Variation of Thermal Structure with Height of a Solar Active Region Derived from SOHO CDS and YOHKOH BCS Observations

Abstract

We present observations of NOAA solar Active Region 7999 when it was near the west solar limb on 1996 December 2 and 3, using data from the Coronal Diagnostic Spectrometer (CDS) experiment on the SOHO satellite. Ratios of intensities of 2 MK material (as observed in CDS Fe XVI images) to 1 MK material (from CDS Mg IX images) indicate that there is a drop in the ratio of the hotter to the cooler material with height in the region, up to an altitude of about 10⁵ km. At low altitudes the relative amount of 2 MK emission measure to 1 MK emission measure ranges from 8 to 10, while the ratio is minimum near 10⁵ km, ranging from 1.3 to 3.5. The decrease with height of the CDS ratio qualitatively resembles the decrease in S XV election temperature with height (measurable up to ~85,000 km) in the same active region obtained from the Bragg crystal spectrometer instrument on Yohkoh. The CDS images indicate that the highest S XV temperatures and largest CDS ratios correspond to regions of microflares, and somewhat lower S XV temperatures and CDS ratios correspond to diffuse regions. Above 10⁵ km, the trend of the CDS ratios changes, either increasing or remaining approximately constant with height. At these altitudes the CDS images show faint, large-scale diffuse structures.