Solar Spectroscopy: Coronium
Thanks to the advent of the spectroscope by two German scientists, Robert Wilhelm Bunsen (1811-1899) and Gustav Robert Kirchoff (1824-1887) in 1859, astronomers began experimenting almost immediately with this new way of analyzing light.
By splitting light into its component wavelengths much as prism splits light into its colors, astronomers soon detected familiar elements like calcium and sodium in the light from distant stars and nebulae.
In 1868, Sir Norman Lockyer (1836-1920) discovered a new line in the solar spectrum called Helium. This element was not detected on Earth until about 25 years later.
Meanwhile, the solar corona was revealing itself as an equally mysterious region.
Discovery of New Solar Emission Line
Charles Augustinus Young (1834-1908) and William Harkness (1837-1903) independently discovered a new bright (emission) line in the spectrum of the Sun's corona. When they pointed their telescopes at the solar corona during the August 7, 1879 eclipse, they saw several bright lines, including a particularly strong green line at a wavelength of 5303 Angstroms (1 Angstrom unit = 0.00000001 centimeters). This green line is so intense that, when eclipses are photographed (like the image above) the green color can easily be seen.
Like Lockyer’s helium, this new atomic line in the solar corona was considered to be from a new element unlike anything seen under laboratory conditions, so they called it Coronium. Young even went so far as to identify it as Iron Line Number 1474, though it didn’t fit the expected pattern of lines from this element.
It wasn’t until 60 years later that Swedish astronomer Bengt Edlin (1906-1993) finally determined that the "Coronium" lines were caused by the element iron seen under very high temperatures. The atom had lost 13 of its 26 electrons (a state that physicists call call ‘Iron-14’ or Fe XIV). One of the remaining 13 electrons, when excited, gives up its energy and produces this green line. A weaker red line also seen in the corona at 6374 Angstroms, was produced by iron atoms stripped of 9 electrons (Fe X).
It would take temperatures near a million degrees C to make iron atoms behave this way, but how could the corona possibly be that hot? Discovering the source of the "Coronium" line only opened up another perplexing question that astronomers were not able to answer until the start of the 21st Century.
The image above was taken in Zambia by John Walker during the solar eclipse of 2001 at an exposure of half a second. This exposure enhanced the streamers and some of the filamentary structure of the corona, particularly near the active region at the top right. The white light which predominates in the inner corona is principally due to light from the photosphere scattered by free electrons stripped from ionised atoms in the corona, where the temperature exceeds one million degrees Kelvin. In the outer corona, light scattered by interplanetary dust also contributes to the continuum emission. This dust pervades the solar system along the plane of the ecliptic to beyond the Earth's orbit and can be seen as the zodiacal light after sunset and before dawn (in a dark and transparent sky).
- "History of Astronomical Spectroscopy, 1869 - 1939 : Coronium" LaserStars.org (University of Ottawa, Faculty of Science, Physics.)
- "In Darkness Africa - Solar Eclipse Zambia 2001" by John Walker.
- "In Darkness Africa - Solar Eclipse Zambia 2001 Photo Gallery" by John Walker.
- "Lockyer spectroscope" from "A New Sun: The Solar Results from Skylab, Chapter 2, The Masks of the Sun." John A. Eddy, NASA-MSFC.
"The Emission Line Corona" - Early observations of the visible spectrum of the corona revealed bright emission lines at wavelengths that did not correspond to any known materials. This led astronomers to propose the existence of "coronium" as the principal gas in the corona. The true nature of the corona remained a mystery until it was determined that the coronal gases are super-heated to temperatures greater than 1,000,000ºC (1,800,000ºF). At these high temperatures both hydrogen and helium (the two dominant elements) are completely stripped of their electrons. Even minor elements like carbon, nitrogen, and oxygen are stripped down to bare nuclei. Only the heavier trace elements like iron and calcium are able to retain a few of their electrons in this intense heat. It is emission from these highly ionized elements that produces the spectral emission lines that were so mysterious to early astronomers. We can now produce artificial eclipses in coronagraphs that cover the disk of the Sun and filter out everything except the emission due to these coronal ions. These coronagraphs produce images of the "emission line corona." (Source: NASA/Marshal Space Flight Center - "Solar Physics, the Corona.)