The 2017 Solar Eclipse and the Chemistry behind Viewing the Sun

 John Hansen, Ph.D.
Professor of Chemistry

 All images, videos and figures in this document are the property of the author.

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Colours which appear through the prism are to be derived from the light of the white one."  
                                                                                                                  Isaac Newton

This was the first total eclipse I experienced, and what an experience! The difference between 95% and 100% coverage by the Moon is beyond words.  I was at Kenlake State Park in Kentucky to witness this celestial event.  I brought with me several telescopes with CCD (charged coupled device) imagers.  These included an 8” Schmidt-Cassegrain to view the Sun in white light, a 40 mm H-alpha telescope to view the Sun with the hydrogen emission at 656.3 nm, and a Ca-K telescope to view the Sun with the Ca (II) emission at 393.4 nm.  I also brought with me a 35 mm digital camera.  However, the experience of being there is a gestalt that can’t be captured on film. 

I have included images and photos of the Sun during the eclipse, which are near the end of this presentation.  I have also included photos of the surrounding park during and coming out of totality that will provide a sense of how dark it got.  However, for one to get a fuller appreciation of just how special this event was, I first want to introduce the fascinating chemistry and physics behind viewing the Sun.

Interpreting the visual images of the Sun requires knowing the chemistry, physics and structure of its atmosphere. The lowest and densest level of the solar atmosphere is the photosphere, which is the source of most of the visible sunlight.  Its density is only one-ten-thousandth that of Earth’s atmosphere at sea level. The photosphere is also quite thin, being only a few hundred thousand meters thick.

The photosphere severely hinders our ability to observe and study the interior of the sun. Its temperature is sufficiently cool to permit protons and electrons to join and form hydrogen atoms.  In fact, a small proportion of those hydrogen atoms can combine with an electron and form negatively charged hydrogen ions. Even at their very low concentration, these hydrogen anions absorb the radiation from the interior and re-emit visible light.  Consequently this thin and low density photosphere is opaque, and makes viewing the interior of the sun impossible. Further, its brightness overcomes the weaker emission from the chromosphere and corona, so viewing the upper atmosphere is seriously restricted.

Viewing the Sun in white light, with appropriate filters, will emphasize features with contributions from a broad range of wavelengths, causing features of the photosphere to be far brighter and dominate over those of the upper atmosphere.  The contrast between these features is enhanced by differences in their temperatures.  For example, sunspots, dark features appearing on the photosphere, arise when magnetic field flux lines concentrate over a region, which inhibits convection and causes local cooling. Sunspots are actually quite bright; however the temperature of a sunspot is about 2000 degrees cooler than the surrounding photosphere.  Consequently, the intensity of light they emit is less than that of the surrounding photosphere and they subsequently appear dark. 

A white light image of the photosphere reveals a seething surface of hot gas, with the occasional dark blemishes due to the presence of sunspots.  Convective currents from beneath the photosphere cause tiny, bright varying regions called granules to form as seen in the video below (left panel).  They are continually created as new hot material rise and reach the visible solar disk.  In the video below (right panel), one observes a sunspot created by a locally powerful magnetic field, which consists of a dark umbra inside a light, filamentary penumbra.  The filaments are aligned with the magnetic field flux lines that pass through the penumbra. 

photosphereVideo of the Photosphere
Sunspot
 Video of a Sunspot

Light emission from the Sun can be modeled by treating the photosphere as a blackbody radiator, which means the intensity and spectral distribution of light is defined by its temperature. In Figure 1, using the accepted temperature of 5800 K for the photosphere, the light intensity against wavelength was calculated using Planck’s law (proposed in 1900). Figure 1 appears as a smooth and continuous curve; however, the spectral distribution of the Sun is not that smooth and continuous.

BlackBody RadFigure 1.  The calculated distribution of radiation intensities
from a blackbody radiator at 5800K plotted against wavelength.
Intensities were calculated using Planck’s Law.

In 1814, Fraunhofer discovered a series of dark lines throughout the sunlight continuum; in other words, at certain wavelengths the light intensity drops abruptly.  Fifty years later Kirchhoff and Bunsen found that the wavelengths where these dark lines appear coincide with the wavelengths of characteristic emission lines identified in the spectra of heated elements.  It was realized that these dark lines reveal the existence of certain elements in the upper atmosphere of the Sun, which absorb only at characteristic wavelengths of light.  In recording these dark lines, Fraunhofer was the first person to have measured a spectrum.

Conversely, sunlight is the radiation emitted from atoms and ions in the photosphere, chromosphere and corona that had absorbed energy from the solar interior. When atoms and ions absorb energy they are raised to higher energy levels referred to as excited states. When an atom or ion relaxes back to a lower energy state, light is emitted at a specific wavelength that depends on the difference in energy between the two states.  Each kind of atom and ion has a unique set of excited state energies, and thus they will emit light at certain wavelengths that form a unique pattern of emission lines referred to as an emission spectrum.  A historically famous example of this is the light emitted from excited state hydrogen.  The spectrum for hydrogen consists of four emission lines in the visible range located at the following wavelengths: 410 nm, 434 nm, 486 nm and 656 nm. These emission lines are known as the Balmer series, which correspond to the light emitted as hydrogen transitions from excited states (n = 6, 5, 4, and 3, respectively) to a lower energy state, n = 2, where n denotes the principle quantum number.  Figure 2 displays three of the four emission lines making up the Balmer series.

