Abstract
Context. The origin of the high temperature of the solar corona, in both the inner bright parts and the more outer parts showing flows toward the solar wind, is not understood well yet. Total eclipses permit a deep analysis of both the inner and the outer parts of the corona using the continuum white-light (W-L) radiations from electrons (K-corona), the superposed spectrum of forbidden emission lines from ions (E-corona), and the dust component with F-lines (F-corona). Aims. By sufficiently dispersing the W-L spectrum, the Fraunhofer (F) spectrum of the dust component of the corona appears and the continuum Thomson radiation can be evaluated. The superposed emission lines of ions with different degrees of ionization are studied to allow the measurement of temperatures, non-thermal velocities, Doppler shifts, and abundances to constrain the proposed heating mechanisms and understand the origin of flows that lead to solar wind. Methods. We describe a slit spectroscopic experiment of high spectral resolution to provide an analysis of the most typical parts of the quasi-minimum type corona observed during the total solar eclipse of Aug. 21, 2017 from Idaho, USA. Streamers, active region enhancements, and polar coronal holes (CHs) are measured well using deep spectra. Results. Sixty spectra are obtained during the totality with a long slit, covering ±3 solar radii in the range of 510 nm to 590 nm. The K+F continuum corona is exposed well up to two solar radii. The F-corona can be measured even at the solar limb. New weak emission lines were discovered or confirmed. The rarely observed Ar X line is detected almost everywhere; the Fe XIV and Ni XIII lines are clearly detected everywhere. For the first time hot lines are also measured inside the CH regions. The radial variations of the non-thermal turbulent velocities of the lines do not show a great departure from the average values. No significantly large Doppler shifts are seen anywhere in the inner or the middle corona. The wings of the Fe XIV line show some non-Gaussianity. Conclusions. Deep slit coronal spectra offered an opportunity for diagnosing several aspects of coronal physics during a well observed total eclipse without extended investments. The analysis of the ionic emission line profiles offers several powerful diagnostics of the coronal dynamics; the precise measurement of the F-continuum component provides insight into the ubiquitous dust corona at the solar limb.
Highlights
Total solar eclipses (TSEs) are rare events that permit the analysis of the solar corona with great contrast and a high signal to noise ratio (S/N) for both imaging and spectra due to the low level of scattered light
These include the following: evidence of high temperatures that often reach 2 MK at a distance of typically 1 arcmin above the limb; the abundance ratio of elements of different first ionization potentials (FIPs) with high degrees of ionization of forbidden emission lines in the corona (Edlén 1943; Edlén et al 1969); the flows suggested by the occurrence of extended streamers on W-L images; and Doppler shifts of its emission lines (Kim 2000; Mierla et al 2008)
For the 2017 TSE, the coordinates of the site of observation situated in Indian Valley (Idaho, USA) are: latitude 44◦ 26.47833 N and longitude 116◦ 28.03167, which are at a distance of less than 20 km from the central line of eclipse totality and an altitude of almost 1000 m
Summary
Total solar eclipses (TSEs) are rare events that permit the analysis of the solar corona with great contrast and a high signal to noise ratio (S/N) for both imaging and spectra due to the low level of scattered light. At the rare TSEs where the continuum dominates in each deep spectrum, we have the opportunity to measure the F-component superposed over the K-corona component where F-lines are almost completely washed out This F-component, which is not to be confused with the scattered light from the terrestrial atmosphere (Stellmacher & Koutchmy 1974), is due to the dust surrounding the Sun and originates in heliospheric comets heated by the Sun, degrading asteroids, and meteorites and small micron size bodies orbiting close to the Sun. At ground-based facilities large aperture Lyot coronagraphs that work under the direct solar light are limited by the parasitic light of different origins, and solar F-lines are imprinted (e.g., Koutchmy et al 1983; Contesse et al 2004). We could extract several high quality spectra as far as the spectral resolution is concerned (Bazin 2013)
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