The “Mystery” of Observing neutral Sodium [NaI] and ionized Calcium [CaII] Solar Absorption Spectra as “Light”

If you look at the spectrum of the Sun, you see there are black bands in the image.  These gaps in the spectrum where there is no light emitted are called absorption lines. Other astronomical sources (and also light sources you can test in a lab) are found to create spectra that show little intensity at most wavelengths but a few precise wavelengths where a lot of intensity is seen. These are referred to as emission lines.

Two questions arise:

(1) How can we observe absorption spectra as “light”?

(2) How can we observe the effects of convection, radiative transfer, magnetic flux, and temperature, on elements in a continuous spectrum of Solar radiation?

Multi-Colours from The Sun (NASA site)


The Sun’s spectrum is continuous with absorption from the Upper Atmosphere

The Sun emits a continuous spectrum of light from its core and lower atmospheres.  This continuum occurs from all the processes within the Sun’c core.  We can extract a temperature from this continuum based on modeling  the Sun’s core as a “black body.”  A black body is a perfect absorber and emitter of light.  For the Sun, T = 5,778 K .

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The upper atmospheres of the Sun are not anywhere near perfect, however. The Photosphere and lower Chromosphere are cooler than the core. They absorb some of the continuum light at specific transitions of elements. like Hydrogen, Calcium, and Sodium.


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In the early days of spectroscopy, experiments revealed that there were three main types of spectra. The differences in these spectra and a description of how to create them were summarized in Kirchhoff’s three laws of spectroscopy:

  1. A luminous solid, liquid, or dense gas emits light of all wavelengths.
  2. A low density, hot gas seen against a cooler background emits a BRIGHT LINE or EMISSION LINE spectrum.
  3. A low density, cool gas in front of a hotter source of a continuous spectrum creates a DARK LINE or ABSORPTION LINE spectrum.

Like Kepler’s laws of planetary motion, these are empirical laws. That is, they were formulated on the basis of experiments. In order to understand the origin of absorption and emission lines and the spectra that contain these lines, we need to first spend some time on atomic physics. Specifically, we will consider the Bohr model of the atom.

Whenever you are studying the light from an astronomical object, recall that there are three things you need to consider:

  1. the emission of the light by the source,
  2. processes that affect the light during its travel from the source to the observer, and
  3. the process of detection of the light by the observer.

A brief description of how the Sun works…


The Bandpass Filters for our Solar Telescope

So, how do we observe light from absorption lines?  We do not. We observe the absence of certain wavelengths of light.  With a luminous light source like the Sun, absorption lines stand out against the continuum light as dim or dark regions.  How do we concentrate on these regions alone?

A bandpass filter rejects other wavelengths of light, but the band of light we are interested in. If we have a narrow bandpass filter centered on these absorption lines, it will image variations in the intensity of the continuum light around where absorption lines lie.  The neutral sodium D-lines are at ~589.9 nm.  We are interested in them.  If the Sun absorbs sodium D-lines then we should see no light using a bandpass of 590nm. However there is a width (FWHM) to this bandpass filter of 10 nm and the peak is accurate to 2nm around ~590 nm.




 Transmission versus wavelength for the bandpass filters we use. Note: even though the bandpass filter wavelengths themselves intersect in there bandwidth, only the 590 nm filter includes the the D-Line absorption which is extremely narrow at 0.2 nm.



IMG_2224Here are the two bandpass filters. Left:  A 590 nm bandpass filter. Right: A 580nm bandpass filter.  Both have a FWHM of 10nm. We can clearly see the colour difference.

What then?  Using these bandpass filters on our Solar Telescope, the Sun’s image will appear in all wavelengths of light capable of passing the bandpass filter.  No absorption lines exist in the photosphere of the Sun in the region ~575-585nm.  With the bandpass filter centered at ~580 nm, if the intensity of light is large, it will begins to saturate the camera/detector and appear as white light uniformly.  If we lower the sensitivity of our camera/detector, the light diminishes uniformly.


580 nm bandpass

The 580 nm bandpass filter with our camera/detector reaching saturation.  A yellow and green edge is caused by observing through living room windows of two-pane thermal glass which diffract the light.


590 nm bandpass NaI

The 590nm bandpass filter provides different results, as shown above, with sensitivity set at the same level as the 580nm filter.

Why do the two filters give different results? Some white light, but mostly yellow light – all flecked and patterned appear after the 590nm bandpass filter.  Where we see white light, it is all the light passing through the bandpass filter and saturating the camera/detector.  Where we see yellow light, part of the light intensity is diminished by the absorbed light of the D-Lines of Sodium.  This is not a linear process, but one which gives insight into the turbulent nature of the photosphere and chromosphere – the upper atmospheres of the Sun.

Since we are using a narrow bandpass filter centered on ~590nm, light between  ~588.9-590.9 nm is completely absent, and the remaining light is allowed to pass by the narrow bandpass (~585-588.9nm 589.9-595nm) to our camera/detector.  Since this is in the “yellow” part of the spectrum, we see that light as yellow.

The yellow light represents the absence and absorption of NaI transition.  The white light, because of convection, and doppler shifting, and other effects, represents the whole continuum of light, like the 580 nm bandpass filter.

It is in this way we can investigate another realm of solar activity…

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Beading or graining of the surface.



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