ABSTRACT
The transition region and corona at several million-degrees emit UV, EUV and X-ray photons. Non-thermal processes associated with energetic transient events lead to sudden, many-fold enhance-ments in soft and hard X-ray emissions, and even produce gamma-rays. Modern solar astronomy involves observing these high energy photons emitted from the Sun. This is a relatively new science as it had to await the access to space by rockets and satellites as these high energy photons are ab-sorbed in the higher layers of Earth's atmosphere. Solar instrument payload on the Skylab space-mission in 1973 gave a major boost in X-ray solar observations. Recent developments of instrumen-tation in these wavelengths has made it possible to observe the corona and the plasma processes oc-curring over the entire solar disk, and not just at the limb. Coronal holes and CMEs have been dis-covered only after the advent of space era. This chapter discusses the observational challenges in detection of these high energy photons, and their imaging techniques. Multi-wavelength observa-tions have led to a better comprehensive understanding of coronal heating problem, and the primary site of energy release in solar flares. Various physical processes involved in emissions from corona downward to the chromosphere, and upward to the interplanetary space in flares are discussed. Some important past and present space missions and their contributions are described.
