K corona, F corona and E corona

[carlos-carlos]

TL;DR Summary

How do we distinguish the K corona from the F corona? Is it not sufficient to say that K corona has a continuous spectrum and F corona Fraunhoffer lines?

Particularly I could read this sentence "In the infrared, however, the small dust grains did not act like perfect mirrors and this is what made it easy to distinguish the F-corona from the K-corona".
I do not understand it. I could understand a problem in separate E corona from F corona, but K corona should be continuous. Should not be a problem.
Here
The Origin of the Corona’s Light | Total Solar Eclipse 2017 (nasa.gov)
you can see the context.
Thanks.

 

[Astronuc]

Summary:: How do we distinguish the K corona from the F corona? Is it not sufficient to say that K corona has a continuous spectrum and F corona Fraunhoffer lines?

From the cited article,

there isn’t simply one corona emitting light but five different solar corona, each produced by different light-emitting processes. To distinguish them they are called the K-corona, E-corona, F-corona, T-corona and the S-corona after ‘Kontinuierlich’ (Continuum), Emission, Fraunhofer, Thermal and Sublimation.

It seems there is a distinction of the different layers by light emission, or scattering, processes based on temperature, ion and electron densities, and composition.

Even during a total solar eclipse, we see that the K-corona outshines the F an E coronas at distances closer than about twice the solar radius, but at greater distances it is the F-corona that completely dominates. The E-corona is the weakest of the three within twice the sun’s radius and is generally not observable except by using a spectroscope, . . .

Look at the temperatures. The photosphere temperature is ~5800 K, the choronosphere temperature is ~10000 K, then there is a transition zone where the temperature changes from 10,000 K (typical of the chromosphere) to nearly a million degrees. The hottest part of the solar atmosphere, which has a temperature of a million degrees or more, is called the corona.
Ref: https://courses.lumenlearning.com/astronomy/chapter/the-structure-and-composition-of-the-sun/

Again from the NASA article,

hydrogen is completely ionized into free protons and electrons, other common elements such as oxygen, nitrogen and iron are not completely stripped of all their electrons. This means that the coronal gas is a high-temperature plasma consisting of numerous free electrons and the ions of many kinds of atoms. The E-corona represents the light from ions in this plasma that absorb light from the photosphere and re-emit it through specific electronic transitions between energy levels in these ions.

and

The E-corona only extends about 700,000 km from the photosphere, but this region includes the majority of the bright corona that you see during a total solar eclipse.

In the discussion of the K-corona

With all those free electrons in the corona, a second form of light emission can take place. Photons of light emitted by the photosphere can interact with these electrons causing the photons to be scattered in different directions. This is a process called Thomson scattering by physicists, and because of this process and the huge number of photons leaving the photosphere every second, we can see the entire extent of this electron-plasma cloud out to 2 million kilometers from the photosphere. This is called the Kontinuierlich (German for ‘Continuum’) or K-corona because it emits a continuum of light frequencies, unlike the E-corona which only emits light at the specific frequencies of the ions it contains.

then there is the comment about distance from the solar surface

Near the photosphere, the density of electrons is about 400 million per cubic centimeter, but by a distance of 3 solar radii (2 million km) it has dropped to only 400,000 per cubic centimeter, which explains the dramatic fading of the K-corona with distance by a factor of nearly 50,000 in intensity.

Then in the discussion of the F-corona,

This ‘new’ coronal gas had virtually the same spectral lines as the solar photosphere observed by Fraunhofer, hence the name F-corona. What was seen were absorption lines from cool gas produced by ions of hydrogen (HI: 121.6 nm), carbon (CIII: 97.7 nm) and silicon (SiIII: 120.3 nm) in the ultraviolet, and calcium and other elements in the visible ‘Fraunhofer’ spectrum. Basically, the F-corona looks like a mirror of the photosphere situated some 1 million km above the solar surface.

and

astronomers determined that this region was populated by interplanetary dust grains falling into the sun!

How far would dust grains penetrate the atmosphere of the sun? Consider the radiative pressure and particle flux flowing out of the sun, and consider that grains would be heated and vaporized.

Then more discussion of distance

The F-corona has been detected to distances of 7 solar radii or nearly 5 million kilometers. The most common way to detect and map the F-corona is by using telescopes equipped with infrared filters during total solar eclipses. Observations at 2.2-microns were performed during the 1998 total solar eclipse and showed that the F-corona was slightly flattened but its brightness declined as nearly the inverse-square of the distance out to about 8 solar radii (6 million km), and the tilt is similar to the ecliptic plane through which the zodiacal dust has the highest density. . . . The K-corona provides most of the coronal brightness between 1 and 4 solar radii, but the F-corona then smoothly takes over until it joins up with the Zodical light at about 80 solar radii at about 20 degrees from the center of the sun.