Presence of solar inner F-corona and coronal heating

TL;DR

The paper investigates whether neutral atoms exist in the solar inner corona and contribute to heating. It uses an eclipse-based raster-scanning spectrograph in the 516–540 nm band to map Fraunhofer-line depression and emission, revealing an inner F-corona dominated by neutral metals with line-dependent diffusion. Fraunhofer line depths peak around 0.25% (average ~0.30–0.32%), with diffuse, asymmetric patterns, and neutral-atom diffusion into coronal loops could boost heating by up to eight orders of magnitude over Spitzer conductivity via Cowling dissipation; line emissions reinforce the neutral corona picture. This finding provides observational constraints supporting a multi-thermal, diffusion-driven heating scenario linking chromosphere, transition region, and corona, and suggests the neutral component is a non-negligible contributor to coronal heating.

Abstract

A new source in solar corona scattering photospheric and chromospheric Fraunhofer spectral lines is detected below a height of one solar radius above solar limb, consisting of tenuous and cool neutral atoms and much fewer once ionized ions. It is demonstrated via maps at the sample Fraunhofer lines within the band from 516.38 to 539.89nm, reconstructed from one set of spatially successive raster scanning data. The dataset was obtained from a spectrograph during the total solar eclipse on April 8, 2024, at Oden, Arkansas, USA. It is revealed from these maps that both the scattering and its spatial distribution depend on spectral lines, yielded from different ionization and excitation states of neutral metal atoms and ions. The distributions show asymmetry and feature of diffusion originated from the photosphere and chromosphere. Ratio of the Fraunhofer line depth to the continuum intensity evaluated over the observational band peaks at 0.25$\%$ and has an average of 0.32$\%$. More discrete and weaker diffusion of emission counterparts of some Fraunhofer lines are detected simultaneously. These properties are critically different from those owned by that F-corona yielded via dust grain scattering beyond heights of about two and half solar radii. Hence a term 'inner F-corona' is dubbed for the assembly of scattering by this new particle source. It becomes definite now that the solar corona consists of not only free electrons and ions but also much fewer yet non-negligible neutral atoms. It is emphasized that global distributions of the outward neutral atom fluxes and coronal magnetic loops can make the abnormal Cowling resistance the most primary mechanism responsible for the coronal heating, via collisions of the neutral atoms injected with ions in the coronal loops. This likes the heating process in Tokamak with neutral beam injection(NBI).

Figures (4)

• Figure 1: Sample spectral images(top panels) and corresponding profiles(lower panels) drawn from them. The observational band spans from 516.38nm to 539.89nm, depicted along the abscissa, and the slit direction is arranged along the ordinate. The top left one was acquired with the slit crossing bright solar disk before the total eclipse as the reference spectrum. Spectral images in the top middle and top right were obtained at the 40th and 628th scanning slit positions respectively above the solar limb, with lunar disk blocking during the totality. The overwhelmingly bright vertical spectral line in the two images is the famous green coronal line(FeXIV530.3nm). In the lower three panels, three spectral profiles are plotted, drawn from the above spectral images respectively after the green coronal line profile is removed for highlighting the Fraunhofer lines.
• Figure 2: Maps of original spectral line intensity(top panels), their adjacent continuum intensity(the second row panels), the 'monochromatic' F-corona(the third row panels) and intensity of the counterpart line emission(bottom panels).
• Figure 3: Multiple faces of solar corona. Top left: the inner F-corona representing the Fraunhofer line depression integrated over the whole observational band. It is formed via scattering by primarily neutral atoms and fewer once ionized ions. The diffusion can be witnessed; Top right: map of line emission intensities contributed from eleven most significant spectral lines yielded by allowed transitions. It shows more discrete diffusion. Bottom left: the E-corona generated from the line wings of the forbidden green coronal line FeXIV530.3nm. It tells us how these coronal magnetic loops spread and thus contribute to the Cowling dissipation via collisions of the neutral atom fluxes(indicated as inner F-corona) with ions in the loops. Bottom right: the K-corona yielded primarily via scattering by free electrons in the solar corona, resulted from an integration of all the continuum intensities within the band for each spatial point in the field of view. It is much brighter than the other kinds of coronae shown here by two to three orders as indicated by the grey scale bars plotted in the bottom part of each panel.
• Figure 4: Solar corona with multiple faces reflecting its multi-thermal essence and probably hinting different coronal heating phases by the abnormal Cowling resistance. These coronal EUV maps were obtained from SDO/AIA respectively at 30.4nm(top left), 17.1nm(top right), 21.1nm(bottom left) and 9.4nm(bottom right). All the maps were selected of acquirement time when the total solar eclipse occurred. Solar corona in these maps corresponding to different formation temperatures increasing from the left to right and from the top to bottom. As the temperature increases, more and more coronal loops were detected, and mosses become more clear till the formation temperature of FeFeXIV21.1nm. And then they become weaker at the formation temperature of FeXVIII9.4nm(bottom right).