the R-DEM method and yields a similar LOSTe distribution to the eclipse data. -
Abstract We present the spatially resolved absolute brightness of the Fe
x , Fexi , and Fexiv visible coronal emission lines from 1.08 to 3.4R ⊙, 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 Fexi line is found to be the brightest, followed by Fex and Fexiv (in diskB ⊙units). All lines had brightness variations between streamers and coronal holes, where Fexiv exhibited the largest variation. However, Fex 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 inferredT 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 inferredT 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. -
Abstract This letter capitalizes on a unique set of total solar eclipse observations acquired between 2006 and 2020 in white light, Fe
xi 789.2 nm (T fexi= 1.2 ± 0.1 MK), and Fexiv 530.3 nm (T fexiv= 1.8 ± 0.1 MK) emission complemented by in situ Fe charge state and proton speed measurements from Advanced Composition Explorer/SWEPAM-SWICS to identify the source regions of different solar wind streams. The eclipse observations reveal the ubiquity of open structures invariably associated with Fexi emission from Fe10+and hence a constant electron temperature,T c=T fexi, in the expanding corona. The in situ Fe charge states are found to cluster around Fe10+, independently of the 300–700 km s−1stream speeds, referred to as the continual solar wind. Thus, Fe10+yields the fiducial link between the continual solar wind and itsT fexisources at the Sun. While the spatial distribution of Fexiv emission from Fe13+associated with streamers changes throughout the solar cycle, the sporadic appearance of charge states >Fe11+in situ exhibits no cycle dependence regardless of speed. These latter streams are conjectured to be released from hot coronal plasmas at temperatures ≥T fexivwithin the bulge of streamers and from active regions, driven by the dynamic behavior of prominences magnetically linked to them. The discovery of continual streams of slow, intermediate, and fast solar wind characterized by the sameT fexiin the expanding corona places new constraints on the physical processes shaping the solar wind.