Coronal patterns of activity from Yohkoh and SOHO/EIT data

Abstract

We have studied the evolution of large-scale coronal structures using soft X-ray data from Yohkoh and EUV data from SOHO /EIT during the rising phase of the current solar cycle 23, and compared with the evolution of the photospheric magnetic field. The poleward-migrating high-latitude coronal structures (“coronal activity waves”) that received significant attention in the past and are represented by the bright polar footpoints of the giant loops in the EUV/EIT data, are visible as the whole loop structures on the solar disk in the soft X-ray Yohkoh data. The Yohkoh data show that these large-scale magnetic connections appear mostly during the rising phase of the solar cycle and its maximum. These connections did not appear or were very weak during the declining phase of the solar cycle. The high-latitude coronal activity waves in the SXT data are closely related to the low-latitude magnetic flux and sunspot numbers and display intriguing quasiperiodic ‘impulses’ with the characteristic period of 1–1.5 years. The relation between the soft X-ray flux, which is considered to be a proxy of the coronal energy flux, and the magnetic flux, can be approximated by a power law with an average index close to 2, and varies with solar cycle.