The first ever image of the full solar corona in the Fe XI 789.2 nm spectral line was acquired during the total solar eclipse of 2006 March 29, by the Institute for Astronomy, University of Hawaii group, led by Shadia Habbal. Funding for this expedition was provided by an NSF grant to the University of Hawaii. Two surprising findings emerged from this image:
These findings were a consequence of the strong radiative excitation
contribution, compared to the collisional excitation, to the intensity of the
line. These observations imply that the localized ion density enhancements can
occur are a consequence of reduced heating to heavy ions in the inner corona.
These observations also enabled the first empirical determination of the
transition from a collision-dominated plasma to a collisionless one in the
corona, hence the determination of the freezing-in distance of the charge states
of Fe, observed in the solar wind. See this ApJ paper
for more details.
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