Temperature structure of the quiet corona: An SXT-SUMER discrepancy

The Soft X-ray Telescope (SXT) on Yohkoh and the Solar Ultraviolet Measurem ents of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheri c Observatory (SOHO) are both capable of providing measures of temperature in the solar corona. A recent SUMER-based study of a quiet coronal region s uggests 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 re sults 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 lighte r elements, an effect that may be attributable to gravitational stratificat ion. Because iron lines contribute heavily to the X-ray emission detected w ith SXT, it is reasonable to suspect that the more rapid decrease in iron a bundance at higher altitudes might be responsible for the SXT-SUMER discrep ancy. We have therefore explored the effect on SXT temperatures of altering individually the abundances of all 14 elements used in standard SXT temper ature 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.