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1. Identifying the Coronal Source Regions of Solar Wind Streams from Total Solar Eclipse Observations and in situ Measurements Extending over a Solar Cycle

   https://doi.org/10.3847/2041-8213/abe775  

   Habbal, Shadia R. ; Druckmüller, Miloslav ; Alzate, Nathalia ; Ding, Adalbert ; Johnson, Judd ; Starha, Pavel ; Hoderova, Jana ; Boe, Benjamin ; Constantinou, Sage ; Arndt, Martina ( April 2021 , The Astrophysical Journal Letters)

   This letter capitalizes on a unique set of total solar eclipse observations acquired between 2006 and 2020 in white light, Fexi789.2 nm (T_fexi= 1.2 ± 0.1 MK), and Fexiv530.3 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 Fexiemission 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 Fexivemission 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.

    

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2. 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 ⊙

   https://doi.org/10.3847/1538-4357/ac8101  

   Boe, Benjamin ; Habbal, Shadia ; Downs, Cooper ; Druckmüller, Miloslav ( August 2022 , The Astrophysical Journal)

   We present the spatially resolved absolute brightness of the Fex, Fexi, and Fexivvisible 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 Fexiline is found to be the brightest, followed by Fexand Fexiv(in diskB_⊙units). All lines had brightness variations between streamers and coronal holes, where Fexivexhibited the largest variation. However, Fexremained 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_evalues 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_ewithin 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.

    

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

   https://doi.org/10.3847/1538-4357/acd10b  

   Boe, Benjamin ; Downs, Cooper ; Habbal, Shadia ( July 2023 , The Astrophysical Journal)

   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 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_edistribution 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_espread in the inner corona, the distribution converges to an almost isothermal yet bimodal distribution beyond 1.4R_⊙, withT_eranging 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 LOST_edistribution to the eclipse data.

    

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4. Early results from the solar-minimum 2019 total solar eclipse

   https://doi.org/10.1017/S1743921320001453  

   Pasachoff, Jay M. ; Lockwood, Christian A. ; Inoue, John L. ; Meadors, Erin N. ; Voulgaris, Aristeidis ; Sliski, David ; Sliski, Alan ; Reardon, Kevin P. ; Seaton, Daniel B. ; Caplan, Ronald M. ; et al ( June 2019 , Proceedings of the International Astronomical Union)

   null (Ed.)

   Abstract We observed the 2 July 2019 total solar eclipse with a variety of imaging and spectroscopic instruments recording from three sites in mainland Chile: on the centerline at La Higuera, from the Cerro Tololo Inter-American Observatory, and from La Serena, as well as from a chartered flight at peak totality in mid-Pacific. Our spectroscopy monitored Fe X, Fe XIV, and Ar X lines, and we imaged Ar X with a Lyot filter adjusted from its original H-alpha bandpass. Our composite imaging has been compared with predictions based on modeling using magnetic-field measurements from the pre-eclipse month. Our time-differenced sites will be used to measure motions in coronal streamers. 

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5. Possible Evidence for Shear-driven Kelvin–Helmholtz Instability along the Boundary of Fast and Slow Solar Wind in the Corona

   https://doi.org/10.3847/1538-4357/ac5cc3  

   Telloni, Daniele ; Adhikari, Laxman ; Zank, Gary P. ; Zhao, Lingling ; Sorriso-Valvo, Luca ; Antonucci, Ester ; Giordano, Silvio ; Mancuso, Salvatore ( April 2022 , The Astrophysical Journal)

   Abstract This paper reports the first possible evidence for the development of the Kelvin–Helmholtz (KH) instability at the border of coronal holes separating the associated fast wind from the slower wind originating from adjacent streamer regions. Based on a statistical data set of spectroscopic measurements of the UV corona acquired with the UltraViolet Coronagraph Spectrometer on board the SOlar and Heliospheric Observatory during the minimum activity of solar cycle 22, high temperature–velocity correlations are found along the fast/slow solar wind interface region and interpreted as manifestations of KH vortices formed by the roll-up of the shear flow, whose dissipation could lead to higher heating and, because of that, higher velocities. These observational results are supported by solving coupled solar wind and turbulence transport equations including a KH-driven source of turbulence along the tangential velocity discontinuity between faster and slower coronal flows: numerical analysis indicates that the correlation between the solar wind speed and temperature is large in the presence of the shear source of turbulence. These findings suggest that the KH instability may play an important role both in the plasma dynamics and in the energy deposition at the boundaries of coronal holes and equatorial streamers. 

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