Solar activity studies through coronal X-ray observations

The solar corona consists of high-temperature plasma that is contained by a wide range of magnetic field structures. The cyclic behaviour of solar act ivity results in continuing evolution of these structures. This evolution c an be well studied by observing the X-ray and extreme ultraviolet (EUV) emi ssion from the hot plasma which delineates the magnetic field in the corona . In this review, the X-ray images obtained from the Yohkoh mission over mo re than half a solar cycle and the information they provide about coronal e volution will be discussed. A variety of short-term transient brightenings observed by Yohkoh, and at EUV wavelengths by the SOHO mission, will be des cribed and their relevance for coronal heating evaluated. Yohkoh observatio ns have advanced our understanding of solar flares. These important results will be summarized and discussed. Finally. the current view of the nature of coronal mass ejections as deduced by Yohkoh and SOHO is presented.