Temperature Structure of the Quiet Corona: An SXT-SUMER Discrepancy

Abstract

The Soft X-ray Telescope (SXT) on Yohkoh and the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) are both capable of providing measures of temperature in the solar corona. A recent SUMER-based study of a quiet coronal region suggests an isothermal plasma in the range from 1.03 to 1.5 solar radii. In this work, we have analyzed part of the same region using SXT data and find a temperature that increases with height. This is consistent with earlier SXT-based coronal temperature studies but obviously disagrees with SUMER results for the same region. The SUMER study reveals a lower scale height for emission in the Fe X, Fe XI, and Fe XII lines than for the lines of lighter elements, an effect that may be attributable to gravitational stratification. Because iron lines contribute heavily to the X-ray emission detected with SXT, it is reasonable to suspect that the more rapid decrease in iron abundance at higher altitudes might be responsible for the SXT-SUMER discrepancy. We have therefore explored the effect on SXT temperatures of altering individually the abundances of all 14 elements used in standard SXT temperature calculations. We find that only iron and carbon have any significant effect but that this effect is not nearly sufficient to account for the SXT-SUMER discrepancy.