APOD: The Flash Spectrum of the Sun (2017 Sep 07)

[APOD Robot]

The Flash Spectrum of the Sun

Explanation: In clear Madras, Oregon skies, this colorful eclipse composite captured the elusive chromospheric or flash spectrum of the Sun. Only three exposures, made on August 21 with telephoto lens and diffraction grating, are aligned in the frame. Directly imaged at the far left, the Sun's diamond ring-like appearance at the beginning and end of totality brackets a silhouette of the lunar disk at maximum eclipse. Spread by the diffraction grating into the spectrum of colors toward the right, the Sun's photospheric spectrum traces the two continuous streaks. They correspond to the diamond ring glimpses of the Sun's normally overwhelming disk. But individual eclipse images also appear at each wavelength of light emitted by atoms along the thin, fleeting arcs of the solar chromosphere. The brightest images, or strongest chromospheric emission, are due to Hydrogen atoms. Red hydrogen alpha emission is at the far right with blue and purple hydrogen series emission to the left. In between, the brightest yellow emission is caused by atoms of Helium, an element only first discovered in the flash spectrum of the Sun.

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[holger.nielsen@skolekom.dk]

An excellent picture! It could be used for a small exercise: Read off the pixel positions of known emission lines (for instance the Balmer lines), calibrate with their wavelengths and use the calibration to calculate the wavelengths of less conspicious emission lines in order to try to identify them.
A minor quibble: The text mentions "blue and purple hydrogen" lines. Purple is not a spectral color, but a mixture of red and blue. The lines in question are violet. On the other hand, violet is a color allmost never seen in dayly life (rainbows are an example). Television and computer screen cannot emit violet light, so what we se in the picture on the screen is purple.

[JohnD]

Neither is Green a spetral colour, but a mixture of yellow and blue, yet to my eye, the brightest part of the spectrum is in the Green (in the Green!)
https://www.youtube.com/watch?v=LPZmFQQu8L0

This agrees with the "Green Flash" that sunset watchers seek.

Please explain the Green Flash, and this spectrum?
John

[Indigo_Sunrise]

This is a very ineresting image.
And I also have a question about the colors: or more specifically, what is the difference between 'hydrogen alpha emission' that are colored red, and 'hydrogen series emission', colored blue and purple?

TIA

[neufer]

This is a very ineresting image.

And I also have a question about the colors: or more specifically, what is the difference between 'hydrogen alpha emission' that are colored red, and 'hydrogen series emission', colored blue and purple?

---

• Balmer_series

<<The visible spectrum of light from hydrogen displays four wavelengths, 410 nm, 434 nm, 486 nm, and 656 nm, that correspond to emissions of photons by electrons in excited states transitioning to the quantum level described by the principal quantum number n equals 2. There are several prominent ultraviolet Balmer lines with wavelengths shorter than 400 nm. The number of these lines is an infinite continuum as it approaches a limit of 364.6 nm in the ultraviolet.>>

<<The name of the culture comes from the often parodied Baltimore accent and slang. "Hon" (/ˈhʌn/, an abbreviation of "Honey") was a common informal name for someone else. It is almost always used at the end of the sentence, e.g., "how bout dem O's, Hon?"* Baltimore's accent exemplifies a dialectal continuum between Tidewater American English, a southern American dialect, and Delaware Valley American English, a common coastal dialect, loosely possessing the vowel shifts of the former and general pronunciation of the latter. For instance, "Baltimore" is pronounced "Balmer", and "Maryland" becomes "Merlin." Other common pronunciations include "ool", "amblance", "wooder", "warsh", "sharr or shaow", "far", "cowny", "tew", and "zinc" (oil, ambulance, water, wash, shower, fire, county, two, and sink respectively).[citation needed] There is also a popular summertime phrase, "goin' downy ayshin" (going down to the ocean, usually referring to Ocean City, Maryland) as well as popular phrases such as, "my (appliance) went up" (meaning died, shortened from "went up to heaven") and "dem O's" (i.e. "them O's", referring to the city's Major League Baseball team, the Baltimore Orioles).>>

[Holger Nielsen]

Green is definitely a spectral color. If the eye receives photons with a wavelength around 530 nanometers, the brain registers this as "green".
If the eye receives a mixture of blue and yellow light, the brain interpretes this as "green" as well, allthough no green photons have been received.