FinalFigure 2. The Balmer series.  Light emitted from a hydrogen gas discharge tube was dispersed into several emission lines using a transmission grating. The intensity of the emission line at 410 nm was too faint to record an image.

We can take advantage of light emission from specific atoms and ions to image the chromosphere and corona.  Filtering out all the light but that emitted when an atom transitions from a particular excited state to lower energy state allows us to see details in the upper atmosphere, which would otherwise be drowned out by the brightness of the photosphere.  The most abundant element on the sun is hydrogen.  The temperature of the chromosphere near the photosphere is around 6,000 K and rises to 35,000 K near the corona.  In this temperature range, most of the excited state hydrogen is in the n = 3 quantum state.  The chromosphere's spectrum is dominated by the emission from 656.3 nm light that arises when a hydrogen atom transitions from the n=3 to the n=2 quantum state, which is referred to as the H-alpha emission.  This emission is in the red part of the spectrum, which causes the chromosphere to have its characteristic reddish color.

Viewing the sun in only 656.3 nm light (see images and video below), we can see the chromosphere without hindrance from the interferingly bright photosphere.  This is particularly good for viewing prominences, flares, spicules, fibrils, and Ellerman bombs, features that reveal the ferocious nature of that otherwise docile looking incandescent orb.  Images were taken using a telescope with a 40 mm aperture and a filter that only lets through the H-alpha emission. Previously, the only way to see the chromosphere was to wait for a total eclipse and view it only along the limb of the Sun.

Flare1

Sunflare

Video of a Solar Flare
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Another useful emission line to image the chromosphere is the K-line of the Ca(II) cation. This emission occurs at 393.4 nm in the near UV portion of the spectrum, and requires a CCD camera to view the image, since the retina is generally insensitive to this wavelength of light. Whereas H-alpha emission is useful in imaging the mid chromosphere, Ca(II) K-line emission is particularly useful for imaging the lower chromosphere and is sensitive to local variations in its magnetic field.  The irregular shaped dark blemishes on the disc are sunspots.  The lighter areas show the more active regions of the Sun (the bright rings about the sunspots, and chromospheric plages, as well as finer granulation).  In the figure below, is a view of the Sun taken in only 393.4 nm light with all other wavelengths of light filtered out.  A telescope with a 60 mm aperture and a filter that only lets through the Ca(II) K-line emission was used to capture this image.

Sun Ca II K line emission

The corona is the most enigmatic part of the solar atmosphere.  Temperatures there reach up to several million degrees.  Explaining how the temperature rises from 6000 K near the outer surface of the photosphere to millions of degrees in the outer atmosphere defies a simple explanation.  At first blush, one might think that thermodynamics is being violated.  How can thermal energy flow from a region of lower temperature to one of higher temperature?  It appears that energy is transferred to the corona via magnetic field flux lines.  At a temperature of several million degrees there is enough thermal energy to strip off many electrons from iron, calcium and nickel atoms present in the upper atmosphere. The light given off by the corona (332.8 nm, 338.8 nm, 360.1 nm, 398.7 nm, 408.6 nm, 423.1 nm and 511.6 nm)  arises from emission of excited state Ca(XII), Fe(XIII), Ni(XVI), Fe(XI), Ca(XIII), Ni(XII) and Ni(XIII) ions, respectively.

Below are images of the Sun recorded during the 2017 solar eclipse. If you look closely at the corona, you can make out the beginnings of fine silvery tendrils emanating from the jet black shroud; defining the magnetic flux lines that whip about the Sun.  Further examination of the corona near the edge of the blotted out sun reveals some red tinges that arose from prominences extending out from the limb. So you are also getting a small view of the chromosphere.  Images of totality were recorded with a 35 mm camera using a f/5.6 aperture lens and an ISO speed of 100. No telephoto lens or filters were used. 

SolarEclipse001 SolarEclipse002 SolarEclipse003
 Solar ElipsaaVideo of the 2017 solar eclipse viewed by the Ca(II) K-line emission

I also assembled a number of the Ca(II) K emission images taken during the eclipse into a video clip shown above. It is interesting that the video reveals the integral nature existing within our solar system. The Sun rotates with a period between 25 days (at the equator) and 31 days (near the poles), varying with latitude since the Sun is a fluid, composed of hot gases and plasma. Notice how the series of sunspots is dispersed along a line in these images. Over time a cluster of sunspots will elongate in the direction of rotation. In fact, these features are nearly aligned along the Sun’s equator. Further, observe that the Moon transits the Sun along this line. In other words, the rotational plane of the Sun is shown to be nearly coplanar with that defined by the Moon’s orbit about the Earth, which is nearly coplanar with the plane of the ecliptic (defined by Earth’s orbit about the Sun).

Below are photographs of Kenlake Park during the eclipse.

nightThe view during totality 

dark

Minutes after totality

duskDawn at midday

daylight

The magic is gone.