Evolution of the Solar Corona from the Maximum to Minimum

Abstract

The evolution of the solar corona from the maximum to minimum phase in the cycle 22 is reported from the soft X-ray observation with Yohkoh soft X-ray telescope (SXT). The variation of X-ray flux is investigated for two coronal components; active regions and other dark features consisting of quiet regions and coronal holes. The X-ray flux from active regions is strongly dependent on the size of active regions and changes with the solar activity cycle in phase. We find that the X-ray flux from the other dark features also changes with the solar cycle, and that there is a strong correlation between the X-ray flux from the dark features and the corresponding photospheric magnetic flux outside active regions. These show that the solar corona is heated in conjunction with the magnetic field, and that there is no possibility of being due to the acoustic heating. The soft X-ray flux does not monotonically decrease. Rather, there is a period of enhancement with about a one-year interval in the whole-Sun X-ray flux. 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 the X-ray intensity of the high-latitude regions fluctuates with a time scale of about one year.