Analysis of a Solar Active Region Extreme‐Ultraviolet Spectrum from SERTS‐97

Abstract

Listen

Translate article

Goddard Space Flight Center's Solar EUV Research Telescope and Spectrograph was flown on 1997 November 18, carrying an intensified CCD detector and a multilayer-coated toroidal diffraction grating with enhanced sensitivity over that of a standard gold-coated grating throughout the instrument's 299-353 A spectral bandpass. Spectra and spectroheliograms of NOAA Active Region 8108 (N21°, E18°) were obtained with a spectral resolution (instrumental FWHM) of 115 mA. Nearly 100 emission lines were observed in the spatially averaged active region spectrum. Spectra and spectroheliograms of quiet areas south of the region were also obtained. An end-to-end radiometric calibration of the rocket instrument was carried out at the Rutherford-Appleton Laboratory in the same facility that was used to calibrate the Coronal Diagnostic Spectrometer experiment on SOHO and using the same EUV light source. The accuracy of this calibration is confirmed by the excellent agreement between the measured and theoretical values of density- and temperature-insensitive line intensity ratios. Nine emission lines of Fe XV are identified in our spectrum; however, large differences between wavelengths in the CHIANTI database and some of the measured solar wavelengths, as well as inconsistencies of various theoretical intensity ratios, suggest a need for improvement in the Fe XV atomic physics parameters and/or the presence of unidentified blending lines. Density-sensitive line intensity ratios of Fe XI λλ308.55/352.67, Fe XII λλ338.27/352.11, Fe XIII λλ320.80/312.17, and Fe XV λλ321.78/327.03 yield logarithmic electron densities (in cm-3) of 9.92 ± 0.28, 9.74 ± 0.28, 9.52 ± 0.30, and 9.62 ± 0.26, respectively. Using the strongest emission line observed for each ionization stage of Fe from X through XVI and Ni XVIII, we find that all of the measured nonthermal line widths yield velocities consistent with 35 km s-1. The differential emission measure curve derived from the observed line intensities exhibits a relative minimum at log T ~ 5.7, a broad maximum centered around log T ~ 6.3, and a rapid decline for temperatures above log T ~ 6.6.