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Eclipses reveal first images of Sun's iron emission
Posted: 05 January 2010

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Ground-based observations captured of the solar corona during recent total eclipses have revealed the first images of the Sun's outer atmosphere in the near-infrared emission line of highly ionized iron.

The first maps of the 2D distribution of coronal electron temperature and ion charge state for the 2006 eclipse (left column) and 2008 eclipse (right column) with white-light images added in the bottom row. Red indicates iron line Fe XI 789.2 nm, blue represents iron line Fe XIII 1074.7 nm, and green shows iron line Fe XIV 530.3 nm. Image: Habbal, et al.

The observations were made during the total solar eclipses of 2006, 2008 and 2009 by NASA Goddard Space Flight Center astrophysicist Adrian Daw along with an international team of scientists led by Shadia Habbal from the University of Hawaii’s Institute for Astronomy and provide an important step in understanding how emissions from the Sun – space weather – effects Earth, since this particular charge state of iron (ten-times ionized, or Fe XI), along with Fe X, are the dominant charge states formed as the expanding corona flows out into interplanetary space towards the Earth).

The emission line for Fe XI, occurs at 789.2 nanometres (nm), and the first images of the corona at this wavelength reveal some surprises, notably that the emission extends out at least three solar radii and has localised regions of enhanced density of iron ions.

Comparison of the first image of the corona in Fe XI 789.2 nm, taken during the 2006 eclipse, with a white-light image taken by Miloslav Drückmuller from Brno University of Technology in the Czech Republic. The coulored circles indicate localised enhancements in Fe XI that have no correspondence in white light, while the bubble structure surrounding the yellow circled region is barely distinguishable in white light. Image: Habbal, et al.

"The complicated magnetic structure of the corona confines localized regions of high density plasma," Daw tells Astronomy Now. "On top of that, many different processes can affect the heating of ions in the corona, which can lead to further density enhancements for particular ions.  Collisional processes, radiative processes, gravity, plasma wave phenomena, and the twisting and merging of magnetic structures can all play a role.  The mechanisms that cause different density enhancements are certainly not all well understood, but through observations such as these, we are advancing our understanding."

Combining the data with observations of other iron charge states, the scientists were able to build up the first two-dimensional map of the distribution of electron temperature and charge-state measurements, establishing a direct link between the distribution of charge states in the corona and in interplanetary space.

"Eclipses are the only tool to explore the physical processes in the first few solar radii, starting from the solar surface, in an uninterrupted manner, and this study establishes the first direct link between the distribution of charge states where they are formed in the inner corona and the distribution that flows into interplanetary space, bombarding the Earth's magnetosphere," adds Daw.