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Early work: identifications of magnetic dipole lines show
that the corona is hot
The first reports of a genuinely coronal emission line are from
the 1869 eclipse over N. America, when Young and Harkness
observed visually and independently the green coronal line, using
spectrographs (see
"The Sun", by C. A. Young). Attempts to study the
corona without an eclipse were made as early as 1877. But, in the
first part of the twentieth century, several technological
breakthroughs were of particular importance to the study of the
corona in general and for magnetic dipole coronal emission lines
in particular:
- In 1907,
Wallace showed how photographic plates of the day could be
sensitized to red light. Earlier, only blue and violet parts of
the spectrum could be readily photographed.
- By 1930, Lyot had invented and successfully demonstrated the
use of the coronagraph from mountain sites (notably Pic du Midi
Observatory), which with photographic plates enabled him to
record coronal spectra out to 1.35 microns (see
Lyot 1933,
Lyot 1939).
- During the 1930s, Edlen and colleagues obtained laboratory
spectra of elements with numerous electrons removed.
- Another important development came in 1944 with the invention
of a novel narrow-band filter utilizing interference and
polarization by
Lyot (1944) building on work in 1937 by Ohman, subsequently
known as a "Lyot filter". This enabled the imaging of
coronal emission lines and prominences in, for example, H alpha,
without the use of spectrographs.
These advances came together in an idea whose time had come by
the late 1930s. Earlier, in 1931, Grotrian suspected that the
corona was anomalously hot on the basis of the huge widths of
electron- scattered light of Frauenhofer's H and K lines seen
in coronal data from 1929. By 1939,
Lyot ) found many unidentified lines, including those at
1074.6 and 1079.8 nm. In 1937, he suggested that the large
apparent width of the green line might be due to thermal motions.
In 1939, Grotrian (Naturwissenschaften 27, 214)
proposed tentative identifications for the red coronal line with
9 times ionized iron, and a line at 789.2 nm with 10 times
ionized iron. In 1943, in a famous paper marking perhaps the
beginning of coronal physics,
Edlen identified 4 of the coronal lines against laboratory
measurements, and many other lines using extrapolation methods
based upon atomic theory applied to the laboratory spectra. All
of Edlen's identifications were with ions which had between 9
and 14 electrons removed. {mospagebreak}
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