TEMPERATURES OF CORONAL HOLES OBSERVED WITH THE YOHKOH SXT

Temperatures of coronal holes have been estimated from several sets of soft X-ray images taken through various broad-band filters with the S oft X-ray Telescope (SXT) aboard Yohkoh. Since coronal holes are dark areas, a detailed examination of the telescope point spread function, which is slightly dependent on the X-ray wavelength, is of crucial imp ortance. The calibration is made using post-launch data, and the effec t of scattered X-rays from bright regions surrounding coronal holes, e specially those from nearby active regions, is carefully removed. An i sothermal approximation is applied to the thus-corrected data. The tem peratures of coronal holes near the disk center are found to be 1.8-2. 4 x 10(6) K, which is almost the same as those derived for quiet regio ns not including active regions. The emission measures in coronal hole s are estimated to be 10(25.5-26.2) cm-5, about ten times smaller than those of quiet regions. We conclude that temperatures in coronal hole s do not differ from those in quiet regions, and that the depression i n the soft X-ray intensity of coronal hole regions results from a lowe r density by a factor of 3 than quiet regions.