Measuring Active and Quiet-Sun Coronal Plasma Properties with Extreme-Ultraviolet Spectra from SERTS
We obtained high-resolution extreme-ultraviolet (EUV) spectra of solar active regions, quiet-Sun areas, and off-limb areas during 1991 May 7 and 1993 August 17 flights of NASA/Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS). The 1991 flight was the first time a multilayer coated diffraction grating was ever used in space. Emission lines from the eight ionization stages of iron between Fe+9 (Fe x) and Fe+16 (Fe XVII) were observed. Values of numerous density- and temperature-insensitive line intensity ratios agree with their corresponding theoretical values.Intensity ratios among various lines originating in a common stage of ionization provide measurements of coronal electron density. Numerous density-sensitive ratios are available for Fe xiii, and they yield active region density (cm-3) logarithms of 9.66±0.49 and 9.60±0.54 for the 1993 and 1991 flights, respectively, and a quiet-Sun density of 9.03±0.28 for the 1993 flight. Filling factors, calculated from the derived densities assuming a path length of 1 × 109 cm, range from several thousandths to nearly unity. Intensity ratios among lines originating in different ionization stages of iron yield measurements of coronal electron temperature in the isothermal approximation. The line ratios yield temperatures ranging from 1.1 × 106 to 5.2 × 106 K for the active regions, and 1.0 × 106 to 2.1 × 106 K for the quiet Sun, depending upon the ionization stages used. The derived temperature diminishes with decreasing ionization stages. Fe XVII emission, detected in the active regions but not in the quiet areas, accounts for the higher maximum active region temperature. Derived active region temperatures are greater than their quiet-Sun counterparts for ratios that include lines from Fe xiv through Fe XVI; however, the derived active region and quiet-Sun temperatures are not statistically significantly different for line intensity ratios that involve only Fe x through Fe xiii. The latter similarity in derived temperatures suggests the presence of similar thermal structures in all the areas observed, although the active regions also harbor hotter material. Differential emission measure (DEM) distributions were constructed for the active region and quiet- Sun observations obtained during both flights. The two quiet-Sun DEM curves and the 1993 active region DEM curve all show peaks between log T = 6.1 and 6.2. The 1993 active region DEM has a second peak between log T = 6.6 and 6.7, and the 1991 active region DEM has only one peak, between log T = 6.5 and 6.6. Thus, the 1993 active region DEM curve appears, in some sense, to be a composite of the quiet-Sun DEM curve and the 1991 active region DEM curve. The 1991 active region exhibited flaring activity, yielded higher line ratio temperatures, and contained greater photo spheric magnetic fields than the 1993 active region.