THE COMPOSITION OF A CORONAL ACTIVE-REGION

The relative abundances of iron, oxygen, magnesium, and neon in a coro nal active region are determined from measurements of soft X-ray line and broadband intensities. The emission measure, temperature, and colu mn density are derived from these measured intensities and are used to place a constraint on the abundances of the heavier elements relative to hydrogen in the corona. The intensity measurements were made on 19 87 December 11, when an active region was observed jointly by the Amer ican Science and Engineering (AS&E) High Resolution Soft X-Ray Imaging Sounding-Rocket Payload and the X-Ray Polychromator Flat Crystal Spec trometer (FCS) onboard the Solar Maximum Mission spacecraft. The coord inated observations include images through two broadband filters (8-29 angstrom and 8-39, 44-60 angstrom) and profiles of six emission lines : Fe XVII (15.01 angstrom), Fe XVII (15.26 angstrom), O VIII (18.97 an gstrom), Mg XI (9.17 angstrom), Ne IX (13.44 angstrom), and Fe XVIII ( 14.21 angstrom). The effects of resonance scattering are considered in the interpretation of the FCS line intensities. We calculated the exp ected intensity ratio of the two Fe XVII lines as a function of optica l depth and compared this ratio with the observed intensity ratio to o btain the optical depths of each of the lines and the column density. The line intensities and the broadband filtered images are consistent with the emission from a thermal plasma where Fe, O, Mg, and Ne have t he ''adopted coronal'' abundances of Meyer (1985b) relative to one ano ther, but are not consistent with the emission from a plasma having ph otospheric abundances: The ratios of the abundances of the low first i onization potential (FIP) elements (Fe and Mg) to the abundances of th e high-FIP elements (Ne and O) are higher than the ratios seen in the photosphere by a factor of about 3.5. This conclusion is independent o f the assumption of either an isothermal or a multithermal plasma. The column densities derived from the Fe XVII line ratio and the geometry of the active region provide a diagnostic of the abundance of hydroge n relative to the heavier elements. We find that the abundance of iron with respect to hydrogen in the corona is higher than the value given Meyer (1985b) by a factor of 8.2 (-5.4, +5.1). This means that, for t he observed active region, the absolute abundances of the low-FIP elem ents (Fe and Mg) are enhanced in the corona relative to the photospher e, while the abundances of the high-FIP elements (Ne and O) are either slightly enhanced in the corona or nearly the same in the photosphere and the corona.