[Chris Peterson]

Please explain the Green Flash, and this spectrum?

The vast majority of the energy emitted by the Sun is thermal continuum: the photoshere is a blackbody radiator. This light is what gets spread out by dispersion into a rainbow, which is what's happening when the Sun sets. Dispersion through the long atmospheric path separates the colors, with green and blue being the farthest from the Sun and therefore the colors seen just as the Sun dips below the horizon. (There is a blue flash, but it's rarely seen because our sensitivity to blue is low and the blue light scatters more.)

The chromosphere is not a thermal emission source because it is too rarified. The light it emits is from discrete spectral emissions, which is what we see in the flash spectrum. There's not enough energy there to be seen over the light of the photosphere. So we only see it during eclipses or when we look at the Sun with narrowband filters.

[Chris Peterson]

Green is definitely a spectral color. If the eye receives photons with a wavelength around 530 nanometers, the brain registers this as "green".
If the eye receives a mixture of blue and yellow light, the brain interpretes this as "green" as well, allthough no green photons have been received. :mrgreen:

It's all semantics. Physically, there's no such thing as "color". "Spectral color" is largely meaningless. Color is physiology: a perceptual response to different wavelengths of light.

[JohnD]

Thank you, Chris!
And yes, I suppose that light of the correct wavelength is perceived as "green" but also light of two blue and yellow wavelengths is "green". But this is not semantics, it's physiology! We have but three types of colour receptors in our eyes, that react to red, yellow and blue. Strangely, the yelloiws are most sensitive, which is why that colour is used in HiViz garments.

John

[Fred the Cat]

If a neutrino detector could view an eclipse what would it see. The “N”oon?

[JohnD]

The flux of solar neutrinos on the surface of the Earth (day or night!) is about 10^11/cm^2. So, an enormous number.
The Gran Sasso (BOREXINO) experiment sent a beam of neutrinos from CERN to near Rome, through 454 miles of the rock of the Earth's crust, about a seventh of the Moon's diameter. Only a very, very few of the neutrinos in the beam were detected. So I estimate that the proportion of the neutrinos that would not pass through the Moon at total eclipse would be seven times a very, very small number. So the neutrino Sun would shine as brightly as ever, despite the Moon in the way.
John

[Chris Peterson]

The flux of solar neutrinos on the surface of the Earth (day or night!) is about 10^11/cm^2.

Per second.

[neufer]

• The Earth & Moon are both extremely transparent to the (65 billion/cm²·s) flux
  of low energy (< 18 MeV) solar neutrinos.
  
  However, high energy (10 to 10000 TeV) cosmic neutrinos might some
  day be used to study the interior structure of the Earth & Moon:

Neutrino absorption tomography of the Earth's interior using isotropic ultra-high energy flux
by Pankaj Jaina, John P.Ralstonb, & George M.Frichterc

Abstract: We study the feasibility of using an isotropic flux of cosmic neutrinos in the energy range of 10 to 10000 TeV to study the interior structure of Earth. The angular distribution of events in a ∼ km³-scale neutrino telescope can be inverted to yield information on the Earth's mass distribution that is independent of other methods. The energy spectrum of the neutrino primaries is also determined from consistency with the angular distribution. It is possible to make a model independent determination of the density profile of Earth's interior, separate from the absolute normalization of the incident cosmic neutrinos.

[4321lynx]

I do not see ANY yellow here. Only a slightly pale green hue that some might call yellowish-green.

I see yellow normally in everyday life, and have no colour-blindness of any sort.

So, is it my laptop? It shows the yellow of all the smilies OK. etc

[Chris Peterson]

I do not see ANY yellow here. Only a slightly pale green hue that some might call yellowish-green.

I see yellow normally in everyday life, and have no colour-blindness of any sort.

So, is it my laptop? It shows the yellow of all the smilies OK. :D :) etc

Well, the He line is at 588 nm, which appears orange-yellow to most people, a color I see on my monitor for that emission line. That said, I assume this was captured with a conventional color sensor, which has integral RGB filters, and is actually a pretty terrible tool for accurately recording spectra. You can see how bad by noting the broad bands of nearly unchanging red, green, and blue. You should really only use the apparent color here as a rough guideline; it's the position of the lines that contain the real information.