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.
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