Solar cycle variation of coronal temperatures and emission measures (1991-2001) observed with Yohkoh Soft X-ray Telescope

The soft X-ray telescope (SXT) aboard the Yohkoh satellite is a grazing-incidence telescope which observed the Sun for a nearly whole solar cycle (Cycle 22 max through Cycle 23 max). Like Hinode/XRT, SXT was equipped several broadband X-ray analysis filters, which were designed to diagnose coronal temperatures(Te) and emission measures(EM) through the filter ratio technique.

We used the whole-sun integrated intensities of the two thin filters (the Al.1 and AlMg filters) to calculate coronal temperatures and emission measures for the whole mission, based on recently re-calibrated SXT response functions and a recent atomic database (CHIANTI ver. 8.0). Our result shows that in the plane of log_Te (K) and log_EM (cm^-3), daily averaged coronal plasma at the solar maximum located around (6.5, 49.5), moving toward around (6.25, 48.7) at the solar minimum then turning back to the previous values at the maximum phase. Our calculation assumed so-called 'hybrid' elemental abundances. By assuming traditional coronal abundances, Te showed no significant difference, while EM resulted in 30 to 45% lower values.

We next applied the filter ratio method to the images instead of integrated intensities, to derive solar cycle variation of the EM of cool(less than 1.5MK), medium(1.5 to 2.5MK) and hot(more than 2.5MK) temperature component of coronal plasma. It turned out that the medium and hot components show similar sunspot-cycle variation as the soft X-ray irradiance. The cool component, on the contrary, keeps flat in descending and rising phase of the activity cycle and shows enhancement at the minimum phase. It also turned out that the EM weighted average of the spatially resolved temperatures are generally 20 to 30% lower than those derived from the integrated intensities, and that the total of spatially resolved EMs are accordingly 1.5 to 2 times higher than the values from the integrated intensities.