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The Sun's corona is primarily composed of hydrogen and helium atoms, which are being impacted by sunlight. The electromagnetic frequencies vary quite a bit, and only a small portion of the frequencies are capable of exciting the helium atoms in the corona. In this paper I found a graph showing the specific frequencies required for helium excitation.

Frequencies required to excite helium

However, I couldn't find any information on the specific percentage of sunlight that falls within the right frequency range to excite helium. The graphs I've seen so far only show the range of frequencies present in sunlight. The few that include percentages have ranges that are too broad to be useful, such as this chart:

the suns electromagnetic spectrum

What percentage of sunlight is in the correct frequency range to excite helium?

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  • $\begingroup$ "right frequency range" isn't quite right. Each absorption line shown on the first diagram is more like a bell curve where it strongly absorbs at one frequency and less so around it, but without a hard cut off. You could get each of these curves. integrate them over the power spectrum of the Sun (given by a black body), and sum it all up. The problem is that the width of these curves aren't fixed, they vary substantially with temperature, pressure, magnetic field, etc... Without fixing those variables, I think the question is probably not well enough defined for a numerical answer. $\endgroup$
    – user54772
    Commented Sep 25 at 17:53
  • $\begingroup$ @ScienceSnake I am mainly interested in the part of the corona which is around 10 million kilometers away from the surface of the sun. A quick google search tells me that the corona is at 2 million degrees Fahrenheit, 1.1 million Celsius nasa.gov/science-research/heliophysics/… and at around 10^15 particles per cubic meter en.wikipedia.org/wiki/Stellar_corona#Physics_of_the_corona I hope that helps $\endgroup$
    – The Rocket fan
    Commented Sep 25 at 19:12
  • $\begingroup$ The Sun's corona contains almost no atoms. It is ionised hydrogen and helium. The corona is optically thin and absorbs almost no light. The corona is not heated by absorbing light. $\endgroup$
    – ProfRob
    Commented Sep 25 at 20:52
  • $\begingroup$ 10 million kilometers? You won't be able to see any spectral lines anymore that far out. Spectral lines are observed in the E-corona, which extends only up to about 1 million kilometers (see eclipse2017.nasa.gov/origin-corona%27s-light ). In any case, the corona is almost 100% ionized, so there are hardly any atoms in the ground state that could be excited. Excited atoms result practically only from recombination of electrons with ions into the corresponding levels. $\endgroup$
    – Thomas
    Commented Sep 25 at 21:07
  • $\begingroup$ @Thomas, thanks for the correction. In this case, I will change the value for the question from ten million to under one million. You can disregard the ten million kilometer value. $\endgroup$
    – The Rocket fan
    Commented Sep 26 at 3:51

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