ABSTRACT
Solar photosphere, or the bright visible disk of the Sun represents only the thin outer ‘skin’ of a much larger Sun extending downwards to the hidden interior, and upwards to outer atmospheric layers which are much too faint to be directly observed. After describing the radiative transfer equa-tion, and formation of spectral lines, the chapter discusses the outer layers of solar atmosphere, viz., the chromosphere, transition region, and the corona. Observed features at various scales, such as, the granules, mesogranules, and supergranules, sunspots in the photosphere, and the active regions, filaments/prominences in the chromosphere, and various components of solar corona are described. The use of optical observations using chromospheric spectral lines are discussed for monitoring chromospheric transient energetic phenomena, such as, flares and prominence eruptions, and possi-ble sources of their energy release. Mechanisms of chromospheric and coronal heating are suggested. The observations have made rapid progress in spatial and temporal resolutions, and have contributed towards our understanding of several aspects of these spectacular solar phenomena. However, the optical observations provide only limited information, and additional simultaneous coverage in other wavelengths is required for the higher layers of the Sun in order to obtain a complete 3-dimensional scenario.
