Variations in the relative elemental abundances of oxygen, neon, magnesium, and iron in high-temperature solar active-region and flare plasmas

Abstract

X-ray line ratios that are weakly dependent on temperature are used here to search for coronal abundance variations of O, Ne, Mg, and Fe. Significant variations are found for the following abundance ratios: Fe/Ne (a factor of 4.1-4.3), Fe/O (2.4-3.1), and O/Ne(2.4). The mechanism, that enhances the coronal abundances of low first ionization potential (FIP) elements apparently operates to a decreasing extent with increasing FIP up to at least 13.6 eV, the FIP of oxygen. The data also indicate that significant abundance changes in the course of a flare are rare, but that changes on a time scale of days are not unusual. It is argued that the minimum observed coronal abundance ratios of Fe/Ne, Fe/O, and O/Ne should be no lower than the photospheric ratios, unless a process that favors the population of the corona with high-FIP elements over lower FIP elements is operative.