Evolution of the solar corona from solar maximum to minimum

A study on solar coronal activities related to the 11-year activity cycle is presented from the Yohkoh soft X-ray observations, Yohkoh was launched in August 1991, just after the solar maximum of the cycle 22 and continues to observe the Sun in the declining phase of the magnetic activity cycle toward the solar minimum. The soft X-ray flux from the whole Sun in the declining phase essentially decreases with the size of active regions. The X-ray intensity in quiet regions in the declining phase decreases with the magnetic flux observed at the photosphere, The whole-Sun soft X-ray flux does not monotonically decrease, but there are periodic enhancements of the flux with about a one-year interval. The activity appears as bright clusters in the butterfly diagram of the soft X-ray intensity and corresponds to the emergence of complexes of activity in the sunspot zones, The high-latitude activity is also studied, and we find that the X-ray intensity of high-latitude regions fluctuates with time scale of about one year. (C) 1997 COSPAR. Published by Elsevier Science Ltd.