Solar Cycle Variation of Coronal Temperature, Emission Measure, and Soft X-Ray Irradiance Observed with Yohkoh Soft X-Ray Telescope
This paper presents the solar soft X-ray irradiance (0.3─3.0 nm) obtained from the mission-long full-Sun X-ray images observed with the Soft X-ray Telescope (SXT) of the Yohkoh mission (1991─2001). The irradiance was calculated using filter-ratio temperatures and emission measures (EMs) from the full-Sun integrated X-ray signals employing a coronal spectrum synthesized with CHIANTI atomic database version 8.0. Dependence of the results on the assumption of elemental abundances is investigated. The SXT irradiance for a shorter wavelength range was also calculated and compared with the low energy flux (0.1─0.8 nm) of the X-Ray Sensor on board the GOES satellite. To incorporate the spatial information of the observed corona, we tried the alternative method to derive irradiance by applying the filter-ratio method to the images of northern hemispheric corona instead of spatially integrated signals from there. The EM weighted average of the spatially resolved temperatures turned out to be generally 20%─30% lower than those derived from the integrated signals, while the total of spatially resolved EMs are accordingly 1.5─2 times higher. This trend is enhanced when solar activity is the lowest in early 1996. The irradiance obtained from the alternative method does not significantly vary from the result from the integrated signals. This provides some validation for the simpler full-Sun integrated method. The temporal variation of the EMs of cool (<1.5 MK), medium, and hot (>2.5 MK) components indicates that the ratio of the hot component relative to the medium component is higher in the ascending phase (mid 1998─2001) than in the descending phase (1992─1995) of the solar activity cycle.