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Understanding the extreme ultraviolet spectrum of the Sun and stars

posted May 24, 2013, 10:18 AM by Jeremy Drake   [ updated Jun 2, 2013, 7:25 AM ]
Perhaps the least explored range in the electromagnetic spectrum of the Sun is the "extreme ultraviolet" - the range of wavelengths whose photons are energetic enough to ionize hydrogen atoms (wavelengths shorter than 912 Angstroms) but not really energetic enough to be called X-rays.  There have been especially very few studies of the Sun in the range near 100 Angstroms, around the rather blurred boundary that separates EUV and X-ray regimes.  Surprisingly, this wavelength range is much better studied in other stars using the now defunct NASA Extreme Ultraviolet Explorer satellite and the flagship Chandra X-ray Observatory.

We looked at the spectrum of the nearby star Procyon, whose outer atmosphere, or "corona", is fairly similar to that of the Sun, and compared the observations with the best possible model spectrum we could make using current knowledge of the atomic emission processes in the EUV spectral range.  The aim of the study was to better understand what is being seen by the EUV filters of the Atmospheric Imaging Assembly instrument on NASA's Solar Dynamics Observatory.  We found that the best model EUV-X-ray spectrum we could make was missing many weak spectral lines that combine to amount to a significant amount of flux.  The plot here shows the ratio of the flux in our model to that observed in the Chandra spectrum of Procyon.  Flux deficiencies in the model are typically more than a factor of 2, and are up to a factor of 3 near 100 AA or so.  This means that current spectral models of the Sun in this range are also missing this amount of flux.  In addition to being important for understanding SDO observations, this is also important for understanding the EUV heating of the Earth's upper atmosphere. This work was lead by Paola Testa at SAO, and was published in the 2012 February 1 edition of the Astrophysical Journal.