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The Flash Spectrum of the Sun

(Suggestion: Print this out for future reference & easier reading.)



Review of the Basics

Because atoms selectively absorb and emit light at particular wavelengths, astronomers can use the light from a distant object to study that object's composition and physical properties. Kirchoff's Laws tell us what type of spectrum to expect: A hot dense gas will emit a continuous spectrum. If, however, that hot dense gas is viewed through a cooler gas, then the atoms in the cooler gas will absorb light at specific wavelengths, and an absorption spectrum results. This is what happens in the atmospheres of the Sun and most stars. The relatively cool atmosphere of the Sun (still quite warm at 5700 K) impresses an absorption spectrum upon the continuous light emitted from the hotter layers below. Thus, we generally classify stellar spectra according to the pattern of absorption lines. We also know that a hot rare (low density) gas emits an emission spectrum with bright lines at the wavelengths characteristic of the atoms making up the gas. We've seen this in the lab in the light emitted by discharge tubes containing specific elements such as hydrogen, helium, neon, etc.


Solar Eclipses and the Flash Spectrum

As described above, light from the visible disk of the Sun, the photosphere, shows an absorption spectrum. (With your spectrometer, you could even see several solar absorption lines in sunlight scattered in the Earth's atmosphere.) During a total solar eclipse, however, an exceptional situation occurs, and for a few fleeting seconds, an emission spectrum can be observed. This happens at the very beginning of totality and just after the last bit of photosphere has been covered by the Moon. For a period of several to perhaps ten seconds the chromosphere is visible as a red arc. (Recall that the chromosphere is a very thin layer just above the photosphere.) Often prominences are seen jutting from the chromosphere. The deep red color of the chromosphere comes from very strong emission in the hydrogen-alpha line at 656 nm. Because the chromosphere is quite rare and hot (with a temperature of about 10,000 K) it shows an emission spectrum in the absence of any light from the much brighter photosphere. Of course, the advance of the Moon soon covers the chromosphere and then the much rarer corona becomes visible as a broad white halo. Within minutes totality reaches completion, the chromosphere reappears on the opposite side of the Sun followed seconds later by the overwhelming brightness of the photosphere.


Because the chromosphere is so thin, it forms an ideal subject for spectral imaging. A diffraction grating inserted in the light path of a telescope separates the light according to wavelength (just as in your spectrometer). An attached camera will then record the chromosphere imaged in each of the component wavelengths in its emission spectrum. This is the flash spectrum, so-called because of its brief accessibility.


In this exercise you will use the Sky Image Processor to analyze a flash spectrum taken at the total solar eclipse of March 7, 1970 near Currituck, NC. The spectrum shows a number of spectral lines that you will identify.


Carefully study the sequence of images from this eclipse which illustrate the relationship of the photosphere, chromosphere, and corona during a total eclipse, and show the important spectra:





The final gleam of brilliant photospheric light immediately before the beginning of totality. This brief phenomenon is called the diamond ring effect. Note that the surrounding corona is just beginning to appear.





The chromosphere. With the photosphere almost completely obscured by the Moon, this very thin layer becomes visible. The red color of the chromosphere is due to emission in the hydrogen-alpha line at a wavelength of 656 nm. Note the jagged upper edge of the chromosphere and the presence of small prominences projecting upwards. This phase of a total eclipse lasts only several seconds. With its hot rare gas and narrow structure, the chromosphere is a natural manifestation analogous to the discharge tubes which you analyzed in lab!




The flash spectrum. Here the light from the chromosphere has been dispersed into its component wavelengths by a diffraction grating inserted into the light path of the telescope. Notice that each spectral line actually appears as a distinct image of the chromosphere in that color. Wavelength increases from left to right spanning the visible spectrum. The strong red line on the far right is the hydrogen-alpha line (656 nm). The objective of this exercise will be to identify the other lines in the spectrum. The four horizontal bands stretching from violet to red are caused by light from photosphere passing through gaps in the jagged lunar limb. (Yes, the edge of the Moon is slightly irregular due to lunar mountains and other features.) It is interesting to note that unlike the light from the chromosphere, the light from the photosphere shows a continuous spectrum (with absorption lines not readily visible here). The image which you will analyze in lab will be in the FITS (Flexible Image Transport System) format used by astronomers, and will appear as a grayscale image, since color representations are not useful in measurement. During lab you may wish to refer back to this webpage to remind yourself of the actual colors.




The solar corona. Just after most of the chromosphere is obscured, the spectacular solar corona appears and dominates most of totality (which was just over 2 minutes long for this particular eclipse). This low density gas is at a temperature of several million degrees Kelvin. The detailed source of heating of the gas remains an area of intensive research. The gas we see in the corona is continuously expanding outward forming the solar wind. It is replenished by gas coming up from the Sun. Note that several prominences are still visible jutting up from the mostly obscured chromosphere.





The spectrum of the corona. Because the corona does not have a sharply defined structure its spectrum appears diffuse, making spectral "lines" hard to measure by this method. Notice, however that the corona does manifest an emission line spectrum, as a hot rare gas should. The green emission (at a wavelength of 530 nm) was discovered in 1869 and its origin remained a mystery for over 70 years. Because it could not be identified with any element known on the Earth, it was suspected that it might be due to a new element, tentatively dubbed "coronium." (Remember, that helium was first discovered in the solar spectrum and named after the Sun.) Eventually, however, the mysterious green line was shown to be due to thirteen-times-ionized iron, that is, iron atoms with 13 electrons stripped off! This was one of the first indications that the corona is extremely hot; indeed temperatures of several million degrees are required to strip 13 electrons from iron. The search for coronal heating mechanisms continues to this day. Notice also that some prominences appear in the same red and violet lines which are seen in the chromospheric (flash) spectrum. The prominences show absolutely no emission in the ionized-iron green line, however. That's because this level of ionization is impossible at the much lower temperatures of the chromosphere (about 10,000 K).





Laboratory spectrum of hydrogen. To measure wavelengths in the flash spectrum it will be necessary to first calibrate the instrument. Thus, the spectrum from a discharge tube filled with hydrogen was obtained. (This is essentially the same hydrogen spectrum that you observed in lab.) You will therefore use this spectrum to deduce a wavelength scale for measuring the spectral lines in the flash spectrum. The spectral lines of hydrogen are denoted as follows (from right to left): hydrogen-alpha (at 656 nm), hydrogen-beta (at 486 nm), hydrogen-gamma (at 434 nm), and hydrogen-delta (at 410 nm). The FITS version of this image that you will analyze in lab will be a greyscale image and thus will not show colors.




All of the images on this webpage and in the associated exercise were taken by Gary Sego, an accomplished photographer of natural phenomena, including active volcanoes as well as eclipses. The members of the expedition also included B. Dennison, M. J. Doyle, J. Morrisey, D. Switzer, and S. Ryan.





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2004 Brian Dennison