Solar uv irradiance, its variation, and its relevance to the earth Original Research Article

Originating in the sunʹs upper photosphere, chromosphere, transition zone, and corona, solar UV (including EUV) emissions have a profound effect on Earthʹs ionosphere, thermosphere, mesosphere, and stratosphere. Through ionization, dissociation and excitation processes, solar UV is the primary source of energy input to the atmosphere and, as a result, it plays a central role in the atmosphereʹs vertical, thermal, and electronic structure. Further, the solar UV irradiance is a principal driver of the strong dynamics in the atmosphere and its cycles of chemical species, especially those of nitrogen and oxygen. Over the past three decades, numerous space-based measurements of solar UV light have been made in order to better understand its nature and effects. They show that the UV irradiance does varies through time, primarily on 27-day solar rotation and 11-year solar cycle time scales, and that this wavelength-dependent variation is primarily associated with the emergence, growth, and decay of active regions on the solar disk. Numerous models and proxies have been developed with some success to better understand and predict solar UV variations and their wavelength dependence. However, in spite of considerable progress, the present estimates of solar variability together with atmospheric models still do not provide complete and accurate descriptions of atmospheric phenomena. Acordingly, we cannot be sure that the accuracy of current measurements is sufficient. Moreover, in some cases, correlation studies of both atmosphere and climate, imply that solar variations are more important than physics-based models would indicate. We discuss the current international UV irradiance measurement and modeling program and make recommendations about its continuation. Also, we outline some evidence for and possible causes of a solar connection in the evolution of atmosphere and climate